Case Report: Acquired Periodic Alternating Nystagmus and Vestibular Weakness in West Nile Virus Encephalitis

Background and Aim: West Nile Virus (WNV) encephalitis is reported to cause “ataxia” in up to 31% of patients, and “vertigo” in up to 14% of patients. The mechanism of these symptoms is unclear. The Case: We report the case of a lady who contracted WNV encephalitis at age 74 years. When evaluated 15 months later she complained both of baseline unsteadiness, and of a fluctuating component of disequilibrium. Instrumented vestibular testing identified bilateral vestibular weakness (in a different pattern on each side). It also identified Periodic Alternating Nystagmus (PAN), presumably acquired (rather than congenital). Acquired PAN has been observed in other encephalidites, but has not been previously reported in association with WNV encephalitis. Conclusion: In this case, WNV encephalitis appears to have caused both vestibular weakness (likely accounting for the baseline unsteadiness) and periodic alternating nystagmus (likely accounting for the fluctuating disequilibrium).