The usual mode of onset is with mainly painless weakness of the proximal limb muscles, especially of the hips and thighs and to a lesser extent the shoulder girdle and neck muscles. Often, the patient cannot easily determine the time of onset of weakness. Certain actions—such as arising from a deep or low chair or from a squatting or kneeling position, climbing or descending stairs, walking, putting an object on a high shelf, or combing the hair—become increasingly difficult. Pain of an aching variety in the buttocks, calves, or shoulders is experienced by approximately 15 percent of patients, and it may indicate a combination of myositis and rheumatoid arthritis, tendonitis, or other rhuematologic disorder.
When the patient is first seen, many of the muscles of the trunk, shoulders, hips, upper arms, and thighs are usually involved. The posterior and anterior neck muscles (the head may loll) and the pharyngeal, striated esophageal, and laryngeal muscles (dysphagia and dysphonia) may be involved as well. In restricted forms of the disease, only the neck or paraspinal muscles (camptocormia) may be implicated. Ocular muscles are not affected but there are rare instances of combined myositis and myasthenia gravis. The facial, tongue, and jaw muscles are only occasionally affected, and the distal muscles, namely the forearm, hand, leg, and foot are spared in 75 percent of cases. The respiratory muscles are weakened to a minor degree and in only an exceptional case is there dyspnea, the cause of which is revealed only by an intercostal muscle biopsy (Thomas and Lancaster). Occasionally, the early symptoms predominate in one proximal limb before becoming generalized. As emphasized further on, onset after age 50 years, normal CK, or aberrant patterns of weakness, such as early wrist or finger flexor, quadriceps, or ankle dorsiflexor involvement, are indicative of IBM (see further on).
The muscles are usually not tender, and atrophy and reduction in tendon reflexes, although sometimes present, are far less pronounced than they are in patients with chronic denervation atrophy, IBM, or Lambert-Eaton myasthenic syndrome (the last of these is discussed in Chap. 46). As the weeks and months pass, the weakness and muscle atrophy progress unless treatment is initiated. Without physical therapy, fibrous contracture of muscles eventually develops. Some elderly individuals with a particularly chronic form of the disease may present with severe atrophy and fibrosis of muscles; the response to treatment in such cases is poor.