Chronic Fatique Syndrome was historically used as catch all diagnose when physicians were unable to determine what the problem was. However, since that time there have been many causes hypothesized to account for the pattern of symptoms.

The Definition

Chronic Fatigue Syndrome (CFS) was originally defined in 1988 when the Center for Disease Control (CDC) in the US brought together a number of researchers who had been investigating a strange syndrome characterized by overwhelming fatigue. This definition however was reviewed by a panel of international experts in 1994 and subsequently revised.

 CFS is very difficult to diagnose because the main symptom of fatigue is present in so many other illnesses. However, once other illnesses have been ruled out through laboratory tests and physical examination, a diagnosis of CFS may be given if the following criteria are met:

1. Clinically evaluated, unexplained persistent or relapsing chronic fatigue that is of new or definite onset (i.e., not lifelong), is not the result of ongoing exertion, is not substantially alleviated by rest, and results in substantial reduction in previous levels of occupational, educational, social, or personal activities.
2. The concurrent occurrence of four or more of the following symptoms: substantial impairment in short-term memory or concentration; sore throat; tender lymph nodes; muscle pain; multi-joint pain without swelling or redness; headaches of a new type, pattern, or severity; unrefreshing sleep; and post-exertional malaise lasting more than 24 hours. These symptoms must have persisted or recurred during 6 or more consecutive months of illness and must not have predated the fatigue.

The full text of the revised definition can be found at the CDC website:
http://www.cdc.gov/ncidod/diseases/cfs/about/definition/index.htm

Chronic Fatigue Syndrome is also known by various other names, most commonly:

Myalgic Encephalomyelitis/Encephalopathy (M.E.)

• Chronic Fatigue Immune Dysfunction Syndrome (CFIDS)
• Post Viral Fatigue Syndrome (PVFS)

These names were given as they describe certain causes and mechanisms proposed to be involved in the illness.

CFS and ME are by far the most widely used names. Using CFS and ME as synonyms to refer to the same disease is controversial in some quarters however, with ME seen by some as a more serious condition with an identifiable trigger (e.g. viral illness) whereas CFS is diagnosed in the absence of such a trigger based on symptoms alone. Indeed ME has been listed as a physical neurological disease by the World Health Organization (ICD G93.3) (34) since 1969 whilst CFS has received only a brief mention in the most recent versions. However, there is seen to be significant overlap in both symptoms and approaches to treatment so most people, both doctors and patients, tend to use CFS and ME to mean the same thing. Recent efforts, primarily in the US have sought to settle the matter by introducing the term 'ME/CFS' and use of this combined name has gained significant popularity in a short period of time and thus will be used for the remainder of this overview.

Symptoms and General Information

Obviously, as is implied by the various names, fatigue is the major symptom in ME/CFS. People often have the misconception that this is the only symptom and hence they assume that sufferers simply like to complain about the normal tiredness that everyone experiences after a day at work etc. ME/CFS is actually much more than fatigue, and the fatigue experienced is a lot more severe than simple tiredness. The following is a list of the major symptoms of ME/CFS.

Fatigue – This can be both physical and mental fatigue. It is a pronounced fatigue that leads to a significant reduction in ability to carry out normal tasks and live your usual lifestyle. The fatigue is usually ever present to some degree and is not relieved by sleep; in fact it may be a lot worse upon waking, especially after a lot of activity the previous day.

• Exercise Intolerance – A major feature shared by ME/CFS sufferers is the lack of exercise tolerance. Although they may feel reasonably good before exercise, shortly after commencing they feel absolutely exhausted and cannot continue. It is the same thing a marathon runner experiences and is commonly known as “hitting the wall”. This phenomenon occurs because the body’s cells requirement for oxygen exceeds the supply so they switch to anaerobic (without oxygen) respiration and lactic acid builds up in the muscles.
• Severe Malaise – A general feeling of being ill. It can be described as a “flu-like” or “hangover” feeling.
• Muscle and Joint Aches – These can occur anywhere in the body but the most common sites are the lower back and legs. The aching can be severe and is aggravated substantially by any exertion, physical or mental. There is significant overlap here with Fibromyalgia and many patients are diagnosed with both ME/CFS and Fibromyalgia.
• Cognitive Dysfunction – Symptoms of cognitive dysfunction are a prominent problem in ME/CFS. They include poor concentration, memory loss (constantly forgetting simple information like names and numbers), inability to take in information (having to read the same thing over and over) and a general reduction in cognitive ability and IQ.
• Chronic Headache – As would be expected in someone who feels “ill all over” and achy in general, headaches are a common complaint in ME/CFS. They are different to headaches experienced before the onset of ME/CFS and their severity usually varies inline with other aches and symptoms.
• Balance Disturbance – An unusual but common symptom is a loss of balance or sensation of dizziness. This most often occurs upon standing up and the sufferer may feel faint and even black out. This is often referred to as 'Neurally Mediated Hypotension (NMH)' or ‘Orthostatic Intolerance’ and is most likely due to low blood pressure or abnormal blood flow to the brain.
• Recurrent Sore Throat – A persistent sore throat and/or swollen glands commonly precedes the onset of ME/CFS and may continue as the illness progresses. This is felt to be an indicator that a viral infection plays a major role in cause of the illness.
• Mood and Sleep Disturbances – Depression, anxiety and irritability are often present which frequently leads to misdiagnosis by doctors. Sleep disturbances are common and may present as hypersomnolence (sleeping more than normal), sleep reversal i.e. sleeping all day and awake at night, or insomnia (inability to sleep). Sleep apnea and Restless Leg Syndrome (RLS) are also common..
• Abdominal Pain/Digestive Disturbances – These are symptoms similar to Irritable Bowel Syndrome and, as with Fibromyalgia, ME/CFS sufferers are often diagnosed with IBS.
• Nausea – Particularly apparent when other symptoms feeling particularly ill in general.
• Heightened Sensitivity to Light and/or Sound – Normal everyday levels of sound and light can seem overwhelming to a ME/CFS sufferer. A busy place like a large shop will leave a sufferer totally exhausted very rapidly.
• Visual Disturbances – These may include a blurring of vision, especially when reading, and eye pain.
• Skin Sensitivity – This is another unusual symptom. Some ME/CFS sufferers experience a sensation where patches of skin become very sensitive to touch and may feel like they are burning. Some people describe this as a “crawling sensation”.

The number and severity of symptoms varies drastically between individuals but the symptoms of fatigue, malaise, cognitive dysfunction and possibly muscle aches, are the core symptoms of the illness.

An important thing to note is that all symptoms are generally made a lot worse by physical or mental exertion of any kind. It is common for the severity of the illness to wax and wane from day to day as well as over longer periods of weeks and months. Patients frequently experience periods of remission and relapse.

http://www.ei-resource.org/illness-information/environmental-illnesses/c...

And the possible causes include:

Type 1 - high levels of depression and anxiety as well as poor sleep and high degrees of pain.

Type 2 - severe post-exertional fatigue, joint and muscle pains.

Type 3 - mildest form of the disease.

Type 4 - moderate levels of body pain and sleep problems

Type 5 - most severe muscle weakness and predominance of gut problems

Type 6 - associated with significant fatigue

Type 7 - most severe form with high levels of pain, swollen glands and headaches.