A 36-year-old female presents with worsening dyspnea, double vision, and dysarthria over the course of the last 3 days, which was preceded by an upper respiratory tract infection that cleared without any treatment. On examination she has mild labial and palatal dysarthria, limited right eye elevation, and abduction with horizontal double vision on right lateral gaze. She is admitted to the neurology service for workup. A nerve conduction study is completed and demonstrates 50% decrement with rapid stimulation. Laboratory workup reveals positive MuSK (muscle specific kinase) antibody.
What is the best treatment plan for this patient?a. Rapid therapy with IVIG and maintenance therapy with oral prednisone
Correct Answer: D
MuSK is a tyrosine kinase receptor found on muscle which is important in the maintenance of the neuromuscular junction. The repetitive stimulation presented in the question demonstrates the typical findings of myasthenia gravis, a decrement of >33% at 3 Hz cycling. Other findings on electrodiagnostic testing include increased jitter on single fiber electromyography. MuSK-positive myasthenia gravis can present with crisis as their initial presentation and typically have predominately ocular and bulbar symptoms. Treatment of crisis is the same as for other myasthenia gravis patients, which is initial treatment with a rapidly acting intervention, either IVIG or plasmapheresis, and concurrent or shortly after with chronic immunosuppressive therapy. MuSK positive myasthenia patients appear to have improved early response with 93% responding to plasmapheresis and only 61% responding to IVIG. Although initiation of steroids is the mainstay of treatment of other myasthenia gravis patients, MuSK antibody patients do not respond as well. These patients respond better to rituximab for chronic immunosuppressive therapy.