Pembrolizumab is an anti-PD-1 monoclonal antibody for selected high-grade malignancies, but its use has been associated with several life-threatening immune-related adverse events. 

We report a case with successful outcome of pembrolizumab associated anti-striational antibody myasthenia gravis (MG) after being treated aggressively with methylprednisolone, Intravenous immunoglobulin (IVIG) and plasmapheresis (PLEX).

A 64-year-old female presented with 14 days of proximal muscle weakness, shortness of breath, left eye ptosis, diplopia, and dysphagia after 2 cycles of pembrolizumab with last dose 2 weeks prior for metastatic non-small cell lung cancer. Examination showed ophthalmoplegia with supraduction and left lateral gaze and proximal muscle weakness. Labs showed CPK of 2,520 and anti striational antibody was 1:15,360 (reference <1:120). Patient received methylprednisolone 1g IV for 10 days and IVIG 2g/kg for 3 days. Following, patient developed chest pain, Troponins were elevated and EKG revealed an inferior myocardial infarction and complete heart block. An echocardiogram and left heart catherization were unremarkable. The patient underwent PLEX for 5 days due to lack of improvement. Cardiology placed a dual-chamber pacemaker for complete heart block. After 3 weeks, the patient discharged home with prednisone taper. 

Pembrolizumab prevents T-cell activation and autoimmunity in lung cancer and malignant melanoma. In a review on anti-PD-1-associated MG, the positive rate for anti-AChR antibody differs widely, from 20% to 73%. Muscle biopsy may show necrotizing myopathy, nonspecific myopathy, myopathy with complement-mediated microvasculopathy and tubular aggregates indicating a myositic component. In a systematic review, anti-striational antibodies were positive in 2 out of 4 patients and were treated successfully with combinations of steroid pulse, IVIG, PLEX, prednisone, immunosuppressant, and cholinesterase inhibitor.

Early aggressive treatment with IVIG or PLEX and steroids, regardless of initial symptoms severity, may lead to better outcomes in patients presenting with pembrolizumab associated anti-striational antibody MG.