Peripheral Nerve Disorders
Peripheral nerve disorders
Approach polyneuropathy by tempo, symmetry, fiber type, and electrodiagnosis. Length-dependent, symmetric distal sensory loss is typical of diabetes, alcohol, B12 deficiency, and chemotherapy; NCS show reduced sural and superficial peroneal SNAP amplitudes first. Demyelinating neuropathy is suggested by conduction velocity below 70% of the lower limit of normal, distal latency above 130% of the upper limit, F-wave prolongation, conduction block, or abnormal temporal dispersion. Uniform slowing suggests a hereditary demyelinating neuropathy such as CMT1A (PMP22 duplication); nonuniform, asymmetric slowing with conduction block suggests CIDP or multifocal motor neuropathy. GBS is acute and often shows prolonged distal latencies, absent F waves, and albuminocytologic dissociation.
Axonal loss produces low CMAP and SNAP amplitudes with preserved velocities until severe. Needle EMG shows large, long-duration, polyphasic motor unit potentials with reduced recruitment.
Focal mononeuropathies have characteristic locations: median at the wrist (carpal tunnel), ulnar at the elbow (cubital tunnel), radial at the spiral groove, and peroneal at the fibular head. Test the relevant sensory nerve and look for conduction block across the site. Traps include Martin-Gruber anastomosis (median-to-ulnar forearm connection) falsely lowering median CMAP, and double-crush lesions.
Worked example: a patient with nocturnal numbness of the thumb, index, and middle fingers has a prolonged median distal sensory latency and normal ulnar studies, indicating carpal tunnel syndrome. If the median sensory response is normal but the median CMAP is low, consider a Martin-Gruber anastomosis or severe axonal loss.
Rule of thumb: in length-dependent neuropathy, the sural SNAP is abnormal before the CMAP.