The Low Prevalence of Inclusion Body Myositis in an Outpatient Rheumatology Myositis Cohort.

Introduction Sporadic inclusion body myositis (IBM) is a rare type of myopathy of unknown etiology typified by the presence of distinctive muscle fiber inclusions. IBM belongs to a larger family of conditions called the idiopathic inflammatory myopathies (IIM). This study seeks to compare the prevalence, clinical manifestations, and disease course of IBM patients to a cohort of polymyositis (PM) and dermatomyositis (DM) patients in an outpatient rheumatology myositis cohort. Methods We conducted a retrospective chart review of all adult patients attending rheumatology clinics at the Cook County Hospital from 2006 to 2011 with the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9) diagnoses of idiopathic inflammatory myopathies (IIM). Data collected included patient demographics, serial muscle strength testing, serial creatine kinase (CK), muscle biopsies, and immunosuppressive therapies received.  Results We identified 112 patients with IIM with the following breakdown: 66 - dermatomyositis, 42 - polymyositis, and four - IBM. These four patients represent the primary cohort. They had a mean age of 67 years (range 62-74), the mean follow-up period of 15 months (range 3-47), mean initial muscle strength in the weakest group of 3.75 (range 2-5, standard deviation [SD]=1.3), mean initial CK of 1,968 U/L (range 313-4,795, SD=1,970), mean final muscle strength in the weakest group of 3.75 (range 3-4, SD=0.5), mean final CK of 1,326 U/L (range 213-2,943, SD=1,200). Conclusions Our IBM patients had many features similar to other described IBM cohorts, including older age, male predominance, slow progression (stable CK and muscle strength), and lack of response to immunosuppressive therapy. Interestingly, we found a low prevalence of IBM in our outpatient rheumatology IIM cohort.