A wolf in sheep’s clothing: myositis mimicking motor neuron disease – a case report

Abstract

Idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of immune-mediated muscle disorders typically presenting with proximal muscle weakness and elevated creatine kinase levels. Atypical presentations, particularly in elderly individuals, may mimic motor neuron disease (MND), creating significant diagnostic challenges. We report a 72-year-old man who presented with a six-month history of progressive weakness beginning in the proximal lower limbs and later involving distal limbs and bulbar muscles. Examination revealed muscle wasting, spasticity, exaggerated reflexes, polyminimyoclonus, and neck flexor weakness, suggesting a mixed upper and lower motor neuron syndrome. Nerve conduction studies were normal, while electromyography demonstrated an irritable myopathic pattern without diffuse neurogenic denervation. Serum creatine kinase levels were normal. Autoimmune evaluation showed anti-thyroid peroxidase positivity, SSA/Ro52 positivity, rheumatoid factor positivity, hypocomplementemia, inflammatory polyarthritis, renal involvement, and interstitial lung disease. Whole-body muscle magnetic resonance imaging demonstrated diffuse symmetrical inflammatory changes, while FDG-PET CT excluded occult malignancy. The patient was treated with intravenous methylprednisolone followed by oral prednisolone and mycophenolate mofetil, resulting in marked clinical improvement. Idiopathic inflammatory myositis may rarely present with upper and lower motor neuron-like signs and mimic MND. Recognition of systemic autoimmune features, appropriate electrophysiological evaluation, and muscle imaging is essential to avoid misdiagnosis of a potentially treatable condition.