Nerve Root and Plexus Disorders
Root and plexus localization
Radiculopathy is suggested by pain in a dermatome, weakness in a myotome, and a depressed reflex, with EMG showing denervation in at least two muscles of the same myotome supplied by different peripheral nerves, plus paraspinal muscles. Sensory nerve action potentials (SNAPs) stay normal because the lesion is proximal to the dorsal root ganglion. The classic trap is relying on EMG too early: fibrillation potentials and positive sharp waves appear about two to three weeks after nerve injury, so a normal study at five days does not exclude radiculopathy. Paraspinal muscle denervation is useful but can be absent in small foraminal lesions.
Plexopathies are preganglionic or postganglionic. Root avulsion (preganglionic) preserves the SNAP; a postganglionic plexus lesion reduces it. Brachial plexopathy from Parsonage-Turner syndrome is painful and patchy, often affecting the upper trunk; radiation plexopathy is painless, affects the lower trunk, and may show myokymic discharges. Neurogenic thoracic outlet syndrome involves the lower trunk (C8-T1), with wasting of the intrinsic hand muscles and a reduced medial antebrachial cutaneous SNAP. Diabetic lumbosacral radiculoplexus neuropathy is painful, asymmetric, and often associated with weight loss, with denervation in multiple root territories and paraspinal sparing.
Key discriminators: root lesions affect paraspinal muscles; plexus lesions spare them. Root lesions follow a myotomal pattern, while plexus lesions follow a trunk or cord pattern.
Worked example: weakness of deltoid, biceps, and brachioradialis with a reduced lateral antebrachial cutaneous SNAP and paraspinal fibrillation potentials points to C5-C6 root disease; the same weakness with an abnormal SNAP and normal paraspinals favors an upper trunk plexopathy.
Rule of thumb: paraspinal fibrillations favor root; SNAP loss favors plexus distal to the ganglion.