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Home , Dorsum sellae, Facial canal, Jugular tubercle, Medial rectus muscle, Superior orbital fissure

Entrapment neuropathy of the Cranial central nervous system. Part II. 1-IV, VI-VIII, XII

HAROLD I. MAGOUN, D.O., F.A.A.O. Denver, Colorado

This article, the second in a series, significance because of possible embarrassment considers specific examples of by adjacent structures in that area. The same entrapment neuropathy. It discusses entrapment can occur en route to their desti- nation. sources of malfunction of the olfactory nerves ranging from the The first cranial relatively rare anosmia to the common The olfactory nerves (I) arise from the nasal chronic nasal drip. The frequency of mucosa and send about twenty central proces- ocular defects in the population today ses through the of the ethmoid to the inferior surface of the olfactory attests to the vulnerability of the optic bulb. They are concerned only with the sense nerves. Certain areas traversed by of smell. Many normal people have difficulty in each are pointed out identifying definite odors although they can as potential trouble spots. It is seen perceive them. This is not of real concern. The how the trochlear nerves are subject total loss of smell, or anosmia, is the significant to tension, pressure, or stress from abnormality. It may be due to a considerable variety of causes from arteriosclerosis to tu- trauma to various bony components morous growths but there is another cause of the . Finally, structural which is not usually considered. influences on the abducens, facial, The cribriform plate fits within the ethmoid acoustic, and hypoglossal nerves notch between the orbital plates of the frontal are explored. bone. Excessive medial compression, caused by trauma to the cranium, either unilaterally or bilaterally, can damp out the nerve function and lead to anosmia. In addition narrowing of the ethmoid notch crowds the ethmoidal and frontal sinuses, with the resulting congestion often causing chronic nasal drip (Fig. 1). The vulnerability of this area is revealed by As was shown in Part 1, entrapment neuro- the fact that, in embalming the dead, the pathy in the cranium can result from bony Egyptians used this avenue to remove the impingement, membranous tension, ligamen- brain without noticeably injuring the skull.2 tous pull, or the pressure of edema, venous The involvement may be with the congestion, and so on. Lymphatic stasis im- alone or include others in the facial area such mediately outside of the skull at the foramina as the vomer, maxillae, or sphenoid. of exit should be included. Space-occupying Anosmia is relatively rare. However, such lesions have not been made a part of this thesis. a possible impingement may readily be dem- Specific examples of nerve involvement remain onstrated. With a finger inserted in the eth- to be considered. moid notch of even a dry, defatted laboratory The first four pairs of cranial nerves arise specimen, it is not difficult to compress the two from the cerebrum ; the last eight, from the orbital plates of the frontal bone sufficiently to hindbrain.1 Their point of emergence can be of feel the bind.

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fluid medium may make itself felt in the func- tion of the optic nerves. From the optic foramina to the cerebrum the nerve tracts are in contact with the , a distance of about 18 mm., passing through between the roots of the lesser wings and then lying in grooves on the su-

perior surface of that bone. Changes in the ETHMOID LATERAL ANGLE NOTCH position of the sphenoid, either of the pre- Fig. 1. The ethmoid notch may be slightly osseous elements before birth or the complete widened or narrowed (see arrows). If the latter, bone in later life, can lead to pathosis. In compression of the cribriform plate and thus the first cranial nerves can occur, as well as func- hyperopia both greater wings may have shifted tional disturbance in the ethmoidal and frontal forward, shortening the orbits and the - sinuses. balls, so that the focal point is behind the retina. In myopia the greater wings may be shifted backard, increasing the anteroposterior The second cranial nerve diameter, so that the focal point is in front of the retina. Should one greater wing move an- The optic nerves (II) are well protected in teriorly and the other posteriorly, one eyeball their short course to the brain. The first milli- will be prominent and the other recessive, meter of their length is intraocular and may be again disturbing function (Fig. considered inviolable. The 30-mm. intraorbital 2). may result from such traumatic portion is covered with a protective sheath malalignment. The child may use the domi- consisting of an extension of the cerebral nant eye exclusively with atrophy of the other meninges. It traverses the orbital fat and so is but this usually does not occur in adults. well cushioned. However, since there is a direct Astigmatism, a defective curvature of the perineural pathway for the cerebrospinal fluid refractive surface of the eye, can result from around the nerves, 1 at least as far as the , the pressure or tension of structural alteration any chemical or trophic disturbance in that of the sphenoid or any of the other mak- ing up the . Each orbit is a flexible, cone-shaped cavity with the optic foramen at the apex of the pyramid between the roots of the lesser wings of the sphenoid. The four walls are made up of seven bones for each orbit: the frontal, sphenoid, , zygoma, palatine, ethmoid, and lacrimal. Fixation of any of these con- stituents can be significant in eye malfunction because of the resulting venous stasis or dural Fig. 2. Prominent right eyeball and recessive left tension. Venous stasis can result from narrow- eyeball. Note also the difference in position. Both situations are due to structural trauma altering bone ing of the with the position and both may be pathogenic. greater wing crowded medially by the tern-

780/102 poral bone to approximate the lesser wing, or from narrowing of the Crania l nerves with a change in the relationship between the greater wing and the maxilla. Occipitotemporal shifts can retard drainage through the jugular 111.•■• • foramina and so cause back-pressure in the 3/(1) orbits by way of the petrosal and cavernous • sinuses and the ophthalmic veins. This is a very common cause of behind the eyeballs. Aft(tglt Myopia may possibly be due to distortion of the occiput, as in the case of the Japanese who YE sleep on a wooden headrest and thus disturb the orbital centers in the occipital lobe near the 1pfernal Carotid calcarine fissure. ery In all races the great preponderance of ocu- Fig. 3. Cross section of the showing lar defects requiring glasses attests to the vul- the relative position of the and nerability of these nerves from many possible cranial nerves III, IV, V(1), and VI. causes. Careful evaluation of visual fields, acuity of vision, and the condition of the optic Within the orbit the third nerve is dis- fundi is the prerogative of the specialist but tributed to seven muscles. Parasympathetic faulty vision is one of the commonest afflic- fibers supply the sphincter of the iris and the tions among patients today. Therefore, along ciliary muscle, to regulate the size and shape with the other measures consideration of the of the pupil, its accommodation, and response structure by the general practitioner, as a cause to light. A branch goes to the levator muscle of the malfunction, is clearly indicated. of the upper , which arises from the Further eye problems may arise from faulty back of the orbit just above the optic foramen action of the orbital muscles due to entrap- and inserts into the of the upper lid. ment neuropathy of the third, fourth, and Thus the eye is opened by the third nerve but sixth cranial nerves. closed by the seventh. Twitching or of the lid indicates trouble in this part of the The third cranial nerve third nerve. A drooping eyelid may accompany Each oculomotor nerve (III) leaves the mesial Bells palsy, in which the seventh cranial nerve side of the cerebral peduncles and runs inferi- is at fault. orly, anteriorly, and laterally in the posterior More frequently involved in entrapment part of the cisterna basalis, and transverses neuropathy are the superior, medial, and in- three vulnerable areas en route : (1) as it ferior rectus muscles of the eyeball, as well as crosses the attached border of the tentorium the . The rectus muscles cerebelli at the side of the , (2) arise in the apex of the orbit from a dural as it passes forward in the outer wall of the sheath surrounding the optic foramen and cavernous sinus (Fig. 3), and (3) as it enters nerve. They insert anterior to the equator of the orbit through the superior orbital fissure. the eyeball. The inferior oblique muscle arises Anyone of these is a potential trouble spot. from the maxilla medially in the margin of

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the floor of the orbit and inserts posterior to the equator of the eyeball. The action of these muscles supplied by the third cranial nerve may be outlined as follows : Petrosphenoid 8 ligament and Eye upward and inward—superior rectus posterior clinoid Eye medially—medial rectus Free border tentorium Eye downward and inward—inferior rectus Medulla Attached border Eye upward and outward—inferior oblique Eye directly upward—superior rectus and Tentorium inferior oblique Eye directly downward—inferior rectus and Fig. 4. Details of possible entrapment of crania superior oblique nerves by the forward reaches of the tentorium. The third nerve crosses the anterior part of the attached Malfunction of the extrinsic muscles include border of the tentorium cerebelli at the side of the a number of manifestations, which may be dorsum sellae to pierce a small triangle between the directly or indirectly attributed to entrapment free and attached borders and enter the outer wall of the cavernous sinus. The fourth nerve runs forward neuropathy. beneath or pierces the free border of the tentorium Strabismus or squint is perhaps the com- and passes over the petrous ridge of the temporal monest type of malfunction. This evidences a bone to enter the lateral wall of the sinus. The sixth nerve passes beneath the petrosphenoid ligament of lack of parallelism between the visual axes or the attached border in a fibro-osseous at the a functional inequality of opposing eye mus- side of the dorsum sellae to enter the substance of the cles, either spastic or flaccid. Nystagmus may sinus. be due to cerebellar disease or involvement of the labyrinth and the vestibular pathway of the eighth cranial which connects with the shifts in sphenoid, temporal, or occipital posi- nerves to the ocular muscles. How are these tion can exert pressure or strain on the oculo- nerves involved? motor nerve amounting to an entrapment. The tentorium cerebelli is attached around Congestion in the cavernous sinus can make the transverse ridge on the inner surface of trouble. The congestion may arise in a variety the occiput, along the petrous ridges of the of ways, from back pressure in the area of and then inserts into the pos- the due to disturbance of the terior clinoid processes on the dorsum sellae occipitotemporal or condylar parts, from a of the sphenoid bone. However, the free border shifting of the sphenoid or frontal bones, thus runs forward and crosses above the attached retarding drainage through the petrosal si- border to insert into the anterior clinoid proc- nuses, or from maxillary interference with esses on the lesser wings of the sphenoid (Fig. pterygoid plexus drainage. 4). A depressed angle is formed cn either side The superior orbital fissure may be nar- between the free and attached borders of the rowed because the temporal bone has crowded tentorium in the middle lateral the greater wing of the sphenoid toward the to the posterior clinoid processes. In this angle lesser wing thus impeding venous drainage or the root of the oculomotor nerve enters the nerve conduction. Dural tension in the sheaths dura in the upper, outer wall of the cavernous surrounding nerves and blood vessels can be a sinus. Excessive tension on the tentorium from factor there or at the orbital foramen from

782/104 which the rectus muscles arise. (A tight sleeve rior to the equator of the eyeball. Thus the or too small a collar is notoriously uncomfort- action of the muscle is to turn the eye down- able.) ward and outward, or directly downward if Within the orbit, bony position and normal its action is combined with that of the inferior motion is important. The extrinsic muscles, rectus muscle. especially the obliques, are lashed to the walls Trochlear paralysis is rare. If the ocular of the orbit by fascial bands. Abnormal rela- divergence is up and in, it may be due to a tions can make trouble. Likewise, any change slight lateral shift of the frontal bone which in the distance between their origin and in- reduces the tension over the "pulley." How- sertion can give rise to faulty function. Sepa- ever, the effect may be minimal because asso- ration may cause the muscles to lose tone and ciated muscles can take over the necessary become flaccid. Approximation may cause con- f unction. traction and spasticity. For instance, a shift A swollen middle turbinate bone may pro- in the position of the maxillae can definitely duce a vacuum in the . This nega- change the tension of the inferior oblique mus- tive pressure on the sensitive walls makes the cles. Increased contraction will fatigue the pull of the trochlear muscle painful in the muscles. Thus altered relations of the frontal, upper inner angle of the orbit but glasses do sphenoid, and maxillary bones, or their adnexa nothing to relieve the discomfort. in the skull, constitute a most important cause As with the oculomotor nerve, the trochlear of entrapment neuropathy and the resulting is involved in fibrillation and nystagmus. It is muscle imbalance. subject to tension as it pierces the tentorium, to pressure in the cavernous sinus, and to dural The fourth cranial nerve stress in the superior orbital fissure, all be- The (IV) emerges from the cause of trauma to the bony components of cerebrum and runs forward along the base of the skull. Perhaps the commonest form of in- the brain in the cisterna basalis. It lies be- jury is a fall on the frontal squama in infancy, neath the tentorium, piercing the free border producing imbalance in the superior oblique behind and external to the posterior clinoid muscle and ocular deviation. This would ex- process and the third nerve to reach the ridge plain why a baby becomes crosseyed immedi- of the petrous portion of the temporal bone. ately after falling off a bed on its head. It then enters the lateral wall of the cavernous sinus and traverses the superior orbital fissure The sixth cranial nerve to reach the orbit and the superior oblique The (VI) emerges from the muscle. ventral surface of the medulla and courses up- This muscle arises from the dural sheath ward and forward in the subarachnoid space surrounding the optic nerve and foramen at between the pons and the basisphenoid. It the apex of the orbit. It also has a filament passes beneath the forward reaches of the ten- arising from the sphenoid bone, medial to the torium cerebelli, the so-called petrosphenoid optic foramen. These two roots unite to pass ligament, through a fibro-osseous canal at the upward and medially through the trochlear side of the dorsum sellae beneath the posterior loop at the superomedial angle of the frontal clinoid process and then over the petrous apex. bone and then downward for insertion poste- This is the most vulnerable area. It may also

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be subject to pressure in the substance of the held in position by the , a cavernous sinus or the superior orbital fissure portion of the dura which covers the fossa before it reaches the . quite securely. Venous drainage from the pi- The lateral rectus muscle arises from the tuitary gland is into the cavernous sinus and apex of the orbit, along with the other recti the basilar plexus. Cerebrospinal fluid circula- muscles, inserts anterior to the equator of the tion is by way of the infundibulum to the third eyeball, and turns the eye outward. Partial ventricle. Both of these channels are subject paralysis of the lateral rectus muscle, with to stress. straight medial convergence, is probably the Because the hypophysis is so firmly held in commonest form of strabismus. This may be relation to the sphenoid, it seems to be par- due to impaired or lost function of the sixth ticularly vulnerable to entrapment, affecting cranial nerve and a flaccidity of the muscle or its motion and so its function in one way or overstimulation and irritation of the third another. At least there are striking examples cranial nerve and spasticity of the medial of improvement in its function when its action rectus. Bony shifts causing pressure through is freed. For example, when the mechanism is the forward reaches of the tentorium are usu- released, uterine inertia can be overcome as ally responsible. A fixed divergent squint quickly as with a hypodermic injection of would be the opposite condition with a spastic Obstetrical Pituitrin. The great advantage of lateral and a flaccid medial rectus muscle. the manipulative approach is that the dosage In esophoria, the external rectus muscle must is always physiologic and never causes dis- contract to restore binocular vision ; in exo- astrous results from violent contractions of phoria, the internal rectus muscle contracts. the uterus. Since these muscles are accustomed to work Growth problems take longer, naturally, but together little strain usually results. However, are no less dramatic. A 2-year-old boy with a slight vertical deviation of one visual axis marked bowing of both legs showed evidence above the other, hyperphoria, can be quite un- of cranial birth injury restricting the normal comfortable since there is an extra tension motion of the sphenoid bone. Because the pat- imposed on all the extrinsic muscles of the tern of abnormality in his skull had become . And, there is tension in the cranial somewhat fixed during his first two years, mechanism as well. weekly osteopathic manipulative treatments Fibrillation in the eyes of the newborn sug- over 13 months were necessary to free this gests an injury to the sixth nerve, possibly area. No other treatment was given nor was from improperly placed obstetrical forceps. there any change in nutrition or other therapy. Any or all of the structural influences men- Successive x-rays during that time showed no tioned in connection with the third or fourth appreciable change in the shafts of the femora cranial nerves are of importance here also. or tibia. However, the legs became perfectly Another structure in the area of the cavern- straight because the epiphyses changed in ous sinus and the ,, the hypo- shape from triangles to rectangles—a simple physis cerebri, is subject to possible entrap- manifestation of normalization of the growth ment neuropathy. The hypophysis rests in the gland in the cranium and development of the hollow of the sella turcica upon the superior medial condyles of the femora (Fig. 5). surface of the sphenoid body and is securely Whiplash-type injuries and the postconcus-

784/106 Fig. 5. The restoration of physiologic motion of the sphenoid bone and normal pituitary function resulted in growth of the epiphyses from a triangular to a rectangular shape with straightening of the legs. (X-rays courtesy of Dr. Glenn Baird.) sion syndrome may both involve a massive cle, in close approximation to the nerve. Dural shock to the central nervous system, as well as tension at the internal acoustic or the its soft tissue constituents, by way of the bony hiatus fallopii is almost inevitable with shifts container. It would be repetitious to mention of the temporal bone. The allows all the possibilities. The same principles apply no room for inflammatory swelling. The fora- here as in entrapment neuropathy elsewhere. men lacerum is always cartilaginous and so is mobile to a slight degree. The vidian canal is The seventh cranial nerve subject to the same dural tension, crowding, The (VII) is a mixed nerve. It or congestion. Narrowing of the pterygopala- leaves the medulla with a sheath of dura and tine fossa has already been mentioned. En- runs forward and laterally in the posterior trapment neuropathy would be manifested by cranial fossa to the internal acoustic meatus. altered salivary flow, disturbance in taste on The motor branch traverses the facial canal the anterior two thirds of the , and so with the eighth cranial nerve, supplies the on. There can be a definite structural reason and, emerging through the for such symptoms. , ennervates the super- However, the most frequent example of ficial musculature of the and , and nerve pathology is Bells palsy, estimated to the platysma, stylohyoid, and posterior belly of make up three fourths of all facial nerve le- the digastric muscles. sions. This entity is an acute facial paralysis The sympathetic portion of the facial nerve which may follow such conditions as respira- runs a devious course. Leaving the medulla it tory infections or exposure to cold. There are crosses the of the occiput to many other possible causes. The pathologic reach the internal acoustic meatus, follows the state is usually peripheral but may include the facial canal to the and facial canal—the higher in the canal the more emerges through the hiatus fallopii into the serious the complications. , crosses the foramen External involvement is indicated by a lacerum beneath the semilunar ganglion, joins flaccid paralysis, lack of or fixity of facial ex- the great deep petrosal nerve and traverses the pression, a flat nasolabial fold, and drooping vidian canal to the sphenopalatine ganglion. of the mouth and cheek on the affected side. The sensory portion of the facial nerve runs The degenerative reaction, following an in- in close company with the motor root until a jury, may be as much as 2 weeks in appear- short distance above the stylohyoid foramen ing. A loss of taste shows that it has reached where the larger part branches off as the the ; hyperacusis, the nerve to chorda tympani nerve to the anterior two the stapedius muscle; tearing or pain, the thirds of the tongue and the parotid gland. geniculate ganglion ; headache, vomiting or All of these portions are subject to entrap- deafness, the internal acoustic meatus. ment neuropathy. Trauma rather easily dis- Trauma looms large as a common etiology. turbs the "rocker bearing" of the temporal Shifts in the position of the occiput or the jugular surface on the occipital jugular tuber- temporal bone bring about tension, edema, and

Journal AOA/vol. 67. March 1968 785/107 Fig. 6. Temporal bone position as incicated by the . Left, bilateral external rotation of the temporal bones. Center, bilateral internal rotation. Right, a combination of the two. restriction of physiologic motion. The dural periarthritis. The etiology is largely an enig- sheath is particularly heavy at the internal ma, except from the structural standpoint. The acoustic meatus. Traumatic irritation to the lateral one third of the 2 1/2 inch eustachian nerve roots may incite antidromic impulses3 tube lies within the petrous portion of the tem- which liberate histamine, giving rise to periph- poral bone. The cartilaginous medial two eral vasodilation and thus exudation, petechial thirds hangs from the lower border of that hemorrhages, and lymphadenitis in the facial bone. Normally the osseous portion is perma- canal. This sequence of events would explain nently open while the rest is closed except the sequellae of ischemia and degeneration during swallowing or yawning, thus equalizing from compression, a true entrapment neu- the pressure on either side of the . ropathy. A slight shift in the position of a temporal bone can alter this physiologic state. The tube may remain open all the time, resulting in a The eighth cranial nerve low roar being heard, or it may be held closed, The acoustic nerve (VIII) is in reality two with a whine or buzz being heard in the upper nerves. The vestibular portion is associated frequencies. This noise, which may be syn- with posture and equilibrium and hence is in- chronous with the pulse, is believed to come volved in problems of balance, vertigo, and from the friction of the blood rushing around nystagmus. The cochlear is the auditory por- the elbow bend in the internal carotid artery, tion and is involved with or its per- immediately adjacent to the internal ear. Un- versions such as tinnitus. Both nerves pass der normal circumstances a person is not con- through the facial canal to the internal acous- scious of this sound, except when it reputedly tic meatus and over the vulnerable area of the is made evident by holding a conch shell to jugular tubercle to the hindbrain. As with the the ear, "to hear the sea roar." A slight shift seventh cranial both are subject to entrapment of the temporal bone holds the tube either open neuropathy. Grant states : or closed so that the noise is intermittently or . . . the anterior inferior cerebellar [artery] . . . runs continuously apparent. Sclerotic changes may tortuously above, below, or between nerves VII and make the condition relatively permanent. VIII. . . . "the arteries may also cause pressure upon Vertigo is another common problem, puz- the nerves and interfere with their functions" (Watt). zling to the patient and physician alike. In the Arteriosclerosis provides an example of such authors experience by far the commonest a situation. Dural stress or pressure neuritis cause is an increase in craniocervical tension lead to malfunction or lowered resistance to and a consequent shift in temporal bone posi- infection. The most striking examples of en- tion from the unequal pull of muscles or from trapment neuropathy concern the organs of trauma. If the organs of equilibrium are to hearing and equilibrium themselves. function normally, the on Tinnitus is characterized by annoying noises either side should be in absolute orientation, in the ear in frequencies varying from low to the one with the other. With the commonly high. It is often accompanied by a postcervical found disharmony in temporal bone position sympathetic syndrome of occipital headache, (Fig. 6) this delicate co-ordination is upset visual trouble, dizziness, neck tension, and and a confused image in the central nervous

786/108 system results. One organ of equilibrium indi- The twelfth cranial nerve cates "straight up" is here and the other indi- The (XII) exits indepen- cates "its over here." Vertigo results. dently from the cranium, through the hypo- Clinical experience would seem to bear out glossal canal but nevertheless suffers involve- this assertion. In World War II candidates for ment, particularly in the newborn. In the flight training, who had "passed" physical infant the canal is only a groove. Hence, examinations were often disqualified by their rotation of the occipital squama produces suf- inability to recover quickly from being whirled ficient malaligmnent of the condylar parts of in a revolving chair. As in many cases of dizzi- the occiput to cause a pressure paralysis on ness, relieving the confused equilibration caused one or both sides. The infant has difficulty in by structural malalignment was the answer. suckling and swallowing and has to be fed with Menieres disease is a more complex involve- a medicine dropper. Improvement can be quite ment of the internal ear. There may be coch- dramatic when the entrapment is released. If lear symptoms such as tinnitus and deafness, the structural relationships are not restored to labyrinthine symptoms such as dizziness and normal, atrophy or fibrillation of the tongue nystagmus, plus a general vasomotor disturb- may follow, with deviation toward the involved ance. Function of the eighth nerve may well side. be traumatically disturbed by increased endo- lymphatic pressure and asphyxia of the labyrin- thine end-organs. The sac and duct do not communicate directly with the sub- 1. Schaeffer, J. P.: Morris Human anatomy. Ed. 10. The Blakis- arachnoid space. However, a functional rela- ton Co., Philadelphia, 1947 2. Kahn, F.: Man in structure and function, edited by G. Rosen. tionship exists with the cerebrospinal fluid be- A. A. Knopf Publishing Co., New York, 1956. vol. 1 cause the proximal third of the sac is in the 3. Norfolk, D. T.: Cranial nerve syndromes. Relationship to musculoskeletal lesions of the cervical spine. J Osteopath 69:17-22, dural portion of the and Aug 62 therefore subject to dural tension with a shift 4. Grant, J. C. B.: A method of anatomy: Descriptive and deduc- tive. Ed. 6. Williams Wilkins Co., Baltimore, 1958 in the position of the temporal bone. Here, as elsewhere, structure and function are two sides of the same coin. Dr. Magoun, 660 Washington Street, Denver, 80203.

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