The Aberrant Course of the Cutaneous Component of the llioinguinal

THOMAS M. OELRICH AND DARVAN A. MOOSMAN Department ofAnatomy, The University of Michigan Medical School, Ann Arbor, Michigan 48109

ABSTRACT The was dissected in 196 adult human cadavers. A normal pattern of distribution, consistent with modern textual de- scriptions, was defined in 60%of inguinal regions. In 35% of the inguinal regions a previously undescribed aberrant course for the cutaneous component of the ilioinguinal nerve was observed in which it is incorporated within the , entered the at the deep inguinal ring, coursed within the cremasteric layer and emerged into the subcutaneous tissue at the superficial inguinal ring on the dorsal side of the or the round ligament of the uterus. The cutaneous component of the ilioinguinal nerve is most frequently found on the ventral surface of the spermatic cord or round ligament during inguinal hernia operations thus an unrecognized aberrant course of this nerve within the spermatic cord is particularly susceptible to acci- dental trauma.

Not infrequently during exposure of the the falx inguinalis and communicate with the inguinal canal structures, the ilioinguinal iliohypogastric and its branches in a plex- nerve cannot be found in its usual or normal iform manner. The main ilioinguinal trunk position on the ventral aspect of the sper- perforates the internal abdominal oblique matic cord or round ligament. In such in- muscle a few centimeters medial to and stances, on retraction of the cord or round slightly below the anterior superior iliac ligament, a nerve is found dorsal to these spine, the exact point of emergence is subject structures emerging from the superficial ring to variation (Jamieson et al., '52). In the and supplying the same cutaneous areas as inguinal region the cutaneous component of the normal ilioinguinal nerve. This unusual the ilioinguinal nerve normally courses be- finding prompted further investigation. The tween the external abdominal oblique aponeu- results are reported herein. rosis and the internal abdominal oblique mus- The normal ilioinguinal nerve arises from cle and continues through the inguinal canal the twelfth thoracic and first lumbar nerve lying upon the creamasteric muscle and fascia components of the . It may arise layer of the spermatic cord or round ligament. in combination with the The nerve usually enters the subcutaneous or it may share distal branches as they course tissue as a single nerve in the region of the between the transverse abdominus and the superficial (clinical synonyms: subcutaneous, internal abdominal oblique muscles. The ilio- medial, external) inguinal ring where it lies inguinal nerve passes from behind the psoas on the anterior surface of the spermatic cord major muscle and courses obliquely across the (fig. 1).As it enters the subcutaneous tissue quadratus lumborum. It continues along the the nerve most frequently pierces the medial crest of the ilium or may course obliquely crus or the external spermatic fascia at the across the to reach and perfor- superficial ring. The cutaneous branches of ate the transverse abdominus muscle near the the ilioinguinal nerve distribute to the mons anterior superior iliac spine. Here it comes to pubis, the (inguinal crease and that por- lie between the transverse abdominus and the tion of the thigh adjacent to the or internal abdominal oblique muscles where it ) and the anterior scrota1 or provides muscular twigs to these muscles. These branches distribute medially supplying Received Nov. 22, '16. Accepted Apr. 29, '17.

ANAT. REC., 189: 233-236. 233 234 THOMAS M. OELRICH AND DARVAN A. MOOSMAN

Fig. 1 Normal location of the cutaneous component of Fig. 2 Aberrant placement of the cutaneous component the ilioinguinal nerve at the superficial inguinal ring. of the ilioinguinal nerve at the superficial inguinal ring. labial to the anterior surface of the onyms: lateral, internal, abdominal) inguinal scrotum or the anterior one-third of the labia ring. It then courses through the inguinal majora respectively. There are minor variant canal between the cremasteric layer and the points and types of emergence of these normal internal spermatic fascia (fig. 3). cutaneous branches (Jamieson et al., '52). Within the inguinal canal the genital There is, however, one previously unde- branch of the genitofemoral nerve separates scribed point of emergence which constitutes to supply the cremasteric muscle, which is its an aberrant course of the cutaneous compo- normal distribution. The aberrant ilioingui- nent of the ilioinguinal nerve. In this instance nal cutaneous component, however, continues this nerve perforates the external spermatic medially, lying lateral and inferior to the fascia dorsally and laterally and leaves the spermatic cord or round ligament on its dorsal side at the superficial inguinal ring (fig. 2). The cutaneous distribution is unaffected, however, the branches to the pubic region usually pass dorsal to the spermatic cord. In addition to the aberrant placement of this cutaneous component of the ilioinguinal nerve at the superficial inguinal ring, the entire course varies from normal. In these the cutaneous component of the ilioinguinal nerve is found to be in combination with the genitofemoral nerve, which usually arises from L1 and L2 of the lumbar plexus thus sharing L1 with the ilioinguinal nerve. This combined cutaneous component of the ilioin- guinal with the genitofemoral nerve follows the usual course of the genitofemoral nerve, perforates the and lies on its ventral side. The combined nerve resem- bles the genital branch of the genitofemoral nerve, courses lateral to the inferior epigas- Fig. 3 Aberrant course of the cutaneous component of tric artery and enters the deep (clinical syn- the ilioinguinal nerve through the spermatic cord. ILIOINGUINAL NERVE 235

TABLE 1

Occurrence of the aberrant course of the cutaneous component of the ilwinguinal nerve Percent of 392 inguinal Percent of No. of spec. areas (sides) 196 bodies Bodies observed 196 Inguinal areas (sides) observed 392 Left side only 30 7.6 15.3 Right side only 18 4.5 9.2 Bilateral (right and left) [45 bodies = 90 sidesl 22.9 22.9 Total left side [45 + 301 75 19.6 38.2 Total right side 145 + 181 63 16 32.1 Overall occurrence left and right inguinal areas 138 35.2 spermatic cord, until it reaches the superficial these aberrant courses relative to inguinal ring where it perforates the cremasteric layer areas or sides. and the external spermatic fascia to enter the In 22 inguinal areas (5.6%) both the normal subcutaneous tissue. At this point the aber- course and an aberrant course of the cuta- rant component is seen on the dorsal side of neous component of the ilioinguinal nerve the spermatic cord (fig. 2). It divides into the were found at the superficial inguinal ring. cutaneous branches characteristic of the nor- Here the normal nerve provides the anterior mal ilioinguinal nerve, namely pubic, scrotal scrotal or labial and pubic branches while the or labial and thigh branches. aberrant course supplies the inguinal crease In the normal ilioinguinal nerve, muscular and the thigh. branches supply the lower and falx inguinalis At the time of this study relatively few fe- fibers of the transverse abdominus and the male bodies were available. Although the internal abdominal oblique muscles, however, numbers are not adequate for percentage when its cutaneous fibers join the genitofem- comparison, the types of occurrence indicated oral nerve, the muscular fibers remain as an for the males in table 1 were also found within independent branch. This muscular compo- the female samples. nent is found in the posterior body wall in the same course and position of the normal ilioin- DISCUSSION guinal nerve but occasionally the muscular branches may also accompany the iliohypo- The lumbar plexus is the least complex of gastric or the lateral femoral cutaneous the major nerve plexuses, however, the result- nerves. In the latter case the muscular ing nerves are probably the most variable in branches leave the nerve as it passes beneath their composition (Bardeen, '01; Webber, '55). the and recur into the The most variable of its individual nerves are lower border of the transverse abdominus and those associated with the abdominal wall the internal abdominal oblique muscles. One musculature. These communicate with adja- hundred and ninety-six male cadavers (392 cent nerves within the layers of muscles and inguinal areas or sides) were observed in labo- result in complexities of their origin. The com- ratory dissection. Thirty-five percent (35%) of binations of nerves of the lumbar plexus have the inguinal areas (sides) demonstrated this been recognized for some time, however, their aberrant position of the cutaneous component terminal course and distribution has not been of the ilioinguinal nerve at the superficial clarified. Griffin (1891) suggested that the inguinal ring. The normal position of the ilio- ilioinguinal nerve might follow the genito- inguinal nerve occurred in 60% of inguinal femoral. In some modern texts and atlases areas. this aberrant course of the cutaneous compo- The statistical occurrence of this aberrant nent of the ilioinguinal nerve is illustrated course may be reviewed in table 1. Not all var- and/or considered incorrectly to be the genito- iations in the human body occur in bilateral (Jamieson et al., '52). In fact, symmetry. Similarly hernias and hernia re- the genital branch of the genitofemoral nerve pair procedures are usually unilateral. Thus has no cutaneous distribution in and of itself. we have chosen to report the occurrence of The arrangement of the upper branches of 236 THOMAS M. OELRICH AND DARVAN A. MOOSMAN the lumbar plexus, specifically the combina- anterior while the aberrant component is tion of the iliohypogastric and ilioinguinal found dorsal to the structures. nerves is not unusual and does not affect the To prevent postoperative peri-inguinal sen- course or terminal distribution of these in a sory deficits, during inguinal hernia opera- significant way. The combination of the ilio- tions, it is important to identify and preserve inguinal and the genitofemoral nerves in both the normal and/or aberrant course of the essence subdivides the ilioinguinal into a cutaneous components of the ilioinguinal muscular and a cutaneous component. The cu- nerve. LITERATURE CITED taneous component accompanies the genito- Bardeen, C. B. 1901 A statistical study of the abdominal femoral nerve whereas the muscular compo- and border-nerves in man. Am. J. Anat., I: 203-228. nent courses independently, resembling the Griffin, M. 1891 Some varieties of the last dorsal and first normal ilioinguinal nerve, or may be incorpo- . J. of Anat., 26: 48-55. Jamieson, R. W., L. L. Swigart and B. J. Anson 1952 rated in other adjacent nerves. The cutaneous Points of parietal perforation of the ilioinguinal and ilio. component has a distinctive course or path- hypogastric nerves in relation to optimal sites for local way and can be identified by its relation to the anaesthesia. Quart. Bull. Northwestern Univ. Med. Sch., spermatic cord (round ligament), and in its 26: 22-26. Webber, R. H. 1955 Some observations on the constitution manner of emergence from the superficial of the lumbar plexus of nerves in man. Anat. Rec., 121: inguinal ring, in which the normal is found 380.