Idiopathic chronic fatigue

Differences from chronic fatigue

ICF differs from other forms of chronic fatigue since it is unexplained rather than linked to a medical or psychological illness (for example, diabetes or depression).^[6] This means that ICF patients have reduced treatment options: there is no underlying disease or known cause that could be treated in order to reduce the degree of fatigue, which results in a poorer prognosis for ICF.^[13]

In ICF, the fatigue lasts for a minimum of six months, but chronic fatigue is usually (but not always) considered to last for a minimum of six months to be considered chronic, and if lasting between one and six months it is considered prolonged fatigue.

Chronic fatigue is the term used when medical tests and a mental and physical assessment has not yet been carried out. ICF can only be diagnosed after these are done and the results show no underlying untreated cause.^[6]

Differential diagnosis from chronic fatigue syndrome

Chronic fatigue syndrome (CFS) requires the additional symptoms of:

1. post-exertional malaise (significantly worsening symptoms with activity which results in a significant reduction in daily activities, which may be delayed by up to 3 days)
2. sleep dysfunction
3. either:
   • cognitive problems, or
   • orthostatic intolerance.

A range of other symptoms commonly result from CFS including headaches, muscle and joint pain and low-grade fever.^[14]

ICF requires:

1. only one symptom: chronic fatigue
2. does not need a significant reduction in activities: some people are able to push through the fatigue to continue activities
3. is only diagnosed if CFS symptoms are not met.^[6]

Prevalence of ICF is between 3–15%, which is two to ten times higher than CFS.^[13] Older age at onset is more common in ICF, particularly from age 50,^[13] while in CFS age at onset is typically 16–35 years old.^[14]

The recovery rate within a year is significantly higher for ICF patients at 30–50% compared to under 10% in CFS.^[13][14]

ICF is categorized within general signs and symptoms by the World Health Organization,^[7] while CFS is categorized as a neurological disease.^[15]

Ability to tolerate exertion including exercise has been shown to be greater in ICF patients compared to CFS patients, particularly on consecutive days, and this applies to both men and women.^[16][17][18]

Severity of illness in ICF is typically less than in CFS, with some relatively small studies finding no severe ICF patients, the same studies found fibromyalgia was significantly less common in ICF.^[17][18]

Differences from neurasthenia

Neurasthenia was a diagnosis in the World Health Organization's ICD-10, but deprecated, and thus no more diagnosable, in ICD-11.^[19][20] It also is no longer included as a diagnosis in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders.^[21]

• Neurasthenia consisted of a large number of symptoms, typically patients had a mix of physical and psychological complaints for example anxiety, stress-related headaches, heart palpitations, depressed mood, fatigue, lethargy, insomnia, restlessness and weariness. Fatigue was common but not essential.^[22][23][19] ICF consists of the single symptom of fatigue, which may be either mental or physical or both, and may be described in many different ways including as "exhaustion".^[6]

• ICF has no known cause, and psychological factors such as stress have been ruled out, but neurasthenia was believed to be caused by the stresses of the modern age and psychological or psychosocial factors were seen as important.^[23][19]
• Neurasthenia has been very rarely reported since the 21st century, and was deprecated in the World Health Organization's most recent International Classification of Diseases, known as the ICD-11.^[7] Neurasthenia had previously been categorized as a psychological illness,^[23] and originally as neurological. ICF is not psychological- the WHO does not have a classification for any fatigue-only psychiatric illness.^[9][7]

Antidepressants

Antidepressants drugs such as tricyclic antidepressants (TCAs) or selective serotonin reuptake inhibitors (SSRIs) may be appropriate if symptoms are exacerbated by suspected or diagnosed serotonin related health issues, such as depression.^[1]

Alternative and complementary treatments

Only limited trials had been conducted for alternative and complementary treatments; there is no clear evidence of these treatments being effective for ICF due to a lack of randomized controlled trials.^[24]