Life-threatening polymyositis with spontaneous hematoma induced by nivolumab in a patient with previously resected melanoma.

Single-agent anti-PD1 antibodies are usually very well tolerated, but serious toxicity can still occur. Despite the PD-1 pathway seems to be relevant in the pathogenesis of immune-related myositis, anti-PD1-related myositis is generally a rare side effect of the treatment and usually not serious. However, its frequency is likely to increase as the use of immune checkpoint blockades. We present here a case of life-threatening polymyositis with associated spontaneous muscular hematoma in a patient treated with single-agent nivolumab in the adjuvant setting. Spontaneous hematoma is an extremely rare complication with unclear etiology of idiopathic myositis. Very few cases have been reported in the literature and their outcome has been often fatal. To our knowledge, this is the first case of autoimmune myositis and spontaneous heamatoma associated with the administration of single-agent checkpoint blockade. Anti-PD1 antibodies have changed the treatment landscape for a number of cancer entities in the past few years. When given as single agent they are usually very well tolerated, but serious rare toxicity can still occur. We present here a case of polymyositis with associated spontaneous muscular hematoma in a patient treated with single agent nivolumab.