[2] Functional symptoms are increasingly viewed within a framework in which 'biological, psychological, interpersonal and healthcare factors' should all be considered to be relevant for determining the aetiology and treatment plans.
[3] Historically, there has often been fierce debate about whether certain problems are predominantly related to an abnormality of structure (disease) or are psychosomatic in nature (secondary gain), and what are at one stage posited to be functional symptoms are sometimes later reclassified as organic, as investigative techniques improve.
For example, symptoms associated with migraine, epilepsy, schizophrenia, multiple sclerosis, stomach ulcers, myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), Lyme disease and many other conditions have all tended historically at first to be explained largely as physical manifestations of the patient's psychological state of mind; until such time as new physiological knowledge is eventually gained.
Studies have shown that unsettled claims affect level of complaints and many medical studies do not include data from cases where outcomes may have been tainted by inclusion of patients involved in worker's compensation cases.
However, unlike these conditions, with functional weakness there is no permanent damage to the nervous system which means that it can get better or even go away completely.
The diagnosis should be made on the basis of positive features in the history and the examination (such as Hoover's sign).