The cause of this condition is irritation of the fifth cranial nerve (the Trigeminal nerve) which is responsible for providing facial sensation. This irritation is occasionally due to benign tumours or multiple sclerosis, either of which can usually be detected by a high quality magnetic resonance imaging (MRI) of the brain.
In the majority of cases, however, imaging of the brain does not reveal a cause of the nerve irritation. In such cases a small vessel (usually an artery but occasionally a vein) is often found at operation to be compressing the root entry zone of the Trigeminal nerve at the brainstem.
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