The main issue here is differentiation of DM and PM from inclusion body myopathy. The specific problem of determining which patients with DM or PM should have an extensive evaluation for a systemic malignancy and for connective tissue disease has been partially settled. We have adopted the practice of careful inspection of the chest radiograph, routine blood tests and stool examination for blood for all patients, and of undertaking a more extensive evaluation in patients older than 55 years and in smokers of any age. The evaluation of patients over 55 and smokers includes chest and abdominal computed tomographic (CT) scans, colonoscopy, pelvic ultrasound, cancer antigen (CA)-125, carcinoembryonic antigen (CEA), as well as other tests. In patients with recent weight loss, anorexia, or other symptoms suggestive of malignancy, we have included upper endoscopy and resorted to a body positron emission tomography scanning.
In addition to these main issues of distinguishing PM and DM from IBM, currently aided by antibody testing, we call attention to the following problems that we have encountered in connection with diagnosis: 1. The patient with proximal muscle weakness is incorrectly diagnosed as having progressive muscular dystrophy (actually, the opposite pertains more often). Points in favor of myositis are (1) lack of family history (although many dystrophies have recessive inheritance); (2) older age at onset; (3) rapid evolution of weakness; (4) evidence, past or present, of other connective tissue diseases; (5) high serum CK values (again, can be high in certain dystrophies); (6) marked degeneration and regeneration in muscle biopsy; and, finally, if there is still doubt, (7) unmistakable improvement with corticosteroid therapy.
2. The patient with a systemic autoimmune disease (rheumatoid arthritis, scleroderma, lupus erythematosus, Sjögren syndrome) is suspected of having PM in addition. Pain in these conditions prevents strong exertion (algesic pseudoparesis). Points against the coexistence of myositis are (1) the inability to document weakness out of proportion to muscle atrophy and the presence of pain on passive movement of the limbs; (2) normal EMG; (3) normal serum CK; and (4) normal muscle biopsy except possibly for areas of infiltration of chronic inflammatory cells in the endomysial and perimysial connective tissue (interstitial myositis).
3. When muscle pain is a prominent feature, polymyalgia rheumatica must be differentiated. This latter syndrome is characterized by pain, stiffness, and tenderness in the muscles of the neck, shoulders, and arms, and sometimes of the hips and thighs; even passive motion of the limbs causes pain because of the periarticular locus of this disease. A high sedimentation rate, usually above 65 mm/h, is a diagnostic feature, but more typically the value is close to 100 mm/h, levels higher than in myositis. Biopsy of the temporal artery frequently discloses a giant cell arteritis. CK levels—and, of course, muscle biopsy—are normal. Rapid disappearance of pain with administration of small doses of prednisone is also diagnostic of polymyalgia rheumatica (see
Chap. 9).
4. The patient has restricted muscle weakness. Weakness or paralysis of the posterior neck muscles, with inability to hold up the head, restricted bilateral quadriceps weakness, and other limited pelvocrural palsies are examples. Most often, the head-hanging or head-lolling syndrome proves to be caused by PM, and the other syndromes are caused by restricted forms of dystrophy or by motor neuron disease. IBM is the main alternative consideration in cases of neck or quadriceps weakness, particularly if the latter weakness is asymmetric; muscle enzymes in the serum are normal or slightly elevated. EMG and biopsy are helpful in diagnosis.