The medical term used for chronic fatigue syndrome is myalgic encephalomyelitis. This term can be divided into “myo” for muscle, “algia” for pain and “encephalomyelitis” meaning inflammation of the brain and spinal cord.

The naming and classification of chronic fatigue syndrome in the medical literature has presented many challenges over the years. This is due to the fact that the exact cause of the disease is not known, the symptoms vary between sufferers and it is difficult to diagnose with confidence. There is a lack of consensus among experts over how the condition should be classified, with some individuals believing the central nervous system is involved and others thinking the underlying pathology is linked to infection, heart disease or an immune system disorder. In some disciplines, the syndrome is also considered to be a psychiatric disorder rather than a physical one.

The name given to chronic fatigue syndrome has varied over the years and by country, with examples including:

• Akureyri disease
• Benign myalgic encephalomyelitis
• Chronic fatigue immune dysfunction syndrome
• Chronic infectious mononucleosis or chronic Epstein Barr virus disease
• Epidemic myalgic encephalomyelitis
• Epidemic neuromyasthenia
• Iceland disease
• Post-viral fatigue syndrome (PVFS)
• Raphe nucleus encephalopathy
• Royal Free disease
• Tapanui flu
• Yuppie flu

Some patients prefer to call the condition myalgic encephalomyelitis or ME rather than chronic fatigue syndrome. However, in 1999, a review from the Royal Colleges of Physicians, Psychiatrists, and General Practitioners suggested that there was not sufficient evidence of a disease involving the muscles and central nervous system to justify the term myalgic encephalomyelitis. This was severely criticised and in 2002, a commentary in The Lancet noted that a report by the CFS/ME Working Group stated that since there was a lack of consensus over which term should be used, both the terms could be used.