Muscle pain (myalgia) may affect the large muscles of the legs and back, or tiny muscles in the hands or face. It may often be mistaken for pain coming from deeper in the body, particularly if the muscles of the belly or chest are affected.
By far the most common and obvious cause of myalgia (muscular pain) is overuse of the muscle with excessive and unaccustomed exercise. This may cause microscopic tears in the muscle, which result in both cramps and pain.
The next cause in order of importance is viral and bacterial infections that involve muscles. Dozens, if not hundreds, of different infections may be responsible. Examples include:
- influenza (muscle pain is a feature that separates influenza from a common cold),
- hepatitis (most forms),
- glandular fever (infectious mononucleosis),
- Ross River fever (joint pains),
- encephalitis (brain infection with neck stiffness),
- dengue fever,
- brucellosis (caught by meat workers from cattle),
- Lyme disease (spread by tics from mice and deer),
- toxoplasmosis (from cats),
- polio (prevented by vaccination, but if caught may cause permanent muscle damage)
- leptospirosis (inflamed eyes and fever).
Fibrositis is the replacement of some muscle fibres with scar tissue after injury to the muscle from over use or repetitive use. The affected muscles may ache, become stiff and harder than usual. The discomfort is eased by heat and worsened by cold.
Less common causes of myalgia (muscle pain) include:
- low blood sugar (caused by overdoses of medication for the treatment of diabetes),
- polymyalgia rheumatica (inflammatory condition involving many muscles),
- motor neurone disease (progressive and permanent degeneration of the nerves that control muscle movement),
- chronic fatigue syndrome,
- severe deficiencies of vitamins B (beriberi) and C (scurvy),
- myositis ossificans,
- rheumatic fever.
Rarer still are causes such as:
- the Guillain-Barré syndrome (progressive symmetrical weakness of the limbs and face),
- eosinophilia-myalgia syndrome (eating excessive amounts of the protein L-tryptophan),
- some psychiatric conditions,
- focal nodular myositis,
- hyperparathyroidism (overactive parathyroid glands),
- Bornholm disease (a viral infection that attacks the membrane surrounding the lungs),
- myofascial pain syndrome
- Weil syndrome (complication of an infection by the bacteria Leptospirosis).