Lumbosacral plexopathies (LSP) represent a distinct group of disorders of the peripheral nervous system due in part to their relative rarity in comparison to other peripheral nerve disorders and also due to their wide array of etiologies. A thorough understanding of the anatomy of the lumbosacral plexus, the most common causes of LSP, and the most currently accepted diagnostic approaches is needed to manage these conditions effectively.
Other neuropathies affecting the lower limbs are reviewed separately. (See "Overview of lower extremity peripheral nerve syndromes" and "Lumbosacral radiculopathy: Pathophysiology, clinical features, and diagnosis" and "Acute lumbosacral radiculopathy: Prognosis and treatment".)
The lumbar and sacral plexuses make up the collective lumbosacral plexus which is formed from the anterior (ventral) rami of the L1 through S4 nerve roots (figure 1). The anterior rami divide within the plexus into anterior and posterior divisions that in turn yield individual peripheral nerves.
The lumbar plexus is derived from the anterior rami of the L1 through L4 nerve roots. These rami pass downward and laterally along the psoas major muscle where they eventually form the plexus. While within the psoas, they divide into anterior and posterior branches. The posterior branches of anterior rami, L2-L4, become the femoral nerve, which exits from the lateral aspect of the psoas, traveling through the iliacus and under the inguinal ligament to the anterior thigh. The anterior branches become the obturator nerve, which exits from the medial aspect of the psoas, and courses over the sacral ala through the obturator foramen to the medial thigh. The lumbar plexus also gives off the lateral femoral cutaneous nerve of the thigh, iliohypogastric, ilioinguinal, and genitofemoral nerves .
The lumbar plexus and sacral plexus are united via the lumbosacral trunk, which is comprised of a portion of L4 nerve root anterior rami, and all L5 anterior rami. The lumbosacral trunk passes over the sacral ala and joins the anterior rami of the S1-4 nerve roots to complete the sacral plexus .