“Inclusion Body” Myositis

Inclusion body myositis (IBM) belongs to the group of IIMs, but differs from PM and DM [1]. Since 1968, when Chou [2] originally described “myxovirus-like structures and accompanying nuclear changes in a patient with chronic PM”, IBM has emerged as an apparently distinct and well-differentiated pathological entity among IIMs, with proposed diagnostic criteria [3] (Table 26.1). The term “inclusion body myositis” was coined by Yunis and Samaha [4] in describing a slowly progressive proximal myopathy affecting a young woman. The findings were similar to those reported 1 year earlier by Carpenter et al. [5], whose patient had both distal and proximal muscle weakness, and depressed or absent tendon reflexes. Subsequently, Carpenter et al. [6] reported six cases with a fairly homogeneous syndrome characterized by a protracted, painless disease involving distal as well as proximal muscles, loss of tendon reflexes, and predominantly affecting elderly men.

Population-based epidemiological data on IBM are scanty. An incidence of 2.2 cases/ million population was reported from Sweden [7] and 4.9 patients/million inhabitants in the Netherlands [8]. When adjusted for age distribution, the prevalence was 16/million inhabitants aged over 50 years. The relative prevalence of disease in all IIMs was reported to vary between 16& and 28& in series from neuromuscular centers [9, 10]. Review of the reported cases revealed a considerable male predominance. The male to female ratio is 3:1 [3, 11]. The condition tends to affect the population over 50 years old; however, it may also occur in younger female, and a bimodal age—sex distribution has been suggested [12, 13]. Recent observations suggest the IBM differs from DM and PM since the epidemiology is more limited, the incidence and prevalence varies, clinic and histology present features of myopathy rather than myositis, as well as the fact that IBM responds only modestly or even not at all to immunosuppressive therapy [13]. Several groups with large number of patients with muscle disease have observed that IBM starts between 30 and 50 years [9, 14], but the most common cases of inflammatory myopathy were in patients over 50 years of age, due to slow development of weakness and difficulties in diagnosis [15]. Some argue that IBM should not be included among the IIMs since inflammation is often sparse and hence may play only a secondary role [16], and the response to anti-inflammatory therapy is recognized as at best modest.