Paralysed by humour? Don't make me laugh

Yesterday the BBC reported the case of Kay Underwood, a 20-year-old student who can't help collapsing if she's made to laugh. Underwood says her friends think she's putting it on, but in fact she has cataplexy. This is a condition that affects a minority of patients with narcolepsy (a tendency to fall asleep in the day). It involves the muscles weakening in response to certain emotional triggers, most often humour. Other documented cases include a man who experienced weakness and trembling when it was his turn during a game of draughts.

Fortunately for those curious to know more about cataplexy, two open-access articles have been published this year, both of which report the results of brain scans taken of patients with the condition while they were exposed to cartoons or funny pictures.

Their findings are slightly contradictory, in that the first by Sophie Schwartz and colleagues found decreased activity in the hypothalamus of the patients compared with controls, whereas the second paper by Allan Reiss' team found increased activity in this brain region.

However, both papers agree that the condition appears to reflect abnormally intense activity in at least some of the brain's emotion network when exposed to humour (for example both studies found exaggerated activation of the amygdala and nucleus accumbens).

The paper by Allan Reiss made several other notable observations: (1) the patients with cataplexy reported finding the cartoons significantly less funny than the controls. This may reflect their attempt to avoid a cataplectic attack by stifling their emotional response to the cartoons. (2) The patients showed greater activity in a region on the right-hand side of the front of the brain, known to be involved in inhibitory control - again suggesting some kind of damping mechanism aiming to keep a lid on the over-reaction of the brain's emotional areas. (3) The researchers managed to grab a brain scan of one patient caught in the midst of a full-blown cataplectic attack. This revealed significantly reduced activity in his hypothalamus. Based on this, the researchers said "massive suppression of hypothalamic activity may be an essential component of a cascade of neural events leading to muscle atonia" (i.e. the weakening that leads Kay Underwood to collapse).

In other words, it sounds like people with cataplexy have an exaggerated emotional response and that sometimes their brains overcompensate for this with such a powerful inhibitory mechanism that it literally leaves them flattened.

Link to BBC News item.
Link to study of man who grew weak when playing draughts.
Link to Brain article on imaging patients with cataplexy (open access).
Link to PLoS One article on the same (open access).

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