Antimitochondrial M2 Antibody-Positive Myositis Presenting With Headache As the Chief Complaint: A Case Report

Antimitochondrial M2 antibody (AMA-M2)-positive myositis is a muscle disease characterized by the presence of serum AMA-M2, which primarily manifests as weakness and atrophy of the trunk muscles, often accompanied by other symptoms such as weight loss, cardiac dysfunction, arrhythmia, and respiratory failure. To date, however, headache has not been reported as an associated symptom. Herein, we present the case of a 46-year-old woman with AMA-M2-positive myositis who was unaware of any notable muscle weakness and mainly complained of headaches. Based on the patient's history and the nature of the headaches-nonpulsatile, constricting, without nausea, and unaffected by position-, a tension-type headache was suspected. However, examination in the supine position revealed a reduction in blood oxygen concentration and trunk muscle weakness, leading to an eventual diagnosis of AMA-M2-positive myositis with type 2 respiratory failure. The patient's headache rapidly improved with the initiation of non-invasive positive pressure ventilation (NPPV), and the cause of the headache was identified as CO₂ retention due to type 2 respiratory failure. In addition, glucocorticoid-based immunotherapy contributed to gradual improvements in weight, muscular strength, and respiratory function, thereby enabling the patient to be weaned off NPPV after 13 months of treatment. This case highlights the importance of considering neuromuscular diseases, including AMA-M2-positive myositis, in the differential diagnosis of headache and emphasizes the necessity of detailed examinations, including those conducted in the supine position.