Have you felt disabled by exhaustion for at least six months with no obvious medical reason? Have you also experienced bouts of weakness, pain, impaired memory or problems sleeping?
Some doctors may diagnose you as having chronic fatigue syndrome (CFS), but others are sceptical that such a condition even exists.
Despite being recognised as a major public health problem since the late 1980s, the lack of uniformity of symptoms and causes among patients has complicated the diagnostic process and development of treatments.
"As family physicians in Hong Kong, we almost never diagnose CFS; it's more a label, a functional diagnosis, a description," says Dr Billy Chiu Chi-fai, a specialist in family medicine at Hong Kong Sanatorium and Hospital.
Instead, when a patient presents with the symptoms of CFS, he says, doctors will test for other conditions before, in most cases, diagnosing the symptoms as evidence of a psychosomatic disorder, anxiety, depression, a mood disorder, neuritis (inflammation of the nerves), insomnia or one of a number of other similar conditions.
In Hong Kong, it's estimated that 10.7 per cent of adults experience chronic fatigue syndrome. This was based on a study published in 2010 of 5,000 Chinese people aged 18 years or older, led by University of Hong Kong clinical psychologist Richard Fielding.
The reported rates of CFS have ranged from 0.037 to 18.3 per cent in the general adult population, according to another HKU study, on the effects of qigong on the symptoms published in the Annals of Behavioral Medicine last year.
Researchers worldwide have paid increasing attention to CFS - known as myalgic encephalomyelitis - in recent years as the search for a definitive cause and cure continues.
In November, Ohio State University researchers reported the discovery of a potential diagnostic method to identify at least a subset of patients with CFS. In a pilot study of six patients, scientists detected specific antibodies linked to the latent Epstein-Barr virus reactivation in blood samples from those who had experienced some key CFS symptoms and had responded to antiviral treatment.
"Part of the problem in trying to identify an agent or biomarkers for chronic fatigue syndrome is the extreme variability among people who say they have CFS," says virologist Ron Glaser, co-author of the Ohio State study. "How to sort that out has held the field back a lot of years."
CFS seems to affect women more than men - Fielding's study found that 13.1 per cent of women and 8.1 per cent of men experienced the disorder. Factors linked with increased odds of CFS included age (being younger increases the odds), being a retiree, having had chronic health problems, poor mental health and a lower quality of life score. Those who exercised three to five times a week were less likely to report chronic fatigue compared to the sedentary.
Other studies show CFS varies in severity: some sufferers have a fairly normal life, while others are reliant on other people for the most basic tasks. Only a fifth of sufferers can work full time, while half can't work at all. For many, it's a life sentence: less than 10 per cent of adult sufferers recover fully, although more than half of children do.
Leading CFS researcher Judy Mikovits says many subclinical CFS cases exist in the population but are not picked up in epidemiological studies. "The disorder of CFS will be harder to separate out in developing countries as it merges with many common and often chronic infections and requires expertise not likely available in such countries."
CFS mimics many other disorders, making diagnosis difficult. Among the conditions and diseases mimicked are: cancer, hypothyroidism, sleep apnea, narcolepsy, hepatitis B and C, substance abuse, mental disorders (major depressive disorder, bipolar affective disorders, schizophrenia) and severe obesity.
But an international study of more than 37,700 people published in 2009 in the Australian and New Zealand Journal of Psychiatry found five domains of chronic fatigue that were robust across cultures and health care settings, and consistent with the 1994 international consensus criteria.
The five are: prolonged fatigue and musculoskeletal pain, impaired neurocognitive function, sleep disturbance and symptoms suggestive of inflammation.
The study authors concluded that chronic fatigue states can be readily identified in the community and at all levels of health care. They add: "Consequently, it is likely that they share common risk factors, are underpinned by a common pathophysiology and may respond to common treatment strategies."
The US Centers for Disease Control and Prevention (CDC) inclusion criteria for CFS remains the most widely used in the field. Apart from fatigue that has persisted or relapsed for six or more months, CFS patients must also have four or more of the following eight distinctive symptoms: impaired memory or concentration capacity, post-exertional malaise, sleep problems, muscle pain, arthralgia (joint pain), headaches, recurrent sore throat and tender cervical or axillary lymph nodes.
Some physicians, however, are not confident enough to make a diagnosis, says Chiu. "Some doctors are not trained to deal with such conditions and feel overwhelmed by the repeated attendance of patients with unspecified symptoms." Those with training in holistic care or a background in psychology or counselling are often better able to manage patients, he adds.
There are other controversial aspects to CFS - its causes. It could be biological, viral, environmental, genetic, psychological, some combination of those or something else entirely. The lack of readily identifiable causes has led many sufferers to feel stigmatised.
"Many patients with chronic fatigue syndrome encounter a lot of scepticism about their illness," says Dr Elizabeth Unger, CFS research chief of the CDC. "They have difficulties getting their friends, colleagues, co-workers and even some physicians to understand their illness."
Attempts to identify a physical cause, however, have so far foundered. Viral suspects to have been suggested include the Epstein-Barr virus, the herpes virus and mouse leukaemia viruses. A 2009 study in the journal Science, of which Mikovits was an author, linked it with a virus called XMRV, but that was retracted when the results turned out to be caused by a lab contaminant.
There are measurable physical changes associated with CFS, but scientists are unsure if they're the cause or an effect. Sufferers often have weakened immune systems or abnormal blood levels of hormones produced in the hypothalamus, pituitary gland or adrenal glands.
Unger led a study last year that showed a decreased response among sufferers in an area of the brain known as the basal ganglia, which is responsible for functions such as motor activity and motivation in response to reward.
"The findings indicate that a biological process involving neurocircuits in the brain that regulate motor activity and motivation as well as fatigue are altered in people with CFS," says Unger. "CFS is not treated as a psychological or psychosocial condition but, similar to other chronic illnesses such as cancer or heart disease, body and mind interactions play an important role in improving functioning and life satisfaction.
"It is most likely that a variety of factors may play a role in CFS and each patient's history of illness differs. Infections, trauma, stress, surgery, hormones and genetics may all interact in ways we don't yet fully understand."
Existing treatments for CFS are limited in their effectiveness. Because Western treatments and medications are often associated with limited clinical benefits and possible undesirable side effects, complementary and alternative therapies are often used by individuals with chronic fatigue to manage their symptoms.
The most common are cognitive behavioural therapy, dealing with the patient's perception of the condition and its symptoms; and graded exercise therapy, a form of physiotherapy involving gradual increases in activity. But evidence for the effectiveness of both is mixed.
From the perspective of traditional Chinese medicine, chronic fatigue and CFS are caused by blood stasis due to qi (vital energy) deficiency and/or emotional constrain; therefore, stimulation of the blood and qi circulation is the core treatment strategy.
The HKU qigong study randomly assigned 64 participants to either an intervention group that did a four-month qigong programme or a control group. Fatigue symptoms and mental functioning were significantly improved in the qigong group compared to the control.
Speaking with a counsellor to work around the limitations of CFS on your life can also help, says Chiu. "Feeling more in control of your life can improve your outlook dramatically."
Mikovits adds: "Controlling stress; boundary setting of physical, emotional and cognitive activities; and attention to proper sleep and diet modification are the first and most important clinical steps to take. A search for the key or keys to individual cases of CFS can turn up … clinical conditions unique to that person.
"Our modern lifestyle involves far more stress than previous generations: oxidative stress in general; increased travel and exposure to different environmental toxins and pathogens from other countries; poorer nutritional value of foods; polluting elements in our environment that used to not be there."