1362

Abstract

Introduction: Inclusion body myositis (IBM) is one of the idiopathic inflammatory myopathies, along with dermatomyositis and polymyositis. It is known to present with insidious onset of weakness, often a slow but progressive course, with limb muscles most commonly affected. Respiratory muscle involvement is rare. We present a case of a 68-year-old man who complained of slowly progressing muscle weakness for one year, characterized as increasing difficulty in walking and lifting objects. He was thought to have chronic inflammatory demyelinating polyneuropathy (CIDP) after a work-up done by a neurologist, which included a lumbar puncture and EMG. He was prescribed prednisone but without improvement of symptoms and hence this was discontinued. His only other co-morbidities were hypertension and dyslipidemia. He then later developed increasing lethargy and confusion and was immediately brought to the emergency room. He had shallow respirations and required intubation. Post-intubation ABG showed a pH of 7.39, pCO2 of 62 mmHg and a bicarbonate of 37 mmol/L, consistent with chronic hypoventilation. Head CT was negative for acute pathology. He was admitted to the intensive care unit and started on prednisone and intravenous immunoglobulin (IVIG) for CIDP. His mental status improved after intubation but remained weak, with notable symmetric limb weakness throughout, with the patient's effort generally about grade 3-4 on the medical research council (MRC) scale for muscle strength. Muscle atrophy especially of the proximal limbs were also notable. Respiratory parameters indicated respiratory muscle weakness. His rapid shallow breathing index was 151 (acceptable is <105) and his negative inspiratory force was -12 cm H20 (acceptable for weaning is generally -30 cm H20 or less). CK was normal (101 units/L). EMG revealed normal conduction velocities and findings inconsistent with CIDP. Steroids and IVIG were discontinued. He then underwent a muscle biopsy of his right deltoid which revealed mixed neurogenic and myopathic changes with few rimmed vacuoles, compatible with the diagnosis of IBM. He subsequently underwent tracheostomy and jejunostomy feeding tube placement. He gradually tolerated tracheostomy mask trials with room air during daytime with mechanical ventilatory support at night. He was later discharged to a long-term rehabilitation facility. IBM classically presents with symmetric proximal lower limb weakness, later involving the upper limbs and distal muscles. CK levels may be normal or elevated. It has no cure, nor standard treatment, and generally no improvement seen with steroids and immunosuppressive therapy. The occurrence of respiratory muscle weakness is unusual and atypical. The lack of awareness of this association, only rarely described in literature, delayed its recognition and diagnosis. Primary respiratory failure from respiratory muscle weakness, although uncommon, may be seen in IBM. Suspicion should be raised and a muscle biopsy may be warranted to correctly diagnose your patient.