Chronic fatigue syndrome (CFS), also known as ME (myalgic encephalomyelitis) is a complex disorder characterised by profound fatigue, but with an uncertain cause or causes.
The ME Association says the condition affects around 250,000 people in the UK.
University of Bristol research in 2011 estimated the number of 11 to 16 year olds with CFS at one in 100.
CFS is assessed as being mild, moderate, severe or very severe, depending on a person's symptoms.
Chronic fatigue syndrome has generated significant debate and controversy in the medical community. It remains a poorly understood and potentially debilitating illness. Scientists believe chronic fatigue syndrome is not contagious.
CFS is one of the conditions that NHS Blood and Transplant permanently exclude people from becoming blood donors. The NHS says this is solely to protect the health of the donor.
Multiple factors including immunological, genetic and environmental ones may be working in combination to cause the development of chronic fatigue syndrome.
Theories include CFS being triggered by viral infections, such as glandular fever.
Possible CFS causes include:
Immune system problems
Genetic or inherited risk
A 2011 study published in the journal Gut suggested a link between being infected with the stomach bug Giardia and an increased risk of having chronic fatigue years later. However, the study couldn't prove a direct cause and effect.
There are no specific medical tests to confirm a CFS diagnosis. Doctors may make a diagnosis after ruling out other conditions which do have more definite tests.
The National Institute for Health and Care Excellence (NICE) guidance for doctors says a diagnosis should be made after symptoms have persisted for four months in adults or three months in children and young people.
A study published in BMJ Open in 2011 suggested the number of children with CFS is underestimated.
Treatment for CFS
Although a CFS diagnosis can be hard to make, NHS doctors are told not to wait for this confirmation before advising a patient on managing their symptoms to help minimise the impact on daily life. NICE says they should:
Acknowledge the reality and impact of the condition and the symptoms
Provide information about the range of interventions and management strategies, including benefits, risks and likely side effects
Provide information on the possible causes, nature and course of CFS/ME
Provide information on returning to work or school/university
Take account of the person's age, particularly for under 12s, the severity of their condition, preferences and experiences, and the outcome of any previous treatment
Offer information about local and national self-help groups and support groups
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