Functional weakness: clues to mechanism from the nature of onset.

BACKGROUND Functional weakness describes weakness which is inconsistent and incongruent with disease. It is also referred to as motor conversion disorder (DSM-IV), dissociative motor disorder (ICD-10) and 'psychogenic' paralysis. Studies of aetiology have focused on risk factors such as childhood adversity and life events; information on the nature and circumstance of symptom onset may shed light on the mechanism of symptom formation. AIM To describe the mode of onset, associated symptoms and circumstances at the onset of functional weakness. METHODS Retrospective interviews administered to 107 adults with functional weakness of <2 years' duration. RESULTS The sample was 79% female, mean age 39 years and median duration of weakness 9 months. Three distinct modes of onset were discerned. These were: sudden (n=49, 46%), present on waking (or from general anaesthesia) (n=16, 13%) or gradual (n=42, 39%). In 'sudden onset' cases, panic (n=29, 59%), dissociative symptoms (n=19, 39%) and injury to the relevant limb (n=10, 20%) were commonly associated with onset. Other associated symptoms were non-epileptic attacks, migraine, fatigue and sleep paralysis. In six patients the weakness was noticed first by a health professional. In 16% of all patients, no potentially relevant factors could be discerned. CONCLUSIONS The onset of functional weakness is commonly sudden. Examining symptoms and circumstances associated closely with the onset suggests hypotheses for the mechanism of onset of weakness in vulnerable individuals.