DEFINITION.—An affection of the voluntary muscles, of which the chief, and often the only, symptom is pain on movement.
SYNONYMS.—Myalgia as a general term has few synonyms. It is sometimes called myodynia. This affection has no essential relation to rheumatism or the rheumatic diathesis; therefore the common use of the term muscular rheumatism as a synonym for myalgia is an error. This error has occasioned much confusion of thought and mistaken medication, and tends to maintain the obscurity which overhangs the subject of the so-called and often miscalled rheumatic affections in general. That true rheumatic processes may extend from serous or fibrous structures to contiguous muscular masses has, in the absence of demonstration, been assumed by many writers of authority, but that acute or subacute rheumatism, with its recognized characters, ever manifests itself primarily or exclusively as an inflammation of muscle-substance is an assumption wholly without clinical or pathological support.
The term myo-rheumatism is as inapplicable as muscular rheumatism, and lacks the sanction of usage. Myositis is a term used to describe (1) an acute inflammation of muscle, often traumatic, and commonly attended by suppuration, and (2) a chronic indurating inflammatory process, not infrequently due to syphilis. Neither of these conditions resembles the affection under consideration in its clinical aspects, nor is allied to it pathologically.
As manifested in particular muscles or groups of muscles myalgia has been described under the terms cephalodynia, torticollis (myalgia cervicalis), pleurodynia (m. pectoralis seu intercostalis), lumbago (m. lumbalis), dorsodynia, omodynia, scapulodynia (m. dorsalis), etc.
This affection must, in the present state of our knowledge, be classified with the diseases of nutrition in the more narrow sense. It is not a diathetic disease.
HISTORICAL CONSIDERATIONS.—To Inman1 of Liverpool is due the credit of having first pointed out the frequency of this malady and the ease with which it may be mistaken for other and much more serious diseases—an error in diagnosis which has been followed by serious results, especially in the case of nervous and self-centred females and other hypochondriacal persons. It cannot, however, be denied that this author, carried away by his enthusiasm, exaggerated the importance of this local affection at the expense of undervaluing the frequency and significance of other painful disorders which have their origin in the nervous system. To Inman we also owe the term myalgia, which has the positive merit of embodying the idea of pain as the chief symptom of the disorder and the muscles as its seat, and the not inferior negative merit of implying no erroneous theory as to its nature and cause.
1 Thomas Inman, M.D., Certain Painful Muscular Affections, 1856; Spinal Irritation Explained, 1858; On Myalgia, its Nature, Causes, and Treatment, 1860.
This affection is described in few even among the recent textbooks; in others it receives merely incidental mention; in the majority of them it is passed over in silence. Yet it is obvious that the descriptions of muscular rheumatism, which are rarely omitted, are based upon and refer to cases of various kinds which for the most part are not rheumatic at all, and very frequently are examples of true myalgia.
ETIOLOGY.—(A) Predisposing Influences.—Myalgia is "essentially pain produced in a muscle which is obliged to work when its structure is imperfectly nourished or impaired by disease." Hence all influences which unfavorably affect the nutrition of the muscles, all diseases which directly affect the integrity of their structure, predispose them to this affection. The defect in nutrition may be only relative to the amount of work the muscle is called upon to do, or there may be absolute malnutrition, implicating the whole body. The muscle may be impaired by a local disease which affects it alone, or it may share in morbid processes which also involve other and distant structures.