Disorders of laughter and crying are very rare. Although strong emotional reactions are not uncommon following brain injury, the most common reaction described is fear. Medical research on laughing and crying disorders consists largely of case reports, with few systematic, large-scale investigations. Abnormal laughter, defined as laughter that is involuntary and inappropriate to the situational context, is most often seen in generalized affective or cognitive disturbances such as psychoses (e.g., schizophrenia). Individual cases of hysterical laughter spells, marked by silly, unrestrained, unmotivated, and unprovoked laughter, have been reported, as has epidemic hysterical laughter.
In many cases of abnormal laughter, the motor act of laughing may be dissociated from its emotional aspect. Such a condition has been termed pathological laughter. Kinnier Wilson defined pathological laughter and crying (PLC) as "a sequel to and consequence of a recognizable cerebral lesion or lesions in which attacks of involuntary, irresistible laughing or crying, or both, have come into the foreground of the clinical picture.'' This definition draws attention to the impaired control and the episodic aspect of the abnormal emotional expression.
In 1994, Shaibani, Sabbagh, and Doody proposed four criteria to distinguish pathological laughter and crying from normal laughter and crying. First, PLC is inappropriate to the situation since it occurs spontaneously or in response to nonspecific stimuli or inappropriate, arbitrary stimuli (e.g., one patient with a left cerellar hematoma showed pathological laughter following left hand tremor). Second, PLC is unmoti-vated; that is, there is no relation between the affect and observed expression, nor is there relief or mood change afterwards. Third, PLC is involuntary; that is, it has its own pattern and occurs against the patient's will. Neither the duration nor the content of PLC can be controlled, and patients do not gradually change from smiling to laughing but have a sudden, brief outburst without any warning. Fourth, PLC differs from emotional lability, which refers to an exaggerated emotional response to a normal stimulus. The latter characterizes patients with multiple sclerosis and
Alzheimer's disease. These patients are overcome by uncontrollable laughter and crying that is usually appropriate to the situation and accompanied by mood alteration.
Very few patients actually meet the criteria for PLC. However, researchers do not concur on the ideal classification system and whether PLC must involve a dissociation between the expressed affect and the mood. It is acknowledged that the degree of volitional control of PLC varies from complete absence to some control.
The most common conditions associated with PLC are summarized in the following sections, subdivided into disregulatory and excitatory conditions.
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