Hypertrophy and involuntary contractions of the trapezius muscle

Hypertrophy and involuntary contractions of the trapezius muscle Teaching Neuro. Images Neurology Resident and Fellow Section © 2013 American Academy of Neurology

Vignette • This 50 year-old man presented with GM 1 antibody-negative Multifocal Motor Neuropathy • The most prominent focal weakness and conduction blocks were in the right radial and fibular territories • His examination had also revealed striking enlargement of the left trapezius, with frequent fasciculations • EMG of the trapezius showed frequent involuntary motor unit potentials presenting as fasciculation potentials and myokymic discharges • With IVIg therapy, both the multifocal motor weakness and the abnormalities in the left trapezius muscle substantially improved © 2013 American Academy of Neurology Bourque et al.

Imaging A © 2013 American Academy of Neurology B Bourque et al.

Trapezius muscle hypertrophy in multifocal motor neuropathy • Neurogenic muscle hypertrophy has been reported in spinal muscular atrophy, radiculopathy, post-polio syndrome, CIDP and multifocal motor neuropathy (MMN). • Muscle hypertrophy in this patient was attributed to constant activation by ectopic motor action potentials generated in the spinal accessory nerve. • The more common manifestation of the nodoparanodopathy of MMN is muscle weakness from conduction block, followed by axonal loss in chronic cases. © 2013 American Academy of Neurology Bourque et al.