Arthrogryposis is another form of fibrous contracture that is found in newborns, involving multiple muscle groups; it occurs in association with several diseases that have two features in common: an onset during intrauterine life and an alteration of the neural or muscular apparatus that results in muscular weakness. In other words, contractures and fixity of the limbs in arthrogryposis are the result of reduced mobility of the developing joints, consequent upon muscle weakness during fetal development. Most often the cause is a loss or failure of development of anterior horn cells, as in Werdnig-Hoffman disease, but the abnormality may be in the nerve roots, peripheral nerves, or motor endplates, or in the muscle itself. The rigid spine syndrome (RSS) in children is yet another form of fibrous contracture, presumably the result of an unusual axial muscular dystrophy.
Notably, most primary muscle diseases are painless. When pain is prominent and continuous during rest and activity, there will usually be evidence of disease of the peripheral nerves, as in alcoholic–nutritional neuropathy, or of adjacent joints and ligaments (rheumatoid arthritis, polymyalgia rheumatica). Pain localized to a group of muscles is more a feature of torticollis and dystonias. Pain tends not to be prominent in polymyositis and dermatomyositis, but there are exceptions, as commented below. Pain tends to be more definite in polyneuritis, poliomyelitis, and polyarteritis nodosa than it is in polymyositis, various forms of dystrophy, and other myopathies. If pain is present in polymyositis, it usually indicates coincident involvement of connective tissues and joint structures. Hypothyroidism, hypophosphatemia, and hyperparathyroidism are other sources of a myalgic myopathy. Certain drugs produce muscle aches in susceptible individuals. They include the “statin” lipid-lowering drugs, clofibrate, captopril, lithium, colchicine, beta-adrenergic blocking drugs, penicillamine, cimetidine, suxamethonium, and numerous others (see the table contained in the review by Mastaglia and Laing).
There are probably a limited number of mechanisms of muscle pain. Prolonged and sustained contraction gives rise to a deep aching sensation. Contraction under ischemic conditions—as when the circulation is occluded by a tourniquet or from atherosclerotic vascular disease—induces pain; the pain of intermittent claudication is presumably of this type and is not accompanied by cramp. It was postulated that lactic acid or some other metabolite accumulates in muscles and activates pain receptors, but there is also evidence to the contrary. The delayed pain, swelling, and tenderness that occur after sustained exercise of unconditioned muscles are evidently a result of fiber necrosis (Armstrong).
Muscle biopsy infrequently reveals the cause of these painful syndromes, but it may be undertaken in cases of suspected metabolic or dystrophic muscle disease. In their retrospective series, Filosto and colleagues determined that the biopsy was most likely to be helpful if there was exercise-induced muscle pain and the creatine kinase (CK) concentration was greatly elevated; even then two-thirds of the entire group had either normal or nonspecific findings on the biopsy.
Having listed all these causes of proximal pains, all physicians are aware that arthritic and mundane musculoskeletal complaints are more common causes of discomfort.