They can also be instantly prevented by teh same treatment that controls trigeminal neuralgia. It is likely that all result from a similar effect of demyelination. The never impulse carried along an axon is normally effectively isolated or insulated from neighbouring axons and indeed, it is difficult to imagine that the nervous sytem could function if this was not so. Loss of the myelin sheath from axons still capable of conducting could lead to the nerve impulse spreading sideways to other axons, causing sudden massive discharge and the paroxysm of pain, itching or muscle contraction. In the days when men wore short hair this was sometimes called the 'barber's chair sign' as it would be experienced when the head was bent forward for the application of clippers.
Many patients become aware of a pronounced effect of temperature on their symptoms.
A hot day, a hot bath, exertion, or even a hot drink may cause temporary weakness or blurring of vision. Some poeple find this so unpleasant that they never have the bath water more then tepid and learn to dread heat waves. The beneficial effect of cold is less obvious, partly because this may actually aggravate spasticity so that any improvement in strength would be masked and such patients actually prefer hot weather. These effects of heat are readily explicable as there is experimental evidence that even a minute rise in temperature will increase the proportion of demyelinated axons that cannot conduct at all. The effect is not harmful and these brief reversible increases in symptoms are quite different from an acute relapse.