To describe the clinical characteristics of patients with anti-mitochondrial M2 antibody (AMA)-positive myositis mimicking amyotrophic lateral sclerosis (ALS).

A 70-year-old female presented with muscle weakness and respiratory distress and was admitted two months ago.  She had a history of heart failure, which was treated 6 years ago. Neurological examination revealed mild consciousness, dysarthria, dysphagia, severe muscular atrophy, moderate truncal and limb muscle weakness, forced grasping, and snout reflex. The neurological symptoms met the updated Awaji criteria for the diagnosis of definite ALS; however, it was atypical for ALS that no fasciculations were observed in atrophic muscles. Despite severe muscular atrophy, tendon reflexes were preserved. Laboratory analysis showed a serum creatine kinase level of 19 IU/L, and blood–gas analysis measurements for pCO₂ and pO₂ were 97.7 mmHg and 44.5 mmHg, respectively. Respiratory function tests reported a vital capacity of 33%. Electromyography could not be performed due to respiratory distress. Her medical history revealed that she had been diagnosed with AMA-associated myositis and had self-interrupted treatment 1.5 years ago. Therefore, she was diagnosed with relapsed AMA-associated myositis. After initiating methylprednisolone pulse and intravenous immunoglobulin therapy, muscle weakness and cardiac function mildly improved. AMA-associated myositis could be misdiagnosed as ALS as the tendon reflexes of patients with AMA-associated myositis may be normal even if muscular atrophy progressed. In this study, 8 of 34 patients with AMA-associated myositis, including our patient, met the updated Awaji criteria for the diagnosis of probable or definite ALS.

AMA-associated myositis can mimic ALS. Upon making the diagnosis of ALS, the clinician should be able to completely differentiate it from AMA-associated myositis.