A group of neurologists from Leiden, writing in the Lancet for September 4, have provided some fascinating information regarding the effects of laughter and its possible hazards. They report that up to 15.5 per cent of individuals experience weakness of skeletal muscles during crises of strong emotional content, and they point out that in several languages the expression "weak with laughter" is current.
The investigators measured H-reflex amplitude in four healthy subjects during the viewing of slides with various emotional content, including humour. This reflex is defined as a monosynaptic reflex in which electrical stimulation of the tibial nerve activates the soleus muscle in the calf of the leg. In all four subjects, the H-reflex was virtually abolished during loud laughter, but decreased far less when the stimulus failed to provoke laughter. A repeated experiment in three of the subjects who were told jokes to make them laugh showed that the amplitude of the H-reflex fell by 88.9 per cent in those who responded with laughter, and by 59.4 per cent in those who did not.
Laughter is generally considered to decrease muscle tone, though this effect is far from clear in its mechanism. The decrease in H-reflex amplitude may be considered to be the neurophysiological equivalent of the condition described as “weak with laughter”. The phenomenon may throw light upon the pathophysiology of cataplexy, a condition in which momentary paralysis is associated with sudden and unexpected emotional reactions, such as mirth, anger, fear or joy. Weakness may be confined to the limbs or be widespread enough to result in a limp fall. Laughing fits are one of the recognised causes of cataplectic incidents. During such attacks the H-reflex is almost abolished.
The findings suggest that hypotonia induced by laughter precipitates further motor inhibition in susceptible people, thus inducing cataplexy. Perhaps a good laugh is not always such a good thing as is generally assumed.