LAUSR

Windmill nystagmus is characterized by a clock-like rotation of the direction of nystagmus [1]. Even though various patterns of variant windmill nystagmus have been described [2, 3], the typical pattern of windmill nystagmus has only once been described in a patient with acquired blindness [1]. We report another case of windmill nystagmus that was observed only 3 months after visual loss. A 47-year-old man was referred for further evaluation of progressive visual loss in both eyes for 3 months. Under the suspicion of atypical optic neuritis, intravenous methylprednisolone, 1 g per day, had been administered for 10 days in another hospital, but his visual acuity had deteriorated. On examination, he was not able to perceive light in the left eye, and only able to count fingers at 50 cm in the right eye. Both pupils were measured at 5 mm and hardly responded to light or near stimuli. Optic discs were pale in both eyes (Fig. 1a). Video-oculography documented intermittent windmill nystagmus with a counter-clockwise rotation of the beating direction (from the patient’s perspective, Video 1, Fig. 1b). The nystagmus was briefly suppressed by changes in gaze direction or body position. Video headimpulse and bithermal caloric tests were normal. Brain MRIs and MR angiography were normal. Genetic analysis for Leber’s hereditary optic neuropathy was negative for mitochondrial DNA 3460, 4171, 11778, and 14484. Antiaquaporin 4 antibodies and other serologic markers for viral or autoimmune diseases were all negative. Our patient showed windmill nystagmus in association with subacute progressive visual loss for 3 months. Windmill nystagmus has been described only once in a blind woman due to an acquired ophthalmic disease. In that patient, the direction of nystagmus rotated to complete a clockwise cycle and then reversed to complete another counterclockwise cycle [1]. Windmill nystagmus resembles periodic alternating nystagmus (PAN) which also changes its direction and amplitude in a certain order [4], but unlike PAN, windmill nystagmus changes the directions also in the vertical plane. Likewise in other abnormal eye movements observed in blind, windmill nystagmus is explained by a lack of visual feedback from the image motion on the retina which stabilizes the velocity storage mechanism (VSM) or gaze-holding network [5]. The VSM enhances the low-frequency vestibulo-ocular reflex (VOR), thereby improving the performance of the VOR during constant head rotation [2, 6]. In normal circumstances, the VSM is calibrated by the visual feedback via the retinal slip and cerebellar nodulus. Due to loss of visual calibration, the VSM and gaze-holding system may become unstable and vulnerable to the “noises”, and ocular drifts may occur alternately in the horizontal and vertical directions [7]. Deprivation of visual inputs may give rise to various involuntary eye movements. By convention, patients who lost their vision in their late age hardly show spontaneous eye movements and the VOR is mostly preserved [5]. In contrast, those who were congenitally blind or had lost their vision early in life mostly show spontaneous nystagmus, and impaired VOR or eye-head coordination [7, Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0041 5-018-9015-4) contains supplementary material, which is available to authorized users.