By Michael Rubin, MD
Professor of Clinical Neurology, Weill Cornell Medical College
Dr. Rubin reports no financial relationships relevant to this field of study.
SYNOPSIS: Inclusion body myositis, while poorly understood regarding cause, is clearly responsive to physical exercise, and patients should be encouraged to maintain robust physical activity.
SOURCE: Paltiel AD, et al. Demographic and clinical features of inclusion body myositis in North America. Muscle Nerve 2015;52;527-533.
Sporadic inclusion body myositis (IBM) is the most common primary myopathy in patients older than 40 years of age and the most common form of inflammatory myopathy in adults. With a prevalence estimated at up to nine cases per one million adults, it has an insidious onset, a slowly progressive course, is often asymmetrical, and has a distally predominant weakness. Etiology and pathogenesis remain a mystery, and many questions remain unanswered. Those addressed by this study encompass the demographics of the IBM population in North America, the nature of their disability and its impact on their activities of daily living and quality of life, how they are diagnosed, and whether the natural history varies between diverse groups.
Under guidance from experts in the fields of medicine, economics, operations research, health care management policy, and registry design, a questionnaire was developed for use in this cross-sectional, self-reporting survey study. Patients diagnosed with IBM were sampled from two patient organizations, the Myositis Association (TMA) and the Muscular Dystrophy Association (MDA), and were sent electronic messages requesting that they voluntarily complete the survey. All records submitted by patients confirming the diagnosis of IBM were included. Informed consent was obtained from all participating patients, and submission of information was voluntary and anonymous. Statistical analysis included column statistics calculated with Microsoft Excel and fixed effects models fitted to the composite index, with all computations performed using Statistical Analysis System 9.3, and 95% confidence intervals and P values for pairwise comparisons generated.
Of 1400 emailed requests, 973 responses were obtained, of which 57 were considered duplicate. Men composed 66.9% of unique respondents, mean age was 70.4 years, and male:female ratio was 2:1. Presenting symptoms included weakness (69.9%), stair-climbing difficulties (59.6%), falling (56.8%), impaired use of arms and legs (53.4%), fatigue (32%), and dysphagia (23%), with diagnosis taking > 2 years in 45.9%, 1-2 years in 22.8%, 6-12 months in 14.3%, and < 6 months in 15.8%. Mean time to diagnosis from initial symptom was 4.7 years. Gait difficulties were reported in 94.3%, and included inability to climb stairs (56.8%), unsteadiness, or needing an assistive device or wheelchair. Two-thirds could not walk more than a block, and reported difficulties eating or swallowing. Activities of daily living were rarely reported as normal. Functional disability was directly related to advanced age, and degree of exercise, with more exercise being associated with higher functional index scores. Patients and clinicians are now better informed regarding prognosis, and IBM patients may now be instructed to “use it or lose it.”
COMMENTARY
Absent a circulating autoantibody, sporadic inclusion body myositis (sIBM) has been considered an idiopathic inflammatory myopathy. However, recent reports of serum autoantibodies directed against a 44-kDa muscle protein in up to 72% of sIBM patients, compared to 5% of polymyositis or dermatomyositis patients, raises the possibility that autoimmunity may be central to its pathogenesis. Cytosolic 5’-nucleotidase 1A (cN-1A) — expressed in skeletal muscle, involved in energy balance control, metabolic regulation, and cell replication, and immuno-histochemically demonstrated to accumulate within rimmed vacuoles — has been identified as the target autoantigen. cN-1A autoantibodies are also found in Sjogren’s syndrome and systemic lupus erythematosus, and thus their diagnostic utility for sIBM remains to be determined.
Inclusion body myositis, while poorly understood regarding cause, is clearly responsive to physical exercise, and patients should be encouraged to maintain robust physical activity.
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