Monocular Downbeat Nystagmus Due to a Posterior Fossa Cyst

This is a 40-yo-man who experienced months of imbalance and was found to have an epidermoid cyst (immediately posterior to the 4th ventricle), which was resected. Months after surgery, he experienced monocular vertical oscillopsia. On examination, there was subtle downbeat nystagmus (DBN) in the right eye only - none was seen OS, even with ophthalmoscopy. There was no palatal tremor. With convergence, DBN OD increased, and subtle DBN OS was provoked. Additionally, he had gaze-evoked nystagmus and hypermetric saccades in addition to mild gait ataxia. Although monocular DBN has most commonly been reported due to lesions affecting the midbrain or mesodiencephalon, it has been proposed that the cerebellar flocculus has regions that are responsible for monocular control. It is therefore possible that specific damage to one of these monocular areas could cause spontaneous monocular DBN, as in our case.Balaban CD, Watanabe E. Functional representation of eye movements in the flocculus of monkeys (Macaca fuscata). Neurosci Lett. 1984 Aug 24;49(1-2):199-205.