EMG in Syringomyelia
Source: Nogues MA, Stalberg E. Electrodiagnostic findings in syringomyelia. Muscle Nerve 1999;22:1653-1659.
The electrodiagnostic distinction of syrinx from motor neuron disease may be impossible. Fibrillation potentials in atrophic arm muscles with sparing of leg muscles is more likely in the former, but both demonstrate normal sensory nerve conduction studies, depressed motor amplitudes, and normal motor conduction velocities. Using multichannel surface EMG recording, as well as routine needle EMG, unique forms of spontaneous activity may be documented in syringomyelia. Among 43 such patients, 10 showed irregular, but continuous, synchronous firing of motor unit potentials in antagonistic muscles, involving finger flexors and extensors (n = 8) or the tibialis anterior and gastrocnemius (n = 2). Five demonstrated respiratory synkinesis ("breathing arm"), with bursts of motor unit activity in one or more arm muscles synchronous with respiration. In three, muscle contraction was visible though none of the patients was aware of any movement. Continuous motor unit activity, most commonly in finger flexors, invariably in weak, wasted muscles, was found in 18, and myokymic discharges were seen in four, in limb (n = 3) or paraspinal muscles (n = 1). Fourteen (33%) showed long latency responses on routine nerve conduction studies, recording from the median nerve/abductor pollicis brevis (mean latency 97.0 ± 34.6 ms) or tibial nerve/abductor hallucis (mean latency 113.3 ± 47.4 ms). Injury and isolation of the lower motor neuron or inhibitory interneuron, and involvement of the descending spinal pathways, including the vestibulospinal, reticulospinal, and rubrospinal tracts, may underlie these phenomena. —mr