This page is provided for your information only. A diagnosis of ME/CFS should be determined only by a suitably qualified medical professional, such as a GP or hospital consultant.

Myalgic encephalomyelitis (ME) is characterised by a range of neurological symptoms and signs, muscle pain with intense physical or mental exhaustion, relapses, and specific cognitive disabilities. Early reports dating from 1934 described epidemics of the illness – such as the 1955 outbreak at the Royal Free Hospital in London – but nowadays it is more common for endemic (sporadic) cases to be identified. The World Health Organisation’s International Classification of Disease lists ME as a disorder of the nervous system, under Post-Viral Fatigue Syndrome (Block 93.3, ICD 10th revision, 2007).

The cardinal symptoms of MEas described in some of the early literature, are profound, generalised post-exertional loss of muscle power (fatigability); muscle pain that may include tenderness and swelling; and neurological signs. Patients are also prone to relapses which may take the form of recurrences of the original systemic illness, or fresh episodes of muscle weakness, neurologic changes or well-defined cognitive problems. As with many chronic illnesses, fatigue may be present, but in ME patients the fatigue is reported to be post-exertional, often delayed, and quite unlike the ‘fatigue’ experienced by healthy people.

What is ME/CFS?

During the 1990s, the term Chronic Fatigue Syndrome (CFS) came into use – see chronic fatigue syndrome for a more detailed description, including various diagnostic classifications.

As there was (and presently remains) no specific diagnostic test for myalgic encephalomyelitis (ME), and as post-exercise ‘fatigue’ was one of its prominent symptoms, people with ME began to be diagnosed with CFS. In recent years, however, there has been a growing recognition that the diagnosis CFS – based on a list of vague non-specific symptoms shared with other illnesses – is so broad that it can encompass a range of diverse patient groups. For this reason, there have been a number of attempts to devise a more sensitive way of diagnosing patients, including a Canadian Clinical working definition of ME/CFS (2003), and a NICE guideline definition for use by GPs in the United Kingdom was produced in 2007 (see below). Efforts to remedy the diagnostic confusion continue, however, and meanwhile the term ME/CFS is frequently used.

Of course, none of the issues surrounding the name alter the reality of the illness for thousands of people, and the lay term ME is still used by patients, patient groups and charities (including ME Research UK) to describe the illness affecting people diagnosed with ME/CFS, including that subgroup of CFS patients with an organic biomedical illness, neurological signs and symptoms.

ME/CFS today: the NICE Clinical Guideline

NICE pathway title

Most patients seeing a GP in the UK today are diagnosed using the criteria outlined in the NICE Clinical Guideline of 2007 (read the Guideline and supporting documentation here). This Guideline makes clear that the illness is recognised on clinical grounds alone (i.e. that there is no specific ‘test’ for the illness) and that healthcare professionals should consider the possibility of ME/CFS if a person has:

a) Fatigue with all of the following features:

  • new or had a specific onset (that is, it is not lifelong),
  • persistent and/or recurrent,
  • unexplained by other conditions,
  • has resulted in a substantial reduction in activity level,
  • characterised by post-exertional malaise and/or fatigue (typically delayed, for example by at least 24 hours, with slow recovery over several days).


b) one or more of the following symptoms:

  • difficulty with sleeping, such as insomnia, hypersomnia,
  • unrefreshing sleep, a disturbed sleep–wake cycle,
  • muscle and/or joint pain that is multi-site and without evidence of inflammation,
  • headaches,
  • painful lymph nodes without pathological enlargement sore throat,
  • cognitive dysfunction, such as difficulty thinking, inability to concentrate, impairment of short-term memory, and difficulties with word-finding, planning/organising thoughts and information processing,
  • physical or mental exertion makes symptoms worse,
  • general malaise or ‘flu-like’ symptoms,
  • dizziness and/or nausea,
  • palpitations in the absence of identified cardiac pathology.

It is important to stress, however, that ME/CFS, whether defined by NICE or using another definition, remains a “diagnosis of exclusion”, which means that other possible causes of a patient’s symptoms should be excluded, often by testing, before this diagnosis is given. As the NICE Guideline says, “A diagnosis should be made after other possible diagnoses have been excluded and the symptoms have persisted for 4 months in an adult [and] 3 months in a child.

PLEASE NOTE – A positive diagnosis of ME/CFS can only be determined by a suitably qualified medical practitioner, such as your own GP or a hospital consultant.

What is the cause of ME/CFS?

virus 1 copy

The cause of ME/CFS is still unknown, but there is unlikely to be a single causative agent. Several past epidemics of ME appear to have been triggered by an outbreak of an infection with enteric organisms or poliovirus which then subsided, and still today many patients report an infectious onset to their illness. However, some people report a variety of contributing factors (including infectious, traumatic, environmental, etc.), all of which can lead on to a condition which shares a set of common symptoms.

Living with ME/CFS

The evidence across many studies is that between 120,000 and 240,000 people in the UK have ME/CFS, a prevalence higher than HIV infection or multiple sclerosis. It is twice as commonly reported in women as men. Studies indicate that most people with ME/CFS are unable to work to full capacity, and that a significant number (from 10 to 25%) are effectively house or bed-bound. Despite early reports about “Yuppie Flu”, it is now known that ME/CFS affects all social groups and all ages, including children. The course of the illness can be extremely variable; some people improve quite quickly, while many others develop stable chronic illness, and a few experience a severe and debilitating downward course over many years. A report to the Chief Medical Officer of England in 2002 stated that ME/CFS “is a genuine illness and imposes a substantial burden on the health of the UK population. Improvement of health and social care for people affected by the condition is an urgent challenge”, and this remains true today.

The role of ME Research UK

ME Research UK exists to fund biomedical research into ME/CFS, to find its cause, to develop effective treatments and ultimately to discover a cure. In 15 years, it has funded 38 specific research projects in the UK and overseas, more than any other single organisation in the world outside the American continent. Read our ‘£1 million of Biomedical Research‘ for an overview of the research we have funded.

Reports and guidelines