In general, inclusion-body myositis (IBM)occurs most commonly in older men and can differ from polymyositis by the additional features of distal (distant, as in forearms and lower legs) muscle weakness, asymmetrical (unequal, not everywhere, not all the same) muscle involvement, neuropathic findings or physical examination and electromyography.
Serum levels of muscle-derived enzymes are elevated at some time during the course of the disease in 99% of the patients.
CPK (Creatine phosphokinase, is useful as a disease activity index, or a gauge of therapeutic response (but not in all patients, at all times).
The enzymes aldolase, aminotransferase (AST and ALT), and Lactate dehydrogenase (LDH) are also considered useful.
There are complicating factors in the use of these enzymes as indicators, especially cardiac involvement. One may need to look specifically at the MB isoenzyme of CPK.
The electromyogram (EMG) is abnormal in 90% of patients (one should not forget about that other 10%).
Classic changes include:
Small-amplitude, short-duration, polyphasic motor unit potentials.
Fibrillation, poitive waves, and increased insertional irritability.
Spontaneous bizarre high-frequency discharges
This complete triad may only be found in some 40% of patients, and in some, the changes are restricted to the paraspinal muscles.
It is frequently necessary to eliminate other causes first, because many of these criteria are not specific or limited to IBM. Circulating autoantibodies are common. The most common Myositis-specific autoantibodies are:
Anti-tRNA synthetases including:
Also: Anti-SRP and Anti-MAS
Reference: Bennett & Plum: Cecil Textbook of Medicine, 20th edition, 1996.