Inclusion body myositis (IBM) is the third major form of idiopathic myopathy and, depending on the care taken with histologic diagnosis, is the most common one in patients older than 50 years. It is often identifiable by certain topographic features of the weakness described below. There is consensus that the disease is immune mediated, even when the inflammatory component is not prominent in biopsy material. A source of confusion has been the separate entity of inclusion body myopathy, a largely hereditary, pauci-inflammatory process, which displays a different pattern of weakness from IBM. Its defining pathologic features, intracytoplasmic and intranuclear inclusions, were first described in 1965 by R.D. Adams and colleagues, who also drew attention to a number of clinical attributes now considered characteristic. By 1994, only 240 sporadic cases had been recorded in the medical literature (Mikol and Engel), but the diagnosis is now made so frequently that this low number almost certainly reflects the misidentification of IBM as PM in the past. Garlepp and Mastaglia concluded that more than one-third of cases of inflammatory myopathy, especially in men, are IBM. Moreover, as alluded to, the majority of myopathies in patients over 50 years, not attributable to medication toxicity, are probably due to IBM. A set of clinical and pathologic diagnostic criteria for the disease have been proposed by Griggs and coworkers and are useful for research purposes.
The myositis, as alluded to, predominates in men (in a ratio of 3:1) and has its onset in middle or late adult life. Diabetes, any one of a variety of autoimmune diseases, and a relatively mild polyneuropathy are associated in approximately 20 percent of sporadic cases of IBM, but associations with malignancy or systemic autoimmune disease have not been established.
The illness is more variable but generally more focal in presentation than is PM and DM. It is characterized by a steadily progressive, painless muscular weakness and modest atrophy, which is usually distal in the arms and both proximal and distal in the legs. In approximately 20 percent of cases, the disease begins with focal weakness of the quadriceps, finger or wrist flexors, or lower leg muscles on one or both sides, and gradually spreads to other muscle groups after many months or years. Selective weakness of the flexor pollicis longus is a particularly characteristic pattern of involvement, and isolated quadriceps weakness or neck extensor weakness should also bring the diagnosis to mind, although IBM is not the exclusive cause of these patterns. In most patients, the deltoids are spared and the thumb flexors are weak, the opposite pattern to PM and DM. The tendon reflexes are normal initially but diminish in about half the patients, especially the knee jerks, as the disease progresses. Interestingly, the knee jerks may be depressed or lost even without much in the way of quadriceps weakness; this is not the case in PM, in which the reflexes are spared until the muscle is extremely weak. These clinical features are well displayed in the series reported by Amato and colleagues. Dysphagia is common (Wintzen et al). Selective or asymmetric involvement of distal muscles, when it occurs, erroneously suggests the diagnosis of motor neuron disease (the reflexes are not, however, enhanced as they are in ALS).
The CK is normal or slightly elevated, generally showing lower levels than in cases of PM with comparable amounts of weakness. EMG abnormalities are much like those found in PM, as discussed earlier. In addition, a small proportion of IBM patients display a more typically neuropathic EMG pattern, mainly with long-duration polyphasic potentials because of the chronicity of the disease, in the distal limb muscles. However, the EMG changes tend to be restricted to weakened muscles, a distinction from ALS.