Hyperventilation-induced nystagmus and vertigo after stereotactic radiotherapy for vestibular schwannoma

OBJECTIVE: To describe the phenomenon of hyperventilation-induced nystagmus (HVN) after stereotactic radiotherapy for vestibular schwannoma. PATIENTS: We present 2 patients with vestibular schwannoma and no pretreatment vestibular symptoms who both received stereotactic radiotherapy. Within 2 months of completing treatment, both patients developed intense vertigo while exerting themselves. Video-oculography demonstrated an intense nystagmus with the fast phase directed toward the side of the schwannoma in both patients. INTERVENTION: Diagnostic. RESULTS: Patients who have undergone surgical resection of their vestibular schwannoma, or have a large tumor, will often demonstrate HVN with the fast phase directed away from the side of the tumor. This is distinct from patients with smaller lesions who have a fast-phase nystagmus toward the tumor's side. This second type of nystagmus is thought to originate from changes in the extracellular calcium concentration secondary to hyperventilation-induced alkalosis. CONCLUSION: We hypothesize that stereotactic radiotherapy induced greater demyelination of the vestibular nerve leading to the observable sign of HVN. These patients represent the first reported cases of HVN after stereotactic radiation and illustrate the pathophysiology of HVN, which may lead to a greater understanding of the effects of stereotactic radiotherapy.