DEVELOPMENT OF COLLATERAL CIRCULATION IN PARTIALLY

EEVASCULARIZED FELINE SCIATIC NERVE

31

JOHN FRANCIS O'KALLEY

A THESIS

Submitted to the Faculty of the Graduate School of the Creighton University in Partial Fulfillment of the Requirements for the Degree of Master of Science in the Department of Anatomy

Omaha, 1 969 V

ACKNOWLEDGMENTS

I am deeply indebted ana grateful to my advisor, Doctor Julian J. Baumel, for his patience, unselfish assistance, and constructive criticism throughout this investigation. I also wish to express my appreciation to Dr. R. Dale Smith for his valuaole assistance, to Dr. John 3arton for his help with the photogranhie representations, to Mrs. Edith Witt for the expert typing of the final manuscript, and to all the members of the faculty of the Anatomy Department for the excellent graduate training I have received. I especially wish to acknowledge my wife, Helen, for her understanding and perseverance through­ out my graduate studies, and it is to her that I dedicate this t h e s i s . vi

TABLE OF CONTENTS

A C KN OWLE DG ï T E N T S ......

LIST OF ILLUSTRATIONS

Chapter

I. INTRODUCTION ...... 1

II. HISTORY ...... 1+

Vasa Nervorum Collateral Circulation

III. MATERIALS AND METHODS ...... 10

Surgical Exposure Devascularization Vascular Injection Clearing

IV. O B S E R V A T I O N S ...... l£

Sciatic Nerve - Anatomy Vascular Supply of Sciatic Nerve Art e r i e s Veins

Development of Collateral Circulation Three-Day ‘/ascular Pattern Six-Seven Day Vascular Patterns Fourteen-Seventeen Day Vascular Patterns

V. D I S C U S S I O N ...... 23

Role of Vasa Nervorum Collateral Circulation

LITERATURE C I T E D ...... vii

LIST OF ILLUSTRATIONS

Figure

1. Sciatic Nerve Bed Showing Relationships to S u r r o u n d i n g M u s c u l a t u r e ...... 33

2. Segment of Normal Sciatic Nerve Showing Linear Pattern of Blood Vessels ...... 36

3. Bifurcation of Sciatic Nerve Into Tibial and Common Peroneal Divisions ...... 37

h. Transitional Zone Between Intermediate and Lower Segments Showing Characteristic Features of Three Day Operated Nerve .... 38

5» Intermediate Segment of Three Day Operated Nerve Demonstrating Incipient Enlargement of Longitudinal Epineurial Vein and Smaller U n f i l l e d Tortuous Veins ...... 38

6. Three Day Operated Nerves Viewed Macro- s c o p i c a l l y ...... 39

7. Segment of Three Day Nerve Demonstrating Poor Filling at Higher Magnification .... 39

8. Comparison Between Six-Seven Day Operated Nerve and N o r m a l N e r v e ...... [¿0

9. Six-Seven Day Nerve in situ With Portion of Enveloping Sheaths Displayed ...... ]RL

10. Plexiform Vascular Pattern in Tibial Division of Fourteen Day Operated N e r v e ...... ¡+2

11. Segment of Six-Seven Day Operated Nerve Demonstrating Comparative Response of Smaller Epineurial Arteries end Veins .... 1+3

12. Segment of Six-Seven Day Operated Nerve Characterized by Very Prominent Longitudinal Epineurial V e i n ...... '\1+l 1.

INTRODUCTION

A survey of the existing literature in the field of peripheral nerve vascularity has established that nerve function is influenced by vascular supply; however, opinion varies considerably concerning the extent of such dependence• Apparently, influence of vascular supply is unquestioned; its importance is suspected, whereas its necessity is highly polemical.

Just how important is vascular supply to a nerve? Its neces­ sity in other body organs is unquestioned; yet its value for normal nerve function is still controversial. Since the discovery of axioplasmic flow (Weiss, 'k3; Weiss and Davis, *U3) two schools of thought have arisen; one holding that the only essential require­ ment for normal peripheral nerve function is the anatomical and physiological continuity of the axon with its cell body and that the primary source of neuronal nutrition is the pulsating flow of axoplasm proceeding distally along the fiber, blood supply being only incidental. The other school claims that vascular supply is essential if normal neuronal structure and function are to be maintained. In all probability both contribute to normal nerve function.

Reports in the literature have shown that ischemia of peripher­ al nerves causes decreased nerve fmiction and have indicated a definite correlation between the vascular state of the nerve and its performance. In an endeavor to verify this, preliminary 2 experiments were conducted on cats, devascularizing the femoral extent of their sciatic nerves. These experiments have demonstrated that ischemia produces alterations in the rate of alpha fiber con­ ductivity. It was consistently observed that during the first three days following controlled devascularization slower conduction velocities were obtained from the devascularized nerve than from the control side. On the other hand, conduction velocities of the ischemic nerves of 5-7 day animals approximated those obtained from the contralateral control side. The pilot studies were consistent with the findings of Gillilan (’66), relating conduction velocity to ischemia.

Inspection of the partially devascularized nerve in injected specimens led to the belief that the return of "normal” conductivity occurred synchronously with the restoration of vascular supply via collateral pathways, evidenced by the enlargement, proliferation, and tortuosity of the epineurial vessels. In other words, it appeared that vascular embarrassment of the sciatic nerve caused decreased conduction velocity initially, whereas collateral circu­ lation, when functionally established, restored, perhaps even enhanced, blood supply, increasing the velocity. This apparent correlation between the vascular competency of the nerve and its functional state required that the details of the development of a collateral circulation be investigated prior to continuation of conduction velocity studies. The research endeavor was now focused on producing "standard" ischemia of the sciatic nerve and studying 3. the reestablishment of circulation on a temporal basis — its mode, extent, pattern, and salient features as contrasted with the vascular pattern of the normal feline sciatic nerve♦ b-

HISTORY

The brief historical review which follows is necessarily two­ fold. It is concerned with the normal vascular supply to peripheral nerves (vasa nervorum), and with the development of collateral circulation evidenced by the changing angioarchitectural pattern that emerges in response to ischemia. A survey of the literature dis­ closes that both topics, the blood supply of nerves, and the development of collateral circulation, have been repeatedly examined and periodically reviewed. However, re-examinâtions must continue as new observations are made and more recent findings become avail­ able.

Vasa Nervorum

It is generally agreed that Albrecht von Haller (1?5>6), be credited the first to consider the vascular supply of nerves in detail. In 1627, Van der Spiegel opined that vasa nervorum might be important in the nutrition of peripheral nerves. Ruysch (1701) was also apparently aware of the potential significance of the vasa nervorum. The first publication devoted exclusively to the subject of peripheral nerve vascularity entitled "De Vasis Nervorum" was co-authored by Isenflamm and Doerffler (1768). Using colored wax they successfully displayed the networks of vessels surrounding nerves. Still another century passed before Ranvier (1878)" delin­ eated these extrinsic vessels in greater detail, in addition demonstrating that nerves housed internal as well as external 5 vascular plexuses which were in communication with one another, anastomoses occurring at both the intrafascicular and interfascic ular levels.

Hyrtl, between 1859 and 186U, published a series of papers which marked the initial attempt to formulate general principles regarding the angio-architecture of peripheral nerves. He established certain principles concerning peripheral nerve vascu­ larity, proposing in essence that:

1) every nerve, regardless of size, receives

nutrient arteries which run to the nerve

without branching and after passing along

its surface, enter it to form internal

capillary plexuses ;

2) these arteries supply only the nerve and

do not provide any branches to adjacent

muscles; and

3) each vessel, although concerned primarily with

the supply of a definite segment of the nerve,

bifurcates into ascending and descending

branches, each of which, in turn, anastomoses

with the corresponding branches of adjacent

nutrient arteries. In this way, a

continuous longitudinal anastomosis is

formed within the nerve ; it was to this

anastomosis that Hyrtl attached great o.

significance.

The monographs of Quenu and Lejars near the termination of the 19th century (1890, 1892, 189U;, deserve special note. Their descriptions are classical in detail and presentation, and their appreciation of functional significance profound. They paid particular attention to the numerous anastomoses between adjacent vasa nervorum both on the surfaces and within the trunks, contend­ ing that such an arrangement would indeed render a total interruption of blood supply to the nerve highly improbable. They demonstrated insight as to the importance of these vessels beyond representing merely potential pathways in the establishment of collateral circulation. For example, they noted specifically that the difficulty sometimes experienced in speech and respiration after a thyroidectoi^y was likely due to diminished vascularity of the vagus and recurrent laryngeal nerves which, during their cervical course, are supplied exclusively by the thyroid arteries.

The work of Quenu and Lejars was characterized by a further elucidation of principles which tended to verify, supplement, rectify and amend the principia of Hyrtl stating that:

1) A nerve is never supplied by a single artery

but rather always receives a number of branches.

2) Arteria nervorum are constant in their origin.

3) Nutrient arteries approach the surface of the

nerve obliquely and upon landing, bifurcate into

ascending and descending rami, each of which 7.

runs for some distance on the surface of the

nerve before entering.

It may also be especially important to note that Quenu and

Lejars appear to be the only investigators up to now who have systematically considered the venae nervorum. They elucidated in some detail the venous drainage of nerve trunks, stressing the existence of venous as well as arterial intercommunications. At the termination of the 19th century there appeared, almost simul­ taneously, two significant communications dealing with the vasa nervorum. These were Bartholdy's account of the arteries of nerves

(1897) and Tonkow's account of the arteries of spinal nerves and their ganglia (1897). Although these investigations were carried out independently, they produced remarkably similar findings of a descriptive nature concerning the vasa nervorum.

Several works dealing with nerve vascularity as well as review articles have appealed in the current century. Those most pertinent to the present study will be cited in the discussion.

The vascular architecture of nerves has been conveniently and efficiently reviewed by Adams (*U2), Sunderlund ('US), Ric h a r d s

( *5>1)} Waksman ('61) and Edsage (*6U). The work of Lundborg and

Branemark (’68) brings this literature up to date.

Collateral Circulation

Over 200 years ago John Hunter noted that ligation of the major nutrient artery to a stag's antler not only did not interrupt 8.

its growth but also produced a prodigious growth of new vessels.

His observations had been anticipated lp centuries earlier by

Antyllus, who noted that the interruption of the artery to a limb

does not necessarily result in its loss. Hunter’s observations

impelled him to undertake numerous experiments as a result of which

he sagely concluded, "vessels go where they are needed."

Thoma (1893), long a student of responses of the vascular

system, is credited with recognizing the molding force of the

circulating blood; from his observations on the development of the

area vasculosa of the chick he codified the original general

principles of what he termed histo-dynamics by stating :

1) The luminal growth of a vessel or the

increase in surface of the wall depends

upon the rate of blood flow.

2) Growth in thickness is determined by the

tension in the wall, and this in turn is

related to the diameter of the vessel and

the blood pressure.

3) Increased blood pressure above a certain

level determines the formation of new

capillaries.

Although Thoma »s laws did not meet with universal acceptance, they nevertheless stimulated much thought and provoked plausible hypotheses regarding the forces and factors responsible for the development of collateral circulation (both proliferation of 9. existing channels and the genesis of new vessels). Clark (*18) asserted that the amount of blood flow rather than the rate of flow promoted arterial enlargement and that perhaps the magnitude of chemical interchange through the wall of a capillary is the determining factor causing the formation of new vessels. Thus, when a certain level is exceeded new sprouts are sent out.

Scholarly reviews concerned with the complex mechanisms of collateral circulation have been published by Kulvihill and Harvey

(*31;9 Quiring (*U9)> Longland ('53), Learmouth ('50), Rau and

Schoop ('60). Liebow (*63) presents an excellent account of col­ lateral circulation in his discussion of situations which lead to changes in vascular patterns. 10.

MATERIALS AND METHODS

Thirty-three cats (Felis catus) were employed in this research; twenty-one were utilized for purpcs es of preliminary experimentation; the other twelve provided the definitive series on which this report is based. The pilot experimentation was threefold: 1) to develop a technique of surgical exposure and method of devascularization which would yield "standardized" ischemic sciatic nerves with minimal bleeding and trauma; 2) to determine by trial the injection method which would most exquisitely display the sciatic vascular tree; and 3) to assess the effect of ischemia on conduction velocity

of the sciatic nerve. An elaboration of the methods of surgical exposure, devascularization, and injection follows ^ general find­ ings correlating conduction velocity changes with vascular deprivation are discussed elsewhere.

Surgical exposure. Prior to surgery animals were arbitrarily assigned to one of three groups. C-roup A (li specimens) animals were permitted to survive three days following devascularization before sacrifice and injection. Group B animals (U specimens) were terminated and injected after six days; Group G animals (U speci­ mens ) after two weeks.

The anesthetic employed was pentobarbital sodium (3f> mg/kg body wt.), intraperitoneally injected. The left thigh was the operated side in the entire series, the right side the control.

The skin incision began in the sacral region, passed between the 1 1 . greater trochanter and ischial tuberosity, paralleled the shaft of the femur posteriorly, and terminated on the lateral aspect of the knee. The deep surface of adjacent skin was freed from underlying fascia and reflected anteriorly. This exposed an area presenting portions of the gluteus maximus, caudofemoralis, tensor fasciae latae, and biceps femoris muscles. This method of reflection also insured the motility necessary to bring the cut ends of the skin into close apposition so that wound clips could effect proper closure following devascularization. After reflection of integu­ ment, the fascia lata was incised; spreaders were employed both proximally and distally exposing the nerve between the biceps femoris and the' vastus lateralis. At first glance the tenuissimus muscle may be mistaken for the sciatic nerve;this band-like muscle arises from the transverse process of the second caudal vertebra, resembling the nerve in all dimensions while paralleling it through­ out its femoral extent. Figure 1 shows the relationship of the sciatic nerve to surrounding, musculature.

Upon exposure, the nerve was traced proximally until it dis­ appeared under cover of the piriformis muscle; it was traced distally into the popliteal fossa. The. removal of adipose tissue was required to display the bifurcation of the nerve (tibial and per­ oneal divisions) as well as the popliteal and saphenous vessels.

It was generally possible to lift a major portion of the nerve from its bed simply by grasping its fascial "mesentery" with forceps; this facilitated identification of vessels supplying the nerve. 1 2 .

Devascularization. Upon exposure, both the popliteal artery and vein were occluded with a single ligature proximal to the origin of the small saphenous branches interrupting a major source of supply to the lower extent of the nerve. All nutrient vessels to the nerve (very few were noted) from this point to the lower border of the piriformis were severed; bleeding being minimal. The only other significant vasa nervorum were encountered proximally where gentle elevation of the piriformis muscle was usually required to detect and occlude the several contributions received from the inferior gluteal artery. Although the segmental supply was inter­ rupted from the popliteal to the gluteal level, in all instances both the epineurial and interfascicular system of anastomosing channels remained intact. Throughout this procedure, saline was applied to prevent drying of tissues. When oozing of the sectioned vessels had ceased, closure was accomplished by repositioning the retracted hamstring musculature without suturing, bringing the skin flaps into apposition and closing with wound clips. The incision was then bathed externally with a 3% solution of hydrogen peroxide which proved effective in preventing infection. This technique of occluding certain vessels resulted in a fairly uniform- preparation from animal to animal.

Vascular injection. Preliminary experimentation with injection techniques was carried out on numerous specimens. Some animals were double injected with red and blue latex, providing excellent con­ trast between arteries and veins. Latex injections were utilized 1 3 .

for exploratory dissections in order to become acquainted with the

vascular patterns in the pelvis, gluteal region, and pelvic

extremity• A second group was injected with Pelikan physiological

ink. Ink injections fill arteries, veins, and capillaries hinder­

ing distinction of arteries from veins. On the other hand, ink

furnished the best possible delineation of angio-architectural

patterns allowing for better appreciation of the minute vascular

plexuses. This had been the prime liability of the double latex

injections, enabling one to distinguish only larger arterioles and

venules, providing poor detailed filling at the microcircul&tory

level.

All cats utilized in the definitive study were injected with

Pelikan physiological ink at sacrifice. The injection procedure

was also executed under pentobarbital sodium (33' mg/kg) anesthesia.

Since only posterior filling was desired, the avenue of injection

was via the abdominal aorta. The abdominal wall was incised and

reflected, bleeders being securely clamped with hemostats.

Abdominal viscera were retracted; the aorta between gonadal and

caudal mesenteric branches was divested of fat, and a ligature

applied to the aorta just below its gonadal branch. After clamp­ ing distally, the aorta was cannulated just cranial to the caudal mesenteric artery with polyethelene tubing to which a 10 cc. syringe was affixed. Upon removal of the distal clamp, ink was perfused in pulsating fasnion. Administration of about 30-u0 cc. provided consistently excellent angioarchitectural displays. The main iu. muscle masses and thorax were injected with 10/6 formalin prior to immersing the animal in this fixative. After one week the animal was removed, transected at the upper lumbar level, and the pelvis split at the symphysis. This facilitated spreading on a dissection board so that comparative observations between the control and

operated sides could be made at the mesoscopic level. After in

situ observations were made the sciatic nerves were excised

bilaterally and cleared for microscopic inspection and photography.

Clearing. The sciatic nerve was removed from the level of

the piriformis to the proximal crus (tibial division where it

passes between the heads of the gastrocnemius and peroneal on the

lateral aspect'of the leg) and cleared according to the method

outlined by Marcarian et al (*67). The nerve was dehydrated in

three changes of acetone and cleared in a single methyl benzoate

immersion (15 minutes each at 37° C.). For microscopic study and

photography, the nerves were submerged in a Petri dish filled with

methyl benzoate in order to prevent drying. Photographs of the

sciatic angio-architecture were taken under a Leitz dissecting

microscope at a magnification of I8x, using a Leica III g. 35 mm.

c a m e r a . OBSERVATIONS

Sciatic Nerve - Anatomy

The sciatic nerve of the cat is organized from the ventral rami of the sixth and seventh lumbar' and first sacral nerves, its major component being the L? root. The intrapelvic part lies

dorsal and medial to the obturator nerve. The sciatic nerve (which is actually two nerves, tibial and peroneal bound together in a common connective tissue envelope) arches out of the pelvis into the gluteal region, emerging beneath the inferior border of the pirifor­ mis muscle, descending in the interval between the ischial tuberosity and the greater trochanter. No specimens were observed in which the tibial and peroneal nerves emerged separately from beneath the piriformis. Throughout its femoral extent, the sciatic nerve is closely applied to the inner aspect of the biceps femoris, resting successively on the adductor femoris and semimembranosus muscles before bifurcating into its tibial and peroneal divisions as it approaches the popliteal region (fig. It). In the proximal thigh, the sciatic nerve provides a prominent leash of fascicles to the hamstring musculature.

The peroneal nerve or lateral branch serves as the source of supply to the peroneus musculature, the anterior crus, and the dorsum of the foot. The tibial nerve enters the popliteal fossa, passes between the heads of the gastrocnemius and supplies the posterior crural musculature. After circumventing the medial 16.

malleolus, it divides into the medial and lateral plantar nerves

which supply the intrinsic musculature of the foot.

Vascular Supply of Sciatic Nerve

Terminology used by lundborg and Branemark (»68} will be

employed. "Lpineural vessels1» are those found on the surface of

the epineurium or within its sheaths ; the term conjunctiva nervorum,

according to Lundborg and Branemark, refers to the outermost

epineurial layer. The 'extrinsic system* of vessels comprises both

the vasa nutritia and epineurial vessels whereas the 'intrinsic

system1 refers to all the vessels of the nerve found inside the

epineurial perimeter. An extrafascicular plexus is one found

around the outside of a nerve fascicle; and intrafascicular plexus

is a similar network within a fascicle.

Arteries. The arteries supplying the sciatic nerve vary only

slightly in the series examined; the foilowing description is a

composite one representing the most common situation. The most

proximal portion of the nerve was supplied by contributions from

the inferior gluteal artery. These nutrient arteries (2-3 in

number) approach the nerve from the medial side; generally two

arteries pass onto the superficial surface and one to the deep

surface of the upper third of the nerve, at the level of the lower border of the piriformis. The often prominent nutrient branch from the inferior gluteal artery is the Arteria comitans nervi ischiadici

between the two main divisions of the nerve. The nutrient arteries

from inferior gluteal generally divide on the nerve into ascending

and descending rami (see Discussion). These, in turn, not only

ramify on the surface but also send penetrating branches into the

nerve. The artery which accompanies the nerve to the hamstrings

is derived from one of the branches of the inferior gluteal.

A consistent supply to the intermediate segment of the nerve

was not observed. There seemed to be no significant nutrient ves­

sels provided for this intermediate segment; the few (3 -2*) m i n u t e

branches noted (generally reaching it via its "mesentery") arose

from perforating rami of the profunda femoris in a somewhat vari­

able manner. These arteries are always accompanied by veins

(usually paired venae comitantes), which are usually readily discernible owing to their larger caliber and complex branching patterns which dominates the vascular scene on the surface of the n e r v e .

Supply to the distal segment is derived from the small saph­ enous branch of the popliteal artery. A branch from this small saphenous artery climbs for some distance in the interval between tiie tibial and peroneal divisions. This was the largest nutrient artery found coursing to the nerve. During its ascent, rami are given ofi to both nerve divisions, the artery continuing to trie lower part of the intact sciatic nerve where it usually divides into two branches. The more prominent lateral branch can be traced up­ ward until it becomes continuous with the main descending branches 18.

of the inferior gluteal ; this then is the companion artery to the

sciatic nerve. These primary arteries give off branches at

essentially right angles which, in turn, give rise to ascending and

descending rami which ramify in a linear pattern paralleling the

nerve fasciculi. (fig. 2)

Veins. The main venous drainage of the sciatic nerve is

proximal into the inferior gluteal system and distal via the saph­

enous vein into the popliteal. Generally, two (sometimes three)

prominent longitudinal channels extend the entire length of the

nerve. Sometimes medial and lateral longitudinal veins reside in

the sulcus between the two divisions of the nerve ; at other times,

only one vein is found in this sulcus, the other paralleling the

posterior edge of the nerve.

The intermediate segment of nerve shows venous connections

with muscular veins. However, the flow appears to be into the

longitudinal veins of the nerve rather than drainage laterally

from the neural veins into muscular veins. Details of variation

in these main venous channels have not as yet been studied.

Development of Collateral Circulation

The principal vascular responses observed in the epineurial angioarchitecture of the partially devascularized sciatic nerves were in all cases much more pronounced in the veins than in the arteries. Comparisons and descriptions of the vascular patterns which follow tend to emphasize the venous configurations since they 19.

display the most striking departure from the normal. Even in the

normal nerve the veins dominate the vascular pattern on the sur­

face; the drastic changes in both size and pattern that they undergo

are much more striking than those of the arterial channels which

exhibit these responses to a lesser degree, (figs. 2,3)

In the normal vascular picture the orientation of surface

vessels is essentially parallel to the long axis of the nerve.

By contrast, the operated nerves in three-day animals presented

enlarged main longitudinal channels, poor filling at mid-thigh

level, and incipient tortuosity proximally and distally. Six

days after surgery venous convolutions had become more pronounced

and were meandering through the entire epineurium; the main channels

were now strikingly enlarged and secondary and tertiary channels

nad followed suit, ihe vessel pattern of the two-week postoperative

nerves is a compromise between a normal and a six-day nerve, a

reduction being evident in size and tortuosity.

Three-day vascular patterns. The operated nerves were loosely

adherent to their bed, requiring little effort to free them. No

connection with any of the neighboring nutrient vessels was observed which would indicate a reestablishment of segmental supply. Meso­ scopic examination showed noticeable enlargement of the two m i n longitudinal venous channels. The medial longitudinal vein showed greater enlargement than the lateral longitudinal vein. Compari­ son of operated and control nerves showed the main longitudinal arterial channels to be slightly enlarged in two cases, not 20.

perceptibly different in size from the normal in the other speci­

mens .

Tortuosity, or meandering of epineurial venous channels, was

commencing in all three-day operated nerves. The intermediate

segment of all nerves was poorly injected with ink without excep­

tion. Tortuosity of veins was apparent mostly in the distal third

of the nerves, more on the peroneal division than on the tibial.

For the most part, the tortuous vessels were filled with clotted

blood in the transitional region between injected and non-injected

areas of the nerve. The blood-filled surface veins of the inter­

mediate segment indicate stasis of flow had occured, resulting in

clot formation, restricting collateral circulation to this part of

the nerve. 3y contrast, the excellent ink filling throughout the

normal nerves verified that the intermediate segment of each

operated nerve was inadequately supplied with blood and improperly

drained, (figs. lj - 7)

Six-seven day vas cular patterns. Figure 8 shows a segment of

six-seven day operated and control nerves. The operated nerves

demonstrate an extremely rich vascular pattern. The intermediate

segments of all except one of the six-seven day operated nerves showed good filling with the ink. This is interpreted as signify- ing that by this time a functional collateral circulation had

become sufficiently established to revascularize the entire nerve.

In the single exceptional case, the intermediate segment resembled 21.

that part of the three-day nerves having poor filling of vessels

because of retention of clotted blood in the veins. Epineurial

vessels at all levels displayed hypertrophy, and the main longi­

tudinal venous channels were huge by comparison with their control

counterparts. Arteries by contrast were only slightly enlarged

and appear patent in the intermediate third. In addition to tortu­

osity per se, the type of interconnections between epineurial

vessels was noticeably different from the familiar angioarchitectural

pattern on the normal side. Whereas the surface vessels on the con­

trol nerve were laid out in essentially linear array with angular

branchings, the epineurial pattern on the devascularized nerve was

distinguished by its arboreal arrangement ; it was marked by

intricacy and complexity of pattern, convoluted vessels, and circu­

itous interconnections.

Generally, at this postoperative stage, the whole nerve has

revascularized exceedingly well. Only in one instance could the

intermediate segment be considered as still deprived because of the clotted blood in surface vessels and inability to accept the ink injectant.

In addition to the vascular appearance of the nerve in this series, the operated nerve was characterized by the presence of a rather tough, thick connective tissue envelope surrounding the nerve. The nerves were firmly "bedded down" by this tissue and could not be easily pulled away from their bed as could those of the short-term groups. (fig. 9) 22.

When this sheath was incised and carefully peeled away it was

seen to be highly vascularized on its inner aspect. In several

cases, small muscular arteries passed into the sheath; these made

connections with the minute epineurial vessels on the nerve.

Fourteen-seventeen day vascular patterns. Channels at all levels were well filled throughout the entire femoral extent of the operated nerves. The main longitudinal venous channels appear slightly larger than normal, but smaller than those of the six-seven day operated nerves. A very definite epineurial hypervascularity still prevails over the entire nerve in a pattern that differs from the normal as well astthat of the operated nerves of earlier stages. The linear type vessels and angular intercon­ nections of the vessels characteristic of the normal nerve are lacking; the meandering tortuosity and convolutions of surface veins characteristic of the six-seven day specimens is missing and has been transformed into yet another pattern. Perhaps the vascular pattern that is exhibited here could best be described as a com­ promise between normal and a six-seven day pattern. In the main, these nerves display an intricate and complex plexiform surface pattern of veins which is much richer than that of the six-seven day operated nerves, (fig . 10)

The tough, enveloping nerve sheath seen in the previous group was present in only one operated nerve of this series; however, all nerves were adherent to the underlying tissue bed. DISCUSSION

hole of Vasa Nervorum

Standard textcooks of anatomy and histology confer only

cursory coverage of the vascular supply of peripheral nerves simply

mentioning the existence of such channels, the vasa nervorum,

citing only instances where an obvious continuous longitudinal

channel exists• The maximum elaboration of such channels is

encountered in the Arteria comitans nervi isohiadici of the pelvic

member and the Arteria comitans nervi mediani of the pectoral mem­

ber. Nutrient vessels are generally derived from adjacent arteries,

exhibiting considerable variation as to source and size. Never­

theless, tiie essential plan is the same — each nerve receiving

along its course a variety of nutrient or so-called segmental

arteries from adjacent vessels. Prominent vessels running on the

nerve for a considerable distance have been shown to represent

persistences of embryological vessels.

In recent years, the most apparent impetus responsible for

revival of interest in the vasa nervorum has been their indictment

as possible causal factors in certain peripheral nerve disorders.

This indictment is not new since Boerhaave (1762) expressed the

opinion that functional disorders of vasa nervorum might well

explain many diseases of peripheral nerves.

Commencing in the 19U0's a combination of time and circum­ stances both warranted and promoted investigations of neurovascular relationships, Basic information was needed as a basis for the

understanding of diagnosis and treatment of nerve injuries incurred

during the second world conflict. Sunderlund, treating peripheral

nerve injuries at a military hospital in Australia, became acutely

aware of the deficiencies in the literature concerning the essen­ tial details of arterial supply to individual nerves. This in turn

led to study of the functional significance of the blood supply

of nerves and the consequences of pathological changes in the vasa.

I n I 9I4O Fetterman and Spitler presented clinical evidence in favor of the hypothesis that vascular disorders are directly responsible for lesions of peripheral nerves or ischaemic neuropathies. Today, the vasa are being implicated in a great many clinical disorders and disturbances involving peripheral nerve elements ; e.g., diabetes mellitus, Buerger's disease, periarteritis nodosa, syphilis, trauma, polycythemia vera, and arteriosclerosis.

Perhaps the controversial role of the vascular requirements of peripheral nerves can best be recognized and appreciated by a sampling of conflicting statements excerpted from the literature.

"An intact blood supply is of importance in

maintaining normal structure and function."

(Roberts, ’ i|6)

"Clinical evidence abounds to show that ex­

tensive mobilization of an injured nerve is

not incompatible with successful repair."

(Bacsich and Wyburn, 'k5) "A constant and adequate blood supply is necessary for the proper functioning of

nerves." (Causey, *55)

Adams (*U3) demonstrated that after the ligation of regional nutrient arteries to the sciatic nerve in the rabbit, there were no harmful effects on structure and f u nction.

"A small but definite circulation is es­ sential for the conduction of impulses along a nerve." (Richards, ‘pi)

"There is evidence that extensive mobil­ ization does not result in significant disturbance of the function of a nerve or produce pathological change." (Blunt, *5?)

"A short period of ischemia produces a temporary disturbance of nerve function which, when the circulation is restored, recovers rapidly and completely."

(Richards, op. cit.)

"The removal of the blood supply cannot be shown to have any significant effect on the rate of regeneration of nerve fibers." (Bacsich and Wyburn, 'U5>)

"Vascular disorders may be directly 2 6 .

responsible for lesions of peripheral

nerves(Fetterman and Spitler, *1*0)

"Ischeraia need not necessarily be com­

plete in order to produce significant

effects on the transmission of a nerve

irnpulse(Adams, 1U2 )

"It is known that functioning of periph­

eral nerves is influenced by the vascular

supply and cessation of .the blood.' . ...

supply to any part of a nerve affects

the passage of a nerve impulse and there­

fore' ultimately induces a nerve block."

(Erhart and Ke.zze, ’66 )

Causey ( ’55 ) states that it is not surprising that such an attitude of mind has arisen which in essence contends that the blood supply of nerves is of relatively little importance. He indicates that the prevalence of such an erroneous belief is quite understandable when we consider that

1) the anastomosis on the nerve is very free;

2) the revascularization is very rapid; and

3) the metabolic requirements are small.

This supposition that the vascular supply of nerves is of no great consequence has probably, at least to some extent, been perpetuated by surgeons, who by manipulation and mobilization of nerves, may have little regard for the role of the vasa — surgical 27. successes or failures being rarely correlated with the extent to which the vascular supply was compromised. Even though peripheral nerves have long been handled surgically with little regard for the vessels associated with them, the findings of this study would tend to indicate that, at least in some instances, such practice may in fact have inadvertently enhanced tne future vascular supply to the nerve in question. (See observations on lp-17 day pattern)

Saltpeter and Singer (*66) hypothesize as follows concerning the vasa nervorum-axioplasmic flow dilemma: "It certainly seems rather absurd that a cell which is characterized by rapid response to environmental changes, (nerve cell) which is capable of immediate recovery following stimulation and which has powers to regrow and remodel its substance should depend entirely for its morphological integrity upon such a distant metabolic source and a slow trans­ port mechanism." It seems highly improbable that the axon would depend entirely upon the cell body for all of its metabolic needs, it appears much more likely that the axon having access to extra­ cellular fluid for snail ions needed in conduction and respiratory gases receive such elements from its local vascular bed. Causey

('5h) employed the follow: j.ng analogy to indicate the possible role of blood supply as it might apply to the more specific problem of nerve fiber regeneration. He speculates that regeneration should not be looked upon as a process whereby the regenerating fiber is pulled out ready ma.de like a strand out of a ball of wool, but rather might be more correctly envisioned as follows. Let the 28.

process be more accurately likened to a new railway line being

pushed out from a large city into a far removed rural area. Ive

must look to the city for the planners, the skilled technicians,

and the specialized materials such as tracks and rails ; these must

travel along the ever extending line from the nerve center of the

enterprise. However, local soil must be used for embankments,

local waste dumps for refuse, and local labor for non-technical

work. It is therefore reasonable to believe that in a similar

manner the nerve cell body might supply the enzymes, nucleic acids,

and other highly specific organized elements whereas such vital

materials as oxygen, sugars and salts might best be recruited from

local vascular channels; these same local channels being the likely

conduit into which metabolic refuse might be discarded.

Assuming such a relationship to exist, what effects then have

resulted from the abolition of these major avenues of supply and

drainage from the sciatic nerve and what compensating adjustments have been made in order to minimize these potentially dangerous effects ? 29.

Collateral Circulation

Collateral circulation as defined by Liebow ('63) is blood

flow that pursues a channel or system of channels which is altern­

ative to, or develops in substitution for, a major vascular

pathway. Collateral channels may be either preformed or newly

formed; the former occur more frequently than the latter.

In the present series of ischemic sciatic nerves, collateral

arterial circulation appears to be well on its way to being estab­

lished by the third postoperative day and is well established by the six-seven day stage evidenced by the fact that the arteries

of the intermediate segment of the operated nerves have developed sufficiently to be completely and fully injected. It will be re­ called that the three-day operated nerves could not be well injected in their middle thirds.

Plexiform iypervascularity of the operated nerves is character­ istic of 1U-17 day specimens. No long term studies have yet been undertaken to determine how long this hypervascularity of the nerves persists, thus it is not known whether this pattern simply repre­ sents a transitory healing mechanism or, is in fact, a permanent addition to the nerve.

i‘.s previously stated the more radical changes in vascular pattern involve mostly veins, especially the superficial or epineurial ones. These findings certainly bear out Learmonth's contention (»£0 ) that "nature has been more prodigal in the 30.

provision of alternative venous and lymphatic routes than she has

been in arranging for arterial collaterals." The intrinsic system

of vessels shows no appreciable deviation from the normal patterns

indicating that perhaps only surface channels had been directly

challenged by the deva.scularization procedure. Furthermore the

epineurial system oi vessels in general are larger than the deeper,

intrinsic vessels. (fig. 1 1 )

The factors causing such pronounced changes in the venae

nervorum and the lack of a similar response in the arteriae ner­

vorum are not known. The work of North et al. («60) states that

venous collaterals develop more quickly and to a larger size than

the arterial ones. Even though no appreciable changes in arterial

size or tortuosity have been noted, it is likely that both dila­

tation of the pre-existing arterialbed as well as enlargement of

the anastomoses between the descending and ascending rami of

successive nutrient arteries has been sufficient to establish ade­

quate arterial flow to the ischemic part of the nerve within a

few days following devascularization. Causey (»53) commented on

the surprisingly short time required to revascularize a nerve,

stating that- in 96 hours a generous vascularization existed in a severely devascularized segment of rabbit sciatic nerve.

It was possible on the normal nerves to confirm the contention of Sunderland (»1+5) and Petrovits and Szabo (»39) concerning the relative sizes of the ascending and descending rami of the nutrient arteries on the nerve. The descending channel is generally, but 31.

not invariably, the longer and larger of the two.

Venous drainage in the lower half of the cat sciatic nerve

appears to be directed inferiorly into the small saphenous vein and

thence to the popliteal vein. With ligation of the popliteal vein

the system of continuous longitudinal epineurial veins on the nerve

provides a substantial alternative route. The obvious enlargement

of the longitudinal veins beginning at three days postoperative indicates that most of venous flow from the lower half of the nerve passed proximally on the nerve, following the course of least resistance. Enlargement of the epineurial longitudinal veins in the early postoperative period agree with North and Sanders (»58) who stated that veins seem to expand within 2k hours. This enlarge­ ment likely came about because of two factors : 1 ) relative immobility of the operated limb reduced the "milking" propulsive forces responsible for venous return centrally and 2 ) the insuf­ ficient number of outflow channels available to accommodate the venous return. Both these factors no doubt contributed to the stasis and venous congestion observed in the three day specimens

(see Observations and also fig. U).

The stagnation of venous blood on the nerve in turn probably resulted in the production of considerable back pressure and venous hypertension. Overfilling and increased pressure distended the veins, leading to progressive enlargement 5 when the limit of trans­ verse extensibility of the veins was reached the longitudinal pressure increased resulting in lengthening and tortuosity of the 3 2 .

smaller veins (Franklin, *37) and enlargement of the main longi­

tudinal venous channels, (fig. 12)

The metamorphosis to tortuosity observed in the six-seven day nerves correlates well with the findings of Franklin and McLachlin

(•36). They noted that compression of the abdominal vena cava in the cat at its proximal end resulted first in dilatation, then in lengthening and shortly thereafter in kinking' of the vein. Tnis coincides with the tortuous, meandering veins characteristic of six-seven day postoperative nerves. Franklin (13 7) also speculated that the tortuosity of vessels found in old people may oe due in part to the longitudinal extensibility remaining after the trans­ verse extensibility had reached its limit.

The reason for the persistence of the hypervascularity exempli­ fied in these 1)4-17 day nerves is unknown. Its presence however allows one to identify the portion of the nerve which suffered from ischemia. Both proximally and distally, abrupt transitional zones between hypervascular pattern and that of normal nerve clearly defines the limits of ischemia. Quenu and Lejars had generalized that the veins of deep nerves always drain into muscular veins or into a venous plexus in the wall of a neighboring artery. They claimed also to have demonstrated an intimate relationship between the veins of muscles and those of nerves, the two forming a neuromuscular venous system. The present observations on sciatic nerve drainage differs from this ; although it first appeared that 33.

lateral drainage from the nerve into neighboring muscular veins

was occurring, closer observation disclosed that the exact oppo­

site was true, that the drainage was in fact proceeding from the

adjacent tissue into the prominent longitudinal venous channels

on the nerve.

The earlier hypothesis correlating conduction velocities of

alpha fibers with the vascular state of the nerve on a temporal

oasis seems to have been well founded. Although the present stucfr

provides no data concerning the effects of partial devasculariza-

tion on either nerve threshold or conduction velocity during the

immediate postoperative period, it will be recalled that alpha

fiber conduction velocity was below normal in three day post­

operative animals. At this time the intermediate segment of the

nerve was vascularly deprived and tortuosity was commencing.

In six-seven day nerves, the extensive vascular pattern

covering the surface attested to the fact that the nerve was well

vascularized. Operated nerves at this postoperative stage once

again demonstrated normal conduction velocities. Although no

detailed comparisons have as yet been attempted to provide further

correlation along these lines on a long term basis, it seems

reasonable to assume that normal conduction velocities would con­

tinue as long as satisfactory vascular requirements were being met.

The present findings concerning the vasa nervorum are neces­ sarily submitted with caution since in biological systems we can never be assured that we possess full appreciation of all existing variables and their possible mechanisms of interaction. Figure 1. Relationships of sciatic nerve, cat •, lateral view, right side j biceps feraoris muscle has been transected and reflected.

M u s c l e s Nerves

a. biceps feraoris 1. n. to hamstrings b. caudofemoralis 2. sciatic n. c. tenuissiraus 3. common peroneal : d . adductor feraoris U. tibial n. e. semimembranosus f. sartorius g. vastus lateralis Figure 2. Segment. o± normal sciatic nerve, ink injected, demonstrating general linear pattern of blood vessels paralleling the nerve fasciculi. .lips of arrows rest on longitudinal epineurial arteries. Note large longitudinal epineurial vein at top of photo. (I8x) 37.

Figure 3. Lower segment of sciatic nerve, ink injected, showing b i f u r c a t i o n .into t i b i a l and p e r o n e a l components. Note longitudinal epineurial veins as they appear in normal nerve. 38.

Figure ij. Lower segment, three day operated nerve ; transitional zone between intermediate and lower segments of nerve. Note early development of tortuous veins ; gray colored veins are blood filled. (I8x)

i

Figure 5. Three day operated nerve, intermediate segment showing poor filling with ink injectant and incipient enlarge­ ment of longitudinal epineurial vein. Compare with' fig. 2, normal nerve. (I8x) Figure 6. (Top) Three day operated sciatic nerve, left side, viewed grossly. Note lack of ink filling in inter­ mediate segment of nerve.

Figure (Bottom) Close up (l8x) shows poor ink filling in segment of three day nerve. bo.

Figure 8. Comparison of six day operated sciatic nerve with normal nerve. Note extreme tortuosity of epineurial veins in segment of operated nerve (top); an example of a well injected normal nerve, (bottom) (o = operated nerve; N = normal nerve) (I6x) la.

gure )« .,xx day sciatic nerve in situ to demonstrate tough sheath enclosing nerve. Arrow points to intact sheath (bottom); portion of sheath opened and pinned back in middle of nerve. Figure 1 0 . Segment of tibia! division of fourteen day- operated nerve showing rich plexiform pattern of epineurial vessels. (I8x) Figure 11. segment of six-seven day operated nerve demonstrating marked changes in both size and tortuosity of veins by comparison with the arteries which appear slightly enlarged. Arrow at top points to artery between venae c o m i t a n t e s ; arrows at bottom point to longitudinal epineurial artery. (l8x) bb.

Segment of six-seven day operated nerve showing huge longitudinal epineurial vein. (iQx) Adams, ¥. E. 19k2 "The blood supply of nerves. 1. Historical Review." J. Anat., 76: 323-339.

______19U3 ''The blood supply of nerves. II. The effects of exclusion of its regional sources of supply on the sciatic nerve of the rabbit." J. Anat., 77: 2U3-291.

Bacsich, P ; ¥yburn, G . M. 1 9k$ "The vascular pattern of peripheral nerve during repair after experimental crush injury." J. Anat. (Lond.), J8: 9.

Bart hoi dy, I(. 1897 "Die arterien der n erven." Morph Arb.. 7: 393.

Blunt, M. J. 19b7 "Functional and clinical implications of the vascular anatomy of nerves." Post grad. Med. J., 33: 68.

Boerhaave, H. 1762 "Praelectiones Academicae de Morbis." Nervorum, F r a n k f u r t .

Causey, G. 1999 "The functional importance of the blood supply of peripheral nerves." Ann. Roy. Coll. Surgeon. 16: 367-383. ~

Clark, 2. R. 1913 "Studies on the growth of blood vessels in the tail of frog larva." Am. J. Anat., 23: 37.

Edshage, S. I 96I4 "Peripheral nerve suture: technique for improved intraneural topography: evaluation of some suture materials." Acta Chir. Scandinav (Supp.), 331: l-10lt.

Srhart, E. W.; Rezze, C. J. 1966 "Effect of experimental devascu­ larization on peripheral nerves." Arg. Neuro-Psiquiat., Sao Paulo, 2k: 7-11.

Fetterman, J. L., Spitler, D. K. 19l|0 "Vascular disorders of peripheral nerves." J.A.M.A., Ilk: 2279.

Franklin, K. J. 1937 A monograph on veins. Charles C. Thomas, Springfield, 111.

Franklin, K. J .5 McLachlin, A. D. 1936 "Dilatation of veins in response to tapping in man and in certain other mammals." J. Physiol., 88: 297. U6.

Gillilan, L.j Dom, R. 1966 "Responses of the venae nerve of monkeys following an embarrassment of circulation." Anat. Rec., 151;: 339.

Haller, A. 1756 leones anatomicae Gottingen, A. Vandenhoeck.

Hyrtl, J. 1859 Oesterr. Ztschr. f. prakt. Heilk.

______1861; Denkschr. d. math, nautrw. Cl. d. La. Akad. d. Wisse n a c h .

Isenflaram, J. F. j Doerffler, J. R. 1768 "De vasio nervorum," Erlangen.

Learmonth, J. 1950 "Collateral Cirdulation, natural and artificial." Surg. Gynecol. Obstet., 90: 385.

Liebow, A. A. 1963 Situations Which Lead to Changes in Vascular Patterns. Handbook of Physiology, Section 2: Circulation, Vol. II. Amer. Physiol. Soc., Wash. D. C.

L o ngland, C.iJ. 1953 "The collateral circulation of the limb." Ann. Roy. Coll. Surg. Engl., 13: 161.

Lundborg, G.; Branemark, P. I. 1968 "Microvascular structure and function of peripheral nerves." Adv. Microcirc., 1: 6 6 - 88 . (Karger, Basel/N.Y.) “

Marcarian, H. Q.; Smith, R. D .; Barton, J. M.j Jenkins, T. W. I 967 "A rapid method for studying vascular patterns three dimensionally and histologically." Am. Heart J., 7I1: 2 1 9 - 2 2 1 . —

Mulvihill, D. A.; Harvey, S . C. 1931 "The mechanisms of the development of collateral circulation." New Engl. J. Med., 20U: 1032-1031;.

North, K. A ., Sanders, A. G. 1958 "The development of collateral circulation in the mouse’s ear." Circulation Res., 6 : 721.

North, K. A. ; Sanders, A. G .; Florey, H. W. I960 "The develop­ ment of an anastomotic circulation to transplanted tissue." Brit. J. Exptl. Pathol., Ijl: 520.

Petrovits, L.; Szabo, Z. 1939 "Die arterielle Versorgung der Gliedmassermerven." Anat. Anz., 8 8 : 392. hl-

Quenu, J.; Lejars, F. 1890 "Les arteres et les veines des nerfs." Compt. rend. Acad. d. Sc. Ill: 608 (Cited by Sunderlund 1 9h$).

______1892 "Etude anatomique sur les vaisseux sanguins des nerfs. Arch. Neurol. (Paris), 23: 1.

______I 89I4. "Etude sur le susteme circulatoire les vaisseaux des nerfs." G. R. Acad. Sei., Paris, III: 6 O 8 . (Cited by Adams 19l*2)

Quiring, D. P. 19U9 Collateral Circulation. Lea and Febiger, Philadelphia.

Ranvier, M. L. 18?8 "Systeme Nerveux", 1, Paris.

Rau, G . 5 Sc ho op, ¥. i 960 "Entwicklung des Kollaternalkreislaufes ,u Arzneimittel-Forsch, lit: 192.

Richards, R. L. 1951 "Ischemic lesions of peripheral nerves: a review. J. Neurol. Neurosurg. Psychiat., II4.: 7 6 .

Roberts, J. T. 19U6 "The structure and significance of blood vessels in peripheral nerves." Anat. Rec., 9I4.: hi-

Ruysch, F. 1701 "Thesaurus Anatomicus Primus", Amsterdam.

S i nger, J .; Saltpeter, M. 1966 "The transport of H-l-histidine through the Schwann and myelin sheath into the axon, including a re-evaluation of myelin function." J. Morph., 120: 281.

Sunderlund, S. 19U5 "Blood supply of the nerves of the upper limb in man." Arch. Neurol. Psychiat., 53• 91-115•

Thoma, R. 1893 Untersuchungen über die Histogenese und Histomechanik des gefasssystems. Stuttgart: Enke.

Tonkow, V. N. 1897 "Oie arterien der Intervertebralganglien und der Cerebrospinalnerven des Menschen." Internat. Monatsehr. f . Anat. u. Physiol., l£: 353.

Van der Speigel, Ä. 1627 De Humani Corporis Falvia 7, Venice.

Waksman 1961 "Experlriiental study of diphteritic polyneur­ itis in the rabbit and guinea pig." J. Neuropath, exp. Neurol., 19: 1|2. Weiss, P. 19^3 "Nerve regeneration in the rat following tubular splicing of several nerves." Arch. Surg., U6: 5>25»

Weiss, P.; Davis, H. 1943 "Pressure block in nerves provided with arterial sleeves." J. Neurophysiol.., 6: 269.