Aug. 16, 2022
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Lumbosacral plexopathy should be suspected when motor and sensory deficits occur in the territory of multiple nerves and multiple spinal nerve roots in one leg. It can be challenging to differentiate lumbosacral plexopathy from lumbar polyradiculopathy or injury to multiple peripheral nerves. Some causes of lumbosacral plexopathy lead to bilateral involvement, further complicating the diagnostic process. Certain etiologies, referred to as radiculoplexus neuropathies, can even affect both the plexus and the spinal nerve roots. Carefully neurologic examination and proper use of electrodiagnostic testing and/or imaging, however, can accurately localize to the plexus and guide the etiologic work-up. Important causes include neoplasm, prior radiation therapy, compression, diabetes, and ischemia. Traumatic causes are uncommon and are mainly due to fractures of the pelvic ring or acetabulum.
• Symptoms of lumbosacral plexus injury include varying degrees of lower extremity weakness, sensation changes, pain, and diminished reflexes.
• Most lumbosacral plexopathies due to trauma are from very violent injuries, such as automobile-pedestrian accidents, high-speed car accidents, or falls from heights, and are often associated with damage to internal organs, blood vessels, and bony structures, especially the pelvic ring.
• MRI of the lumbosacral plexus and EMG/NCS are important diagnostic tools. CT or PET can be additive in certain cases.
• Because most traumatic plexopathies improve spontaneously, at least to some extent, they usually are treated conservatively.
The lumbosacral plexus comprises 2 distinct portions: the lumbar plexus and the sacral plexus, each innervating a different part of the lower limb. The lumbar plexus connects with the sacral plexus via the lumbosacral trunk. The lumbar plexus is primarily derived from branches of the T12-L4 nerve roots. The lumbar plexus takes shape in the retroperitoneum, below and within the psoas muscle. It should, thus, come as no surprise that pathology within the retroperitoneum (eg, hematoma, abscess) is a frequent cause of lumbar plexopathy. The predominantly sensory iliohypogastric (L1), ilioinguinal (L1), genitofemoral (L1-2), and lateral cutaneous nerve of the thigh (L2-3) arise from lumbar plexus. Contributions from L2-L4 split into anterior and posterior divisions; the anterior division forms the obturator nerve, and the posterior division forms the femoral nerve. The obturator nerve innervates the adductor muscles of the thigh and the skin of the medial thigh. The femoral nerve innervates the psoas muscle and the iliacus muscle proximally. Once it passes under the inguinal ligament, it innervates the quadriceps muscles and provides sensation to the medial thigh and lower leg via medial and intermediate cutaneous nerves of the thigh and saphenous nerves, respectively.
The caudally located sacral plexus is formed by the lumbosacral trunk (L4-5) and branches from S1-S4. The lumbosacral trunk crosses over the pelvic brim near the sacroiliac joint and is vulnerable to compression in this location. The largest branch of the sacral plexus is the sciatic nerve (L4-5, S1-3), which is comprised of tibial, fibular, and peroneal divisions. These innervate the hamstring muscles, all the muscles below the knee, and all the skin below the knee except the area supplied by the saphenous nerve. The superior gluteal nerve (L4-5, S1; innervates the gluteus medius and minimus), inferior gluteal nerve (L5, S1-2; innervates gluteus maximus), the posterior femoral cutaneous (S1-3), and pudendal (S2-4) nerves all also arise from the sacral plexus.
Strictly speaking, a lumbosacral plexopathy should affect only those nerve fibers within the plexus itself. Certain pathological processes, however, can extend beyond the plexus to the level of the spinal nerve roots and/or the peripheral nerves. When this occurs, the term radiculoplexus neuropathy is most appropriate. This distinction can be helpful when seeking an etiologic explanation, as inflammatory and infiltrative disorders are especially prone to extending beyond a “pure” plexopathy in this manner.
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