We reported a 63-year-old woman, suffered from
myasthenia gravis with
thymoma who later developed subacute motor neuronopathy after
thymectomy. She noticed distally dominant
muscle weakness and
atrophy of bilateral upper extremities without sensory loss 4 month after thymomectomy. Her
muscle weakness did not improve by
intravenous administration of
anti-cholinesterase (
Tensilon test). Electrophysiological examinations showed no decremental response of examined muscles during repetitive nerve stimulation, nor motor nerve conduction block nor
demyelination of affected peripheral nerves. Laboratory study demonstrated positive anti-
acetylcholine receptor, anti-nuclear and
SS-A antibodies. On immunohistochemistry, the patient's sera positively stained human and rat anterior horn cell cytoplasm as well as axoplasm of spinal white matter and root nerve axon, suggesting the presence of anti-axon antibody, possibly against neurofilament or
tubulin components. The biopsied muscle specimen showed neurogenic muscle changes, but with no evidence of
vasculitis nor cellular infiltration. Therapeutic trial of
plasmapheresis was effective for her
muscle weakness. Further recovery of weakness and
muscle atrophy of hand muscles was obtained by combined
therapy of intravenous and oral
corticosteroid administration and
plasmapheresis. These clinical, electrophysiological and histological findings suggested that
antibodies against neuronal component might be responsible for her motor neuronopathy associated with
myasthenia gravis. The findings of our case study may support the idea that some cases of
motor neuron disease are caused by auto-immune mechanism.