Specific Disorders Of The Brachial Plexus

Brachial plexopathies are most commonly caused by trauma, including closed traction injuries as well as penetrating trauma and dislocation of the humerus. The position of the arm and head is important in determining susceptibility of the brachial plexus structures during a closed injury: with the arm down at the side, a force causing excessive shoulder depression will be transmitted to the upper trunk (C5-6 roots), whereas with the arm overhead, force applied to the axilla is transmitted to the lower trunk (C8-T1 roots). Iatrogenic brachial plexopathies are not uncommon; for example, lower trunk plexopathies can be encountered as a complication of median sternotomy procedures. Obstetric paralysis may occur in newborns following difficult vaginal deliveries; upper trunk lesions (Erb's palsy) are more common than isolated lower trunk lesions (Klumpke's palsy). Infiltrating tumors and other masses, including those in the apex of the lung, can also cause severe, usually very painful, plexopathies.

Neuralgic amyotrophy goes by many names, including idiopathic brachial neuritis and Parsonage-Turner syndrome. This immune-mediated disorder has a characteristic clinical and electrodiagnostic pattern which is distinctly different from structural lesions involving the brachial plexus. Rather than an upper trunk brachial plexopathy, the electrodiagnostic pattern in neuralgic amyotrophy is one of multifocal peripheral nerve involvement, with a predilection for motor nerves innervated by the upper trunk, as well as the anterior interosseous nerve, long thoracic nerve, and proximal median nerve. Often there are minimal sensory abnormalities on both the physical examination and the sensory nerve conduction studies may be normal, in contrast to structural lesions of the brachial plexus.

Radiation plexopathy is a rare complication of radiation therapy. It is usually seen as a late sequelae, years after treatment, most often in women receiving axillary irradiation for breast cancer. Either the upper or lower trunk may be involved; the absence of pain differentiates this from recurrent tumor. If present, myokymic discharges may be identified on EMG and are a useful diagnostic feature distinguishing radiation plexopathy from metastatic tumor.

True neurogenic thoracic outlet syndrome (TOS) is a rare disorder, distinct from the more commonly diagnosed symptomatic TOS, in which pain is a prominent feature, without focal neurological symptoms. In neurogenic TOS a fibrous band compresses the proximal aspect of the lower trunk of the brachial plexus, producing a clinical picture of intermittent pares-thesias progressing to intrinsic hand weakness and atrophy with sensory loss in the medial forearm and hand. Electrodiagnostic studies in neurogenic TOS demonstrate a lower trunk brachial plexopathy.

Lumbosacral Plexus Schematic
Fig. 2. Schematic of anatomy of the lumbosacral plexus.

5. LUMBOSACRAL PLEXOPATHY

5.1. Lumbosacral Plexus Anatomy

The lumbosacral plexus may best be considered as two distinct plexuses, the lumbar and the sacral (Fig. 2). The lumbar plexus is derived from L1-L4 nerve roots and gives rise to two major nerves: the femoral nerve (L2-L4) that innervates the quadriceps muscle and the skin of the anterior thigh and medial lower leg, and the obturator nerve (L2-L4) innervating the thigh adductor muscles and the skin of the medial thigh. Branches to the iliopsoas muscle arise directly from the plexus, traveling alongside the femoral nerve. The lumbar plexus also gives rise to several sensory nerves: the iliohypogastric (L1) supplying the anterior and lateral lower abdominal wall, the ilioinguinal (L1) to the upper medial thigh and root of the penis or labium majus, the genitofemoral nerve (L1-L2) to the upper anterior thigh and scrotum, and the lateral cutaneous nerve of the thigh (L2-L3) to the lateral thigh.

The sacral plexus derives from L4 through S4 nerve roots and gives rise to the largest nerve of the plexus, the sciatic nerve. The sciatic nerve supplies the hamstring muscles and all the muscles below the knee through its two branches: the peroneal and tibial nerves. The superior and inferior gluteal nerves arise from the sacral plexus proximal to the sciatic nerve and supply the gluteus muscles.

5.2. Pathophysiology

The lumbosacral plexus is well protected by the pelvis, making it less susceptible to traumatic injury than the brachial plexus. It is, however, a favored site for involvement in diabetes, in contrast to the brachial plexus, which is rarely affected by diabetic amyotrophy. The LSP can be also affected by malignant invasion, radiation, retroperitoneal hematoma, and less frequently by trauma. As in the brachial plexus, the lesions that can be identified on electrophysiological testing are largely axonal in nature and occur distal to the dorsal root ganglion, as opposed to radiculopathies. Hence, as with brachial plexopathy, sensory NCS abnormalities are typical and help differentiate lumbosacral plexopathy from radiculopathy.

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