Viral infection is a significant risk factor for CFS, with one study finding 22% of people with EBV experience fatigue six months later, and 9% having strictly defined CFS. A systematic review found that fatigue severity was the main predictor of prognosis in CFS, and did not identify psychological factors linked to prognosis.Another review found that risk factors for developing CFS after mononucleosis, dengue fever, or Q-fever included longer bed-rest during the illness, poorer pre-illness physical fitness, attributing symptoms to physical illness, belief that a long recovery time is needed, as well as pre-infection distress and fatigue. The same review found biological factors such as CD4 and CD8 activation and liver inflammation are predictors of sub-acute fatigue but not CFS. However, these findings are not generally accepted due to the use of the Oxford criteria in selecting patients. The CDC does not recognize attribution of symptoms as a risk factor.A study comparing diagnostic labels found that people labelled with ME had the worst prognosis, while those with PVFS had the best. Whether this is due to those with more severe or longer-lasting symptoms results in a label with the description of ME, or if being labelled with ME adversely causes a more severe or prolonged illness is unclear. Pathophysiology
Neurological
A range of neurological structural and functional abnormalities is found in people with CFS, including lowered metabolism at the brain stem and reduced blood flow to areas of the brain; these differences are consistent with neurological illness, but not depression or psychological illness.
Use of a heart-rate monitor with pacing to monitor and manage activity levels is recommended by a number of patient groups, and the CDC considers it useful for some individuals to help avoid post-exertional malaise. Energy envelope theory
Energy envelope theory, considered to be consistent with pacing, is a management strategy suggested in the 2011 international consensus criteria for ME, which refers to using an "energy bank budget". Energy envelope theory was devised by psychologist Leonard Jason, who previously had CFS. Energy envelope theory states that patients should stay within, and avoid pushing through, the envelope of energy available to them, so as to reduce the post-exertional malaise "payback" caused by overexertion. This may help them make "modest gains" in physical functioning. Several studies have found energy envelope theory to be a helpful management strategy, noting that it reduces symptoms and may increase the level of functioning in CFS. Energy envelope theory does not recommend unilaterally increasing or decreasing activity and is not intended as a therapy or cure for CFS. It has been promoted by various patient groups. Some patient groups recommend using a heart rate monitor to increase awareness of exertion and enable patients to stay within their aerobic threshold envelope. Despite a number of studies showing positive results for energy envelope theory, randomized controlled trials are lacking. Exercise
Stretching, movement therapies, and toning exercises are recommended for pain in patients with CFS, and pain medication is also suggested.