Variations of the Lumbar Plexus

Lumbar Plexus

The lumbar plexus is located in the posterior abdominal wall, in front of the transverse processes of the lumbar vertebrae and deep inside or posterior to the psoas major muscle (▶ Fig. 11.1). Variations in the position of the plexus are usually accompanied by variations in the vertebral column itself. While the origin of the various branches of the lumbar plexus varies according to whether the plexus is prefixed, ordinary, or postfixed, it is typically by the union of the anterior rami of the first three lumbar nerves and usually a part of the fourth lumbar nerve, and it projects laterally and caudally from the intervertebral foramina. A communicating branch from T12, also known as the subcostal nerve, often joins the first lumbar nerve. The range of variations in the formation of the lumbar plexus is summarized in ▶ Table 11.1. ⁷^, ⁸^, ⁹^, ¹⁰^, ¹¹

Fig. 11.1 Posterior dissection of the right-sided lumbar plexus following a hemilaminectomy and exposure of the lumbar dura mater. The thecal sac has been filled with saline and the lower ribs, posterior sacrum, and upper iliac crest have been removed. To the right, the lower right diaphragm is seen with overlying parietal pleura. The psoas major muscle is seen anterior to the major components of the lumbar plexus, which, in this specimen, has a “normal” composition.

**Table 11.1** Range of variation of the lumbar plexus ⁵
Nerve	Prefixed plexus (high form)	Usual pattern	Postfixed plexus (low form)
Lateral femoral cutaneous	L1, L2	L2, L3	L3, L4
Femoral	T12, L4	L1, L4	L1, L5
Obturator	L1, L4	L2, L3	L2, L5
Furcal	L3, L4	L4	L4, L5

The lumbosacral plexus is usually bilaterally asymmetric. In its usual form, the L2–L4 ventral rami first bifurcate into anterior and posterior primary divisions. The L1 nerve splits into cranial and caudal branches after receiving a contribution from T12. The cranial branch is thicker and bifurcates into the iliohypogastric and ilioinguinal nerves; the former is also formed by the subcostal nerve in individuals where this nerve contributes to the lumbar plexus. The caudal branch of the L1 nerve unites with the anterior division of L2 to form the genitofemoral nerve. The anterior divisions of the L2–L4 roots form the obturator nerve. The posterior divisions of L2 and L3 further split into two. The thinner branches unite to form the lateral femoral cutaneous nerve; the thicker ones, together with a contribution from the posterior division of L4, join to create the femoral nerve. The psoas major and quadratus lumborum are innervated by the T12 and L1 nerves and the L2–L4 anterior primary divisions. ⁷^, ⁸^, ⁹^, ¹⁰^, ¹¹^, ¹² The anterior division of L4 is commonly divided between the lumbar and sacral plexuses, hence it is called the furcal (meaning forked) nerve. ³ Fibers from the anterior division of L4 join the anterior division of L5 and create the lumbosacral trunk to join the sacral plexus. Eisler found the furcal nerve to be formed by L5 in 19 out of 22 variations noted in the formation of the lumbar plexus; in only two cases was it formed by L3. The proportion of L4 contributing to the sacral plexus is also variable. The common formation of the lumbar plexus is the manner of division of this nerve: in a postfixed plexus, it contributes all or most of its fibers to the lumbar plexus, while in a prefixed one it contributes few or none. More rarely, an appreciable part of the 12th thoracic or a part of the 5th lumbar nerve participates in the formation of prefixed and postfixed plexuses, respectively. ⁷^, ⁸^, ⁹^, ¹²

Matejcík reported the variations of the lumbosacral plexus in 50 cadavers and found the prefixed type in 19 cases and postfixed in 5, the rest being the ordinary form. He also reported that the T12 and L1 roots are thicker in the prefixed type. In this type, the L4 root contributes more significantly to the sacral plexus and the S3 and S4 roots are absent. If the L4 root does not contribute to the sacral plexus or contributes only minimally, the L5 root is also thinner, S1–S3 are thicker, and S4 is also present. As expected, in the postfixed type, the L5 root is the largest contributor to the sacral plexus. These variations at the level of neural root formation are reported to be the most common variations of the lumbosacral plexus. ¹³ Mine Erbil et al described a rare variation: occurrence of the prefixed type on one side and the postfixed type on the other. ¹⁴

The lumbosacral trunk, either a single trunk formed by the union of L4 and L5 or two parallel but nonunited trunks, also appears medial to the psoas muscle, usually within the pelvis itself, on the anterior surface of the sacrum. It lies medial and posterior to the obturator nerve and is usually not visible from the abdomen, since it is covered first by the psoas muscle and then by the hypogastric vessels. The iliolumbar artery typically passes between the lumbosacral trunk and the obturator nerve. ⁷ Urbanowicz studied the connections between the lumbar and sacral plexuses bilaterally in 122 subjects. ¹⁵ The plexuses were joined by a single nervus furcalis in 91.8% of subjects and by a doubled one in 0.8%. The single furcal nerve was formed by the abdominal nerve L4 in 80% and L5 in 7.7%. The doubled furcal nerve arose from L3 and L4 in 0.4% of cases and from L4 and L5 in 0.4%. Usually, the major part of the furcal nerve arising from L4 went to the lumbar plexus, and from L5 to the sacral plexus. In 7.4% of cases, no connection between the plexuses could be found. ¹⁵ In a series of 20 fetal specimens, Yasar et al observed a single furcal nerve, which originated only from the L4 spinal nerve. ¹⁶ A sixth lumbar nerve that contributes to the sacral plexus has been described. ¹⁷

11.3 Branches of the Lumbar Plexus

11.3.1 Iliohypogastric Nerve

The iliohypogastric nerve usually originates from the ventral ramus of the L1 spinal nerve. It emerges from the upper lateral border of the psoas major, crossing obliquely behind the lower renal pole and in front of the quadratus lumborum. Above the iliac crest, it enters the posterior part of the transversus abdominis. Between the transversus abdominis and internal oblique, it divides into lateral and anterior cutaneous branches. The lateral cutaneous branch runs through the internal and external oblique above the iliac crest, a little behind the iliac branch of the T12 spinal nerve. It is distributed to the posterolateral gluteal skin. The anterior cutaneous branch runs between the internal oblique and transversus abdominis and innervates both muscles. It runs through the internal oblique approximately 2 cm medial to the anterior superior iliac spine (ASIS) and through the external oblique aponeurosis approximately 3 cm above the superficial inguinal ring, and is then distributed to the suprapubic skin supplying sensory branches. The iliohypogastric nerve usually gives communicating branches to the subcostal and ilioinguinal nerves. ¹⁰^, ¹¹ The iliac branch of the iliohypogastric nerve can be absent, replaced by the lateral cutaneous branch of the 12th thoracic nerve. The hypogastric branch can supply the pyramidalis muscle and be joined with the 12th thoracic nerve. The iliohypogastric nerve is sometimes derived from the 12th thoracic nerve and can also receive a root from the 11th. ⁷^, ⁸^, ⁹ Anloague and Huijbregts demonstrated the absence of the iliohypogastric nerve in 20.6% of the lumbar plexuses they investigated. ¹⁸ Sometimes its anterior branch is replaced by the ilioinguinal nerve just before the former exits from the external inguinal ring. ¹⁹

11.3.2 Ilioinguinal Nerve

The ilioinguinal nerve is usually smaller than the iliohypogastric nerve and arises with it from the first lumbar ventral ramus. It emerges from the lateral border of the psoas major, with or just inferior to the iliohypogastric nerve. It passes obliquely across the quadratus lumborum and the upper part of the iliacus and enters the transversus abdominis near the anterior end of the iliac crest. Here, it sometimes connects with the iliohypogastric nerve. It pierces the internal oblique lower than the iliohypogastric, supplies it, and then traverses the inguinal canal below the spermatic cord. Sometimes it passes deep to the inguinal ligament. ¹⁹ It emerges with the cord from the superficial inguinal ring and supplies the proximal medial skin of the thigh and the skin over the root of the penis and upper part of the scrotum in males. In females, it innervates the skin covering the mons pubis and the adjoining labium majus. The ilioinguinal and iliohypogastric nerves are reciprocal in size. The ilioinguinal is occasionally very small and ends by joining the iliohypogastric, a branch of which then takes its place. ¹⁰^, ¹¹

This nerve can be small, terminating near the iliac crest by joining the iliohypogastric nerve, as they exit through the psoas major muscle. In this case, the iliohypogastric nerve sends branches to replace the absent terminal part of the ilioinguinal nerve. The ilioinguinal nerve can be entirely absent, its distribution being taken over by the genital branch of the genitofemoral nerve; the iliohypogastric nerve supplies its territory. The ilioinguinal nerve can also be replaced by either the genital (more commonly) or femoral branches of the genitofemoral nerve. The ilioinguinal can provide a lateral cutaneous or iliac branch to supply the skin in the region of the ASIS. It can partially or completely replace the genital branch of the genitofemoral nerve or the lateral femoral cutaneous nerve. It sometimes arises from L2 instead of L1. Sometimes it gives a communicating branch to the lateral femoral cutaneous nerve. If such a branch is present, it pierces the external oblique aponeurosis near the ASIS and joins the nerve in that vicinity. ⁷^, ⁸^, ⁹^, ¹⁹

The ilioinguinal and iliohypogastric nerves sometimes arise as a common trunk, usually separating between the transversus and internal oblique muscles. The ilioinguinal nerve can be derived from the last thoracic nerve (T12), from a loop between the L2 and L3 nerves, or even from the L2 and L3 nerves directly. It can also supply branches to the rectus abdominis muscle. In a study of 200 cadavers, the ilioinguinal nerve was reported to arise from the lumbar plexus in 72.5% and by a common trunk with the iliohypogastric nerve in 25%; it was absent in 2.5%. It was formed from one root in 92.5% and from two roots in about 5% of cases. In 86%, it carried fibers from one spinal nerve (primarily from L1), and in 11% from two (T12–L1; L1–L2; or L2–L3). Within the inguinal canal, the nerve usually lies ventral to the spermatic cord (60% of cases) but it can lie posterior the cord or within it. It usually leaves the superficial inguinal ring medially but sometimes through its lateral aspect. In some cases, the nerve runs outside the inguinal ring. ⁷^, ⁸^, ⁹^, ¹⁹