Multichannel somato sensory evoked potential study demonstrated abnormalities in cervical cord function in brachial monomelic amyotrophy

Background: Brachial monomelic amyotrophy (BMMA) is known to affect the central cervical cord gray matter resulting in single upper limb atrophy and weakness. Settings and Design: Case series of BMMA patients who underwent somatosensory evoked potentials (SEP) studies at a tertiary referral center. Aims: We proposed to record Multichannel Somatosensory Evoked Potentials (MCSSEP) from median and ulnar nerves with neck in neutral and neck fully flexed position in 17 patients with classical BMMA seen over three years. Materials and Methods: Recordings were done from both median (MN) and ulnar nerves (UN). N9, P9, N13, N20 potentials were recorded and amplitudes measured. SSEPs were performed in 22 age-matched healthy men. Amplitudes of cervical response were calculated by N13/P9 ratio and compared in both positions. Results: Among the controls N13 amplitude was always normal {MN: mean N13/P9 - 0.96 in neutral; 0.95 in flexed}{UN: mean N13/P9 - 0.82 in neutral; 0.83 in flexed}, and mean amplitudes did not reveal any difference in both conditions ( P > 0.05). Among 17 patients N9, P9 and N20 responses were normal in neutral position. Flexion showed no change in latency or amplitude of N9 and N20 responses (P-0.63) whereas the N13 response was abnormal in at least one tested nerve in the affected limb (MN: P0.01; UN: P 0.01). During flexion, N13 response was abnormal in 14 (82) patients after MN stimulation and in all 17(100) after UN stimulation {MN: mean N13/P9 - 0.62 in neutral; 0.38 in flexed}{UN: mean N13/P9 - 0.55 in neutral; 0.31 in flexed}. Conclusion: MCSSEP in BMMA with neck flexion caused a significant reduction of the cervical N13 response indicating segmental cervical cord dysfunction.