Myalgia

Myalgia (from the Greek myos — muscle and algos — pain) is a pain syndrome affecting striated (skeletal) muscle. In infectious disease medicine, this symptom is one of the principal markers of a systemic inflammatory response or a sign of direct parasitic involvement of muscle tissue.

Etiology and pathophysiology

The mechanism underlying the development of myalgia is divided into two types:

• toxic-infectious — this type of myalgia (characteristic of influenza, coronavirus infection, and dengue fever) occurs without the physical presence of the virus in the muscles. It is caused by a massive release of pro-inflammatory cytokines into the bloodstream (especially tumor necrosis factor and interleukin-6). These molecules markedly increase the sensitivity of nociceptors (pain receptors) in muscle fibers, causing a sensation of aching throughout the body;
• invasive — comprises an entirely different pathogenesis. This type of myalgia is typical of helminthiases, particularly trichinellosis. In this case, the parasite larvae physically penetrate muscle fibers via the bloodstream (most commonly the calf, masticatory, and deltoid muscles). The larva coils into a spiral and becomes enclosed in a dense capsule (encapsulation). This causes a local inflammatory process (myositis), tissue edema, and severe debilitating pain with the slightest movement.

Clinical Significance

The nature of the myalgia helps the physician make a differential diagnosis:

• diffuse aching (a sensation of dull pain or discomfort in muscles and joints throughout the body) — when combined with high fever and catarrhal symptoms, indicates an acute respiratory viral infection (ARVI). In this case, therapy consists of administering nonsteroidal anti-inflammatory drugs to inhibit prostaglandin synthesis;
• localized, progressive muscle pain — accompanied by facial edema, fever, and marked eosinophilia, is a pathognomonic sign of trichinellosis. In this case, immediate administration of anthelmintic drugs is required, sometimes (in severe cases) in combination with systemic glucocorticosteroids.