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LettersMagnetic Resonance Imaging Criteria to Differentiate Inclusion Body Myositis from Polymyositis To the Editor: It was with great interest that I read the article by Dion, et al about the differences seen by magnetic resonance imaging (MRI) between inclusion body myositis (IBM) and polymyositis (PM)1. The article emphasizes the value of MRI in differentiating between IBM and PM. There are, however, several items of concern. First, in any study that examines the value of a diagnostic test, the issue of the gold standard arises. For IBM and PM there is no clearly defined gold standard. Therefore, complex diagnostic criteria have been drawn up, usually during consensus meetings. The Bohan and Peter criteria used by Dion, et al are outdated and were defined at a time when IBM was not fully recognized to be a separate disease entity2. Following these criteria most patients with IBM will be diagnosed as having PM. Especially in a study that wants to differentiate between IBM and PM, the use of the Bohan and Peter criteria is not very wise. IBM was diagnosed by Dion, et al according to the preliminary criteria published by Calabrese, et al in 19873. These criteria have never been accepted internationally and have been replaced by better defined criteria that distinguish between sporadic IBM and hereditary IBM (which the Calabrese criteria do not, even though Dion, et al present them as criteria for sporadic IBM) and that strongly emphasize the specific clinical syndrome of IBM4. Second, the authors report that no difference in muscle strength was observed between PM and IBM, and that muscle strength was not correlated with MRI findings. Unfortunately, the authors measured strength of the proximal muscle groups only. IBM is characterized by a predominantly distal muscle weakness4. If proximal and distal muscles had been measured, a difference between IBM and PM might have been found, and there might have been a correlation between MRI findings and muscle strength. Third, I would like to stress that IBM is characterized by a very specific clinical syndrome that usually can be distinguished from PM by experienced clinicians based on clinical signs and symptoms, electromyographic abnormalities, and muscle biopsy findings. In only a few cases can a clear distinction not be made, and it is in those cases that we might need the extra information that MRI can provide. It is therefore interesting to consider what the clinical characteristics are of the one patient described by Dion, et al in their Table 2 with PM that has fatty infiltration exclusively in the anterior muscle groups and an asymmetrical distribution of the fatty infiltration. My hypothesis is that this patient has steroid-resistant PM — in other words, a probable case of IBM. Nevertheless, I thank the authors for conducting this time-consuming study, which can serve clinicians in situations of doubt, and which emphasizes the potential value of MRI in neuromuscular disorders. GERALD J.D. HENGSTMAN, MD, Neuromuscular Centre Nijmegen, Department of Neurology, University Medical Centre Nijmegen, PO Box 9101, 6500 HB Nijmegen, The Netherlands. E-mail: g.hengstman@neuro.umcn.nl REFERENCES 1. Dion E, Cherin P, Payan C, et al. Magnetic resonance imaging criteria for distinguishing between inclusion body myositis and polymyositis. J Rheumatol 2002;29:1897-906. 2. Bohan A, Peter JB. Polymyositis and dermatomyositis (first of 2 parts). N Eng J Med 1975;292:344-7. 3. Calabrese LH, Mitsumoto H, Chou SM. Inclusion body myositis presenting as treatment-resistant polymyositis. Arthritis Rheum 1987;30:397-403. 4. Griggs RC, Askanas V, DiMauro S, et al. Inclusion body myositis and myopathies. Ann Neurol 1995;38:705-13. |