Chronic fatigue syndrome is an illness characterised by extreme fatigue of uncertain origin which has been present for at least six months. The fatigue should have a sudden onset and be severe enough to substantially reduce the patient’s day to day activities. A number of other symptoms are associated with chronic fatigue syndrome including complaints of impaired memory, difficulty making decisions, poor attention, and reduced concentration. Many patients complain that it is these cognitive symptoms which cause them the greatest frustration and disability.
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Source: Moss-Morris R, Petrie KJ, Large RG, Kydd RR. Neuropsychological deficits in chronic fatigue syndrome: artifact or reality? J Neurol Neurosurg Psychiatry. 1996 May;60(5):474-7. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC486357/