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Background IMNM is a rare autoimmune myopathy characterised by proximal muscle weakness and necrotic muscle fibres with minimal inflammatory cell infiltrates on muscle biopsy. This case describes a patient presenting with seronegative IMNM initially diagnosed with MG due to overlapping clinical features, including respiratory failure, fatiguability and diplopia.Case report An 82-year-old female presented with a two-week history of dysarthria, dysphagia and generalised weakness. On examination, her speech was noted to be hypophonic with a degree of fatigability. Proximal muscle weakness was prominent. Diplopia was present, worse on upward gaze.She was initially treated for MG with pyridostigmine. Despite treatment, she developed type 2 respiratory failure and required non-invasive ventilation (NIV). A subsequent muscle biopsy depicted features in keeping with IMNM. Her myositis panel was negative.She was initiated on steroids but her prognosis was poor given her age and the severity of weakness. However, she had a remarkable recovery, allowing discontinuation of her NIV. She continues to live independently five years later and enjoys a good quality of life.Conclusion This case emphasises the importance of considering alternative diagnoses in patients presenting with neuromuscular weakness and respiratory failure. Early recognition of IMNM is essential to prevent delays in treatment.nazifa.ullah2@nhs.net