A baby's first laugh should bring only joy.
But for a small number of children, some early grins and giggles can signal a rare brain abnormality characterized by seizures and an unprovoked urge to burst out laughing.
It's caused by a congenital brain lesion called a hypothalamic hamartoma. And neurologists writing in Neurology, the journal of the American Academy of Neurology, say there's nothing funny about it.
The urges don't always produce laughter. And ''many patients find the feeling pleasant,'' said Dr. Samuel Berkovic, of the University of Melbourne, Australia. ''However, they are aware that sometimes a seizure may follow, and this is frightening.''
One 25-year-old man described in the report has had giggle fits since infancy. At 14, he began having epileptic seizures, sometimes preceded by the urge to laugh.
Drugs controlled the seizures, but he is still coping with an urge to laugh up to 10 times a day. He learned as a child to bite his lip to suppress it, letting it out only when laughter was appropriate.
Berkovic wrote the article with Dr. Jonathan W. Sturm, also of the University of Melbourne, and Dr. Frederick Andermann, of McGill University in Montreal.
''This is not strictly speaking a tumor,'' Andermann said. A hamartoma is a collection of nerve cells that appears in the wrong place. Hypothalamic hamartomas are attached to the hypothalamus, a structure deep in the brain that regulates thirst, temperature, appetite and emotions.
The hypothalamus also plays a role in natural laughter, joined by other parts of the brain involved with thought, emotions and the muscles of the chest and throat.
The hamartomas don't grow, but they do generate electrical discharges that produce epileptic seizures. And hamartoma cells can eventually provoke epileptic discharges elsewhere in the brain.
Patients with larger hamartomas -- typically the size of a grape -- suffer unmanageable epileptic seizures, deteriorating mental abilities and behavioral problems.
''The only way to deal with these, if they're sufficiently bad, is to try to get them out,'' Andermann said. Surgery, by destroying the lesions with electricity or radiation, can provide ''considerable'' improvement.
But a rare minority of patients have tiny hamartomas, barely two-tenths of an inch in diameter, that produce only a milder form of epilepsy and the urge to laugh.
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