Primary trigeminal neuralgia
Evidence suggests that in up to 95% of cases, trigeminal neuralgia is caused by pressure on the trigeminal nerve close to where it enters the brain stem, the lowest part of the brain that merges with the spinal cord.
This type of trigeminal neuralgia is known as primary trigeminal neuralgia.
In most cases the pressure is caused by an artery or vein squashing (compressing) the trigeminal nerve. These are normal blood vessels that happen to come into contact with the nerve at a particularly sensitive point.
It's not clear why this pressure can cause painful attacks in some people but not others, as not everyone with a compressed trigeminal nerve will experience pain.
It may be that, in some people, the pressure on the nerve wears away its protective outer layer (myelin sheath), which may cause pain signals to travel along the nerve. However, this does not fully explain why some peeople have periods without symptoms (remission), or why pain relief is immediate after a successful operation to move the blood vessels away from the nerve.