History and prevalence of involuntary emotional expression disorder.

The syndrome now known as involuntary emotional expression disorder (IEED) is a condition characterized by uncontrollable episodes of laughing and/or crying. It has been known for more than a century, but confusing and conflicting terminology may have hampered the progress of physicians in recognizing this condition. IEED is associated with various neurological disorders and neurodegenerative diseases, including amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease, Alzheimer's disease and other dementias, and neurological injuries such as stroke and traumatic brain injury. It is hoped that better defined terminology for IEED may help in the future diagnosis of this debilitating condition, the establishment of accurate prevalence rates for IEED in the varying underlying conditions, and also in removing blame and stigma from sufferers by providing reassurance about the nature of their condition.