Updated:    Tuesday, October 5, 1999


Recent discussions at the Brussels conference and elsewhere have helped to clarify that myalgic encephalomyelitis (M.E.) and CFS are not the same entity. There has been confusion about this, even in Britain where M.E. was first defined and studied under that name.

What exactly is M.E.? According to M.E. experts the key difference between M.E. and chronic fatigue syndrome is that M.E. requires the criterion of easy fatiguability following minimal exertion and a delay in recovery of muscle strength.


M.E. has been described several times in medical literature. It was first defined in an editorial published in the Lancet in 1956 which discussed several epidemic outbreaks of prior years. This first description was rather loose and was not very specific. In later years Ramsay, EG Dowsett and others refined the definition of M.E. in various published papers.

After Ramsay died in 1990, Dowsett et al. wrote the latest version of the M.E. definition now known as the "London criteria". These have been used in recent papers by Costa (Brainstem perfusion is impaired in patients with CFS, QJM 1995; 88:767-773) and Scholey (A comparison of the cognitive deficits seen in M.E. to Alzheimer's Disease, Proceedings of the British Psychological Society, 1999, January, 12).

Below are the text of definitions and descriptions of M.E. as published in six papers since the name was coined in 1956.

Leading article. A New Clinical Entity? Lancet May 26, 1956, pp. 789-90.

Note: this description is taken directly from the article cited above.

* symptoms and signs of damage to the brain and spinal chord, in a greater or lesser degree

* protracted muscle pain with paresis and cramp

* emotional disturbances in convalescence

* normal cerebro-spinal fluid

* involvement, in some variants, of the reticuloendothelial system

* a protracted course with relapses in sever cases

* a relatively benign outcome

Acheson ED. The clinical syndrome variously called benign myalgic encephalomyelitis, Iceland disease, and epidemic neuromyasthenia. Am J Med 1959;26:569-95.

Note: this fair summary of Acheson's description is taken from the National Task Force Report, 1994. The full text of the original article is included in Hyde's textbook.

This is a paralytic illness of world-wide distribution.

The epidemiological features of 12 outbreaks, including that at the Royal Free Hospital:

* A high attack rate

* A predilection for residential communities

* A higher attack rate in women than in men

* A tendency to occur more commonly in young adults

* Commencement of most outbreaks in the summer months

Severely Affected Patents

Severely affected patients show a characteristic clinical picture.

After an acute or subacute onset with headache, symptoms of gastro-intestinal or upper respiratory upset, muscular pains and low or absent fever, an unusual type of paresis develops which is rarely associated with the classic signs of lower motor neurone or pyramidal tract involvement. This is often accompanied by paraesthesiae, sometimes by sensory loss, and occasionally by painful muscular spasms. myoclonus or other types of involuntary movement. As the paresis recovers a curious jerky muscular contraction on volition has been noted in some instances. Involvement of the cranial nerves and the bladder may occur.

Recovery has usually been completed within three months. In a proportion of cases, a well defined state of chronic ill health has developed, characterised by fluctuating myalgia and paresis, partial remission and exacerbation, and depression with emotional lability and lack of concentration.

Mild Cases

In mild cases, when paresis and other signs of neurological involvement are absent, the illness has few differences in its early stages from the many short-lived infections which are characterized by headache and generalised aches. However, the degree of malaise and the severity of the pains are disproportionate to the disturbance of temperature and pulse.

In the convalescent stage in such cases the easy fatiguability, the aches and pains, and the emotional disturbances without definite physical signs lead to difficulty in differentiation from psychoneurosis. (Indeed, in the absence of an epidemic such a differentiation may be impossible.)

Many authors stress the importance of rest. This should be advised as early in the disease as possible, and be as absolute as practical. The association of premature rehabilitation with relapse is well described, therefore bedrest should probably be maintained for some time after the disappearance of symptoms.

The disease is probably due to infection by an unknown agent, or group of related agents. it is not a manifestation of mass hysteria.

A. Melvin Ramsay. Myalgic Encephalomyelitis and Postviral Fatigue States: The Sage of Royal Free disease. 2nd edition. Gower Medical Publishing, London 1988.

Note: this fair summary of Ramsay's 1988 description is taken from the National Task Force Report, 1994.

ME is an endemic disease which is subject to periodic epidemics.


Onset may be sudden and without apparent cause, for example a sudden attack of acute vertigo. There is usually a history of infection of the upper respiratory tract or, occasionally, the gastrointestinal tract. All cases have low grade pyrexia (up to 38 deg. C) usually subsiding within a week.

Subsequently, there is a persistent and profound fatigue, accompanied by a medley of symptoms such as headache, giddiness and a number of muscle symptoms such as pain, cramp, twitching, tenderness and weakness (especially after exercise). Other symptoms include paraesthesia, frequency of micturition, blurred vision and/or diplopia, hyperacusis (sometimes alternating with deafness or normal hearing) tinnitus, fainting attacks which may be the result of hypoglycaemia and a general sense of "feeling awful".

Established Syndrome

Once the syndrome is fully established there are three groups of symptoms: --

Muscle Phenomena

* Muscle fatiguability. Even after a minor degree of physical exercise, 3 or more days elapse before full muscle power is restored. This feature is unique and is the "sheet anchor" of diagnosis. In moderate cases there may be normal muscle power in remission.

* Muscle spasms and twitching. In sever cases there may be swollen and very tender bands of muscle including minute foci of exquisite tenderness in trapezii and gastrocnemii (the muscle groups most commonly involved).

Circulatory impairment

* Cold extremities

* Hypersensitivity to climatic change

* Ashen grey facial pallor, 20 to 30 minutes before patient complains of being ill

Cerebral dysfunction

    Cardinal features
* Impairment of memory
* Impairment of power of concentration
* Emotional lability

    Other Common features
* Using wrong words
* Frequency of micturition
* Hyperacusis
* Episodic sweating
* Orthostatic tachycardia

Other features

* Variability of both symptoms and clinical findings during the day

* Tendency to become chronic. Estimate of at least 25%

This summary was taken from:

Report from The National Task Force on Chronic Fatigue Syndrome (CFS), Post Viral Fatigue Syndrome (PVFS), Myalgic Encephalomyelitis (ME). 1994. Copies of the full report are available from Westcare, 155 Whiteladies Road, Clifton, Bristol BS8 2RF. Tel. 0117-923-9341. Price (U.K.) 9.95 pounds including p&p.

Dowsett EG, Ramsay AM, McCartney RA, Bell EJ. Myalgic Encephalomyelitis (M.E.) -- A Persistent Enteroviral Infection? Postgraduate Medical Journal, 1990;66:526-530.

Note: this description is taken directly from the article cited above. The full text of the original article is included in Hyde's textbook.

We adopted the following clinical criteria for investigation of M.E.: a syndrome commonly initiated by respiratory and/or gastro-intestinal infection but an insidious or more dramatic onset following neurological, cardiac or endocrine disability occurs.

The pathognomonic features are:

- a complaint of general or local muscular fatigue following minimal exertion with a prolonged recovery time;

- neurological disturbance, especially of cognitive, autonomic and sensory functions;

- variable involvement of cardiac and other systems;

- a prolonged relapsing course.

Ramsay AM, Dowsett EG. Myalgic Encephalomyelitis -- Then and Now: An Epidemiological Introduction. pp. 81-84 in Hyde's textbook, 1992.

Note: this description is taken directly from the article cited above. The full text of the original article appeared in Hyde's textbook.

1. Generalised or localised muscle fatigue after minimal with prolonged recovery time.

2. Neurological disturbance, especially of cognitive, autonomic and sensory functions, often accompanied by marked emotional lability and sleep reversal.

3. Variable involvement of cardiac and other bodily systems.

4. An extended relapsing course with a tendency to chronicity.

5. Marked variability of symptoms both within and between episodes.

Dowsett EG, et al. London criteria for M.E. In: Report from The National Task Force on Chronic Fatigue Syndrome (CFS), Post Viral Fatigue Syndrome (PVFS), Myalgic Encephalomyelitis (ME). Westcare, 1994. pp. 96-98.

Notes: this description appeared in the National Task Force Report, 1994. Also, in recent years PVFS (postviral fatigue syndrome) has become synonymous with M.E.

All three criteria must be present for a diagnosis of ME/PVFS to be made:

1. Exercise-induced fatigue precipitated by trivially small exertion (physical or mental) relative to the patient's previous exercise tolerance.

2. Impairment of short-term memory and loss of powers of concentration, usually coupled with other neurological and psychological disturbances such as emotional lability, nominal dysphasia, disturbed sleep patterns, disequilibrium or tinnitus.

3. Fluctuation of symptoms, usually precipitated by either physical or mental exercise.

These symptoms should have been present for at least 6 months and should be ongoing.

Although ME/PVFS typically follows an infection, usually a virus illness (which may be subclinical) in a previously fit and active person, it has also been observed to be triggered by other factors such as immunisations, life traumas and exposure to chemicals. Furthermore, in a minority of patients, ME/PVFS has a gradual onset with no apparent triggering factor. For these reasons proof of a preceding viral illness is not a prerequisite for diagnosis.

Many symptoms are experienced by people suffering from ME/PVFS and in the right symptomatic context they contribute to the validity of the diagnosis. Nevertheless, not all people suffering from ME/PVFS experience all these symptoms and their absence does not exclude the condition.

These can be subdivided into the following two categories:


* bouts of inappropriate night or day-time sweating;

* Raynaud's phenomenon; postural hypotension;

* disturbance of bowel motility manifesting as recurrent diarrhoea or occasionally constipation (these symptoms are frequently indistinguishable from those of irritable bowel syndrome);

* photophobia; blurred vision due to disturbed accommodation;

* hyperacusis;

* frequency of micturition; nocturia.

Immunological (Symptoms suggesting persistent viral infection):

* episodes of low-grade fever (not exceeding an oral temperature of 38.6C) combined with feeling feverish, (i.e. a down-regulated 'thermostat');

* sore throat which may be persistent or recurrent (i.e. present for at least one week per month);

* arthralgia (fixed or migratory)

This list is by no means exhaustive. Headaches, nausea and bloating, for instance are common symptoms in many patients but are not sufficiently discriminative because of their widespread occurrence in many other disorders. The curious intolerance to alcohol and hypersensitivity to drugs are highly specific in this context. It should also be emphasised that the symptoms of ME tend to vary capriciously from hour to hour and day to day. Nevertheless it is absolutely characteristic that they tend to be exacerbated by physical or mental exertion and the association should always be sought whilst taking the history.


Hyde's textbook

A nearly indispensable resource for studying M.E. is Byron Hyde's anthology:
Hyde BM, Goldstein JA, Levine P, eds. The Clinical and Scientific Basis of Myalgic Encephalomyelitis / Chronic Fatigue Syndrome. 750 pp. Nightingale Research Foundation, 1992.
Many of the papers mentioned above are included in this textbook. It can be purchased by sending $39 to The Nightingale Research Foundation, 121 Iona St., Ottawa, Ontario, Canada, K1Y 3M1 or use the online order form at

Another useful reference is:

National Task Force Report, 1994

Report from The National Task Force on Chronic Fatigue Syndrome (CFS), Post Viral Fatigue Syndrome (PVFS), Myalgic Encephalomyelitis (ME). Westcare, 1994.
Its Appendix B contains the various published definitions for M.E., CFS, PVFS and epidemic neuromyasthenia. Copies of the full report are available from Westcare, 155 Whiteladies Road, Clifton, Bristol BS8 2RF. Tel. 0117-923-9341. Price (U.K.) 9.95 pounds which included postage and handling.

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