Lumbosacral Plexopathy

In contrast to the brachial plexus, traumatic lesions of the lumbosacral plexus are relatively uncommon due to its protective location within the pelvis. Injuries such as gunshot wounds, falls, and motor vehicle accidents, which often result in pelvic fractures, can cause an acute and diffuse lumbosacral plexopathy via blunt force or nerve avulsion.

Malignancy may also cause an extensive lesion of the lumbosacral plexus. Tumor invasion from lymphoma, metastatic disease, or malignancies arising from neighboring structures, for instance, gynecologic tumors and colorectal cancer, may cause compression or direct infiltration of the plexus. Another cancer-related cause is radiation injury, which is a slowly progressive lesion that occurs several years after radiation treatment for cancers within the pelvis. Radiation injuries typically affect the sacral plexus (lower portion), causing weakness, sensory loss, and sometimes pain in the corresponding distributions.

In diabetic patients, an immune-mediated vasculitic lesion may lead to a syndrome called diabetic lumbosacral radiculoplexus neuropathy, which causes severe pain and muscle weakness in lumbosacral plexus–innervated muscles, with corresponding sensory loss (see Plate 4-11). Also known as diabetic amyotrophy, the syndrome may initially mimic a femoral neuropathy, but soon progresses to involve more distal segments and eventually the contralateral lower extremity. Nondiabetic patients may experience a similar clinical syndrome, which is also thought to be due to an immune-mediated microvasculitis. Spontaneous recovery over several months typically occurs in both types, although immune-modulating therapies have been shown in small studies to improve pain and weakness.

A more recently described etiology is maternal lumbosacral plexopathy, which occurs primarily in petite women due to compression of the lumbosacral trunk (L5 nerve root with posterior portion of L4) by the descending fetal head at the pelvic brim. Following a difficult or prolonged labor that, in many cases, necessitates a forceps delivery or cesarean section, the patient will notice a footdrop on attempts to stand or walk. Fortunately, the underlying pathophysiology of the lesion is thought to be demyelinating conduction block, and most patients spontaneously recover within 3 months.

Retroperitoneal hematomas in patients receiving anticoagulation or after experiencing blunt trauma may compress the lumbar plexus and result in acute weakness of hip flexors, with pain in the groin, hip, or lower abdomen. The hemorrhage is usually seen on computed tomography (CT) of the abdomen and pelvis, and can be accompanied by a sudden drop in the hematocrit level.

Finally, infection may cause a lumbosacral plexopathy through a compressive mechanism due to an infra-abdominal abscess or infiltrative lesion, such as that seen with herpes simplex or cytomegalovirus infections.

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