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.