Lumbosacral Plexopathy

Anatomy

The lumbosacral plexus is usually thought of anatomically as consisting of an upper lumbar plexus and a lower lumbosacral plexus ( Fig. 35.1 ).

Upper Lumbar Plexus Nerves

The lumbar plexus, formed from the L1–L4 roots, lies in the retroperitoneum behind the psoas muscle. Several important nerves are derived from the lumbar plexus.

Femoral Nerve

The anterior rami of the L2–L3–L4 roots divide into anterior and posterior divisions. The three posterior divisions unite to form the femoral nerve, which runs through the pelvis and exits into the thigh under the inguinal ligament. Muscular innervation is supplied to the iliopsoas (hip flexion), pectineus, sartorius, and quadriceps (knee extension) muscles. In addition, sensory branches innervate the medial calf (saphenous nerve) and anterior-medial thigh (medial and intermediate cutaneous nerves of the thigh).

Obturator Nerve

The anterior divisions of the L2–L3–L4 anterior rami form the obturator nerve. The obturator nerve descends through the pelvis to exit through the obturator foramen, supplying muscular innervation to the thigh adductors (adductor longus, adductor brevis, adductor magnus, and gracilis) and sensation to a small area of skin on the medial thigh.

Iliohypogastric and Ilioinguinal Nerves

These two paired nerves, derived from the L1 root, are similar to the thoracic intercostal nerves. Both run around the pelvic crest to supply muscular innervation to the transverse and internal oblique muscles. In addition, the iliohypogastric nerve supplies sensation to a strip over the lower anterior abdomen. Just inferior to this, the ilioinguinal nerve supplies sensation to (1) an area of skin over the inguinal ligament, (2) a small area of skin over the rostral medial thigh, and (3) the upper part of the scrotum in males or labia in females.

Genitofemoral Nerve

This small nerve is derived from both the L1 and L2 roots. It descends in the pelvis and divides into a genital and a femoral branch at the level of the medial inguinal ligament. The genital branch provides muscular innervation to the cremasteric muscles in males and sensation to the skin over the lower part of the scrotum in males or labia in females. The femoral branch supplies sensation to the area of skin over the femoral triangle.

Lateral Cutaneous Nerve of the Thigh

The lateral cutaneous nerve of the thigh (LCNT) is a pure sensory nerve that is derived from the L2–L3 roots. This nerve is also known as the lateral femoral cutaneous nerve, but as it has no relationship to the femoral nerve, the preferred and anatomically correct name is the LCNT. It emerges laterally from the psoas muscle and then crosses obliquely over the pelvic brim, superior to the iliacus muscle, and toward the anterior superior iliac spine (ASIS), where it passes under the inguinal ligament. It is here at the ASIS and inguinal ligament that the nerve is susceptible to injury and compression. The average distance between the inguinal ligament and the point at which the LCNT emerges distally from the underlying fascia is 10.7 cm with a range of 10–12 cm. At this point, the nerve typically then divides into anterior and posterior branches that supply sensation to a large oval area of skin over the lateral and anterior thigh. Among individuals, there can be significant anatomic variation to where the nerve crosses in relationship to the ASIS and the inguinal ligament ( Figs. 35.2 and 35.3 ). However, in the vast majority, the nerve is within 2 cm medially from the ASIS.

Lower Lumbosacral Plexus Nerves

The lower lumbosacral plexus is formed primarily from the L5–S3 roots, with an additional component from the L4 root. This L4 component joins the L5 root to form the lumbosacral trunk ( Fig. 35.4 ), which then descends below the pelvic outlet to join the sacral plexus. The remainder of the lower extremity nerves are derived from the lower lumbosacral plexus.

Sciatic Nerve

Most of the fibers in the lower lumbosacral plexus are destined for the sciatic nerve, which receives innervation from the L4–S3 roots. Leaving the pelvis through the greater sciatic foramen, usually under the piriformis muscle, the sciatic nerve supplies muscular innervation to the knee flexors (hamstrings: semimembranosus, semitendinosus, and long and short heads of the biceps femoris), the lateral division of the adductor magnus muscle, and all muscles innervated by the peroneal and tibial nerves. Sensory innervation is provided to the entire lower leg below the knee, with the exception of the medial calf, which is innervated by the saphenous nerve.

Superior Gluteal Nerve

The superior gluteal nerve ( Fig. 35.5 ), derived from L4– L5 –S1 fibers, leaves the greater sciatic foramen to supply muscular innervation to the tensor fascia latae, gluteus medius, and gluteus minimus muscles (hip abduction and internal rotation). This nerve usually carries no cutaneous sensory fibers.

Inferior Gluteal Nerve

The inferior gluteal nerve ( Fig. 35.5 ), derived from L5– S1 –S2 fibers, supplies only the gluteus maximus muscle, which subserves extension of the hip joint.

Posterior Cutaneous Nerve of the Thigh

The posterior cutaneous nerve of the thigh ( Fig. 35.5 ) is derived principally from the S2 root but also has a component from S1 and S3. It leaves the pelvis adjacent to the sciatic nerve to supply sensation to the lower buttock and posterior thigh. Given its proximity, traumatic injuries to the sciatic nerve commonly damage this nerve as well.

Clinical

Lumbosacral plexus lesions usually are divided clinically into those affecting the upper lumbar plexus and those affecting the lower lumbosacral plexus, analogous to the underlying anatomic division. Lumbar plexopathies affect predominantly the L2–L4 nerve fibers, resulting in weakness of the quadriceps, iliopsoas, and hip adductor muscles (femoral and obturator nerves). The knee jerk is frequently depressed or absent. Pain, if present, usually is located in the pelvis with radiation into the anterior thigh. Sensory loss and paresthesias occur over the lateral, anterior, and medial thigh and may extend down the medial calf ( Fig. 35.6 ).

Lesions of the lower lumbosacral plexus predominantly affect the L4–S3 nerve fibers. Patients describe a deep boring pain in the pelvis that can radiate posteriorly into the thigh with extension into the posterior and lateral calf. The ankle jerk may be depressed or absent. Sensory symptoms and signs may be seen over the posterior thigh and posterior-lateral calf and in the foot ( Fig. 35.7 ). Proximally, weakness may be present in the hip extensors (gluteus maximus), abductors, and internal rotators (gluteus medius and tensor fascia latae). In the leg, weakness may occur in the hamstrings, as well as in all muscles supplied by the peroneal and tibial nerves. Nerve fibers destined for the peroneal nerve often are preferentially affected in lumbosacral plexopathies, similar to the preferential involvement of peroneal nerve fibers seen in sciatic nerve and L5 root lesions. Accordingly, patients may present with foot drop and sensory disturbance over the dorsum of the foot and lateral calf. In some cases, the pattern of weakness and numbness may be difficult or impossible to differentiate clinically from an isolated lesion of the common peroneal nerve. It is in such cases that electrodiagnostic studies are crucial.

Etiology

Similar to diseases of the nerve roots, lumbosacral plexopathies can be divided into those caused by structural and those caused by nonstructural lesions ( Box 35.1 ). Structural lesions include pelvic tumors, hemorrhage, aneurysms, endometriosis, and trauma. Among nonstructural causes of lumbosacral plexopathy, the most common is diabetes mellitus, known as diabetic amyotrophy. Known also as proximal diabetic neuropathy or plexopathy, diabetic amyotrophy classically affects the lumbar plexus. Lumbosacral plexopathy can also occur on a nonstructural basis from radiation damage, usually in the context of prior treatment for a pelvic, abdominal, or spinal tumor. In addition, the lumbosacral plexus may be injured during pelvic or orthopedic surgery, especially when retractors are used. Other nonstructural causes of lumbosacral plexopathy include inflammation, infarction, and postpartum injuries.

Common Lumbosacral Plexopathies

Retroperitoneal Hemorrhage

Retroperitoneal hemorrhage is most commonly seen as a complication of anticoagulation, either with low-molecular-weight heparin (e.g., enoxaparin), unfractionated heparin, warfarin, or the novel oral anticoagulants, but it may also occur in the setting of hemophilia or as a result of an aortic aneurysm rupture. Such hemorrhages usually are located within the psoas muscle itself, where they can compress the lumbar plexus ( Fig. 35.8 ). Patients present acutely with significant pain and often hold the hip flexed and slightly externally rotated. Although the entire lumbar plexus is compressed, the major neurologic deficit usually is in the femoral nerve territory, with weakness of hip flexion and knee extension and a reduced or absent knee jerk. However, close examination often reveals some dysfunction beyond the femoral distribution, either in the obturator or LCNT territories, or both.