Upbeat nystagmus is a useful sign in the regional diagnosis of trigeminal nerve disorder with multiple sclerosis.

Trigeminal nerve disorder is an important neurological sign that is often seen with multiple sclerosis (MS). We investigated eye movements in three MS patients with trigeminal disorder due to pontine lesions near the trigeminal root entry zone (REZ). Upbeat nystagmus was observed in all MS patients with trigeminal REZ lesions. We conjecture that trigeminal nerve disorder and upbeat nystagmus appeared due to simultaneous damage to both the trigeminal nerve and the vestibulo-ocular reflex pathway. If upbeat nystagmus appears in MS patients exhibiting a trigeminal nerve disorder, such as trigeminal neuralgia, and paralysis, pontine lesions near the trigeminal REZ should be considered. Upbeat nystagmus can be understood as a useful sign for the clinical regional diagnosis of trigeminal nerve disorder.