Approach to Anti-MuSK Myasthenia gravis: The ultimate mimicker

Background

Anti-Muscle-specific tyrosine kinase – Myasthenia gravis (MuSK-MG) is a rare neuromuscular junction (NMJ) disease subtype with variable clinical presentation and often atypical electromyography findings. While amyotrophic lateral sclerosis (ALS) can present with respiratory failure, its median respiratory insufficiency onset is estimated at six months from the onset of diagnosis, with variability predicted by baseline functional vital capacity (FVC) and bulbar onset-ALS. Anti-MuSK-MG presentation with the predominately irritable myopathic diaphragm is rarely reported.

Clinical presentation

We report a case of Anti-MuSK-MG presenting with persistent respiratory insufficiency and bulbar dysfunction initially misdiagnosed as bulbar-type ALS due to bulbar findings and tongue atrophy. Electromyography (EMG) and single fiber EMG (SFEMG) defied former diagnosis (ALS) with findings of asymmetrical right ulnar and spinal accessory decrements on slow rate repetitive nerve stimulation (RNS), abnormal jitter on SFEMG, and irritable myopathy pattern of the diaphragm and proximal muscles. The serology marker is positive for Anti-MuSK Antibody, and negative AhCR anti-body. With supportive care and Rituximab, the patient's bulbar and respiratory function gradually improved.

Conclusions

Anti-MusK-MG presenting with persistent respiratory insufficiency has been reported with atypical electromyography findings of myopathy and denervation. We report an intriguing case of MuSK-MG with irritable diaphragm myopathy pattern presenting with myasthenic crisis mimicking bulbar subtype ALS.