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Purpose: The aim of the study is to examine the baseline intraocular pressure (IOP) and its changes after performing a water drinking test (WDT) in patients with unilateral hemifacial spasm (HFS). Patients and Methods: In this prospective observational study, patients aged 21 years and above diagnosed with unilateral HFS were recruited from the Singapore National Eye Centre between January 2015 and August 2016. The unaffected eye of each patient served as a matched control. An interviewer-administered standardized questionnaire on HFS symptoms and ophthalmic examination was performed. Automated perimetry, optical coherence tomography (OCT) of the optic nerve head, color disc stereophotography and water drinking test (WDT) were done. The primary outcome measure was the difference in IOP between eyes affected by HFS and fellow eyes at baseline and at 15, 30 and 45 minutes of the WDT. Results: Fifty-four patients with unilateral HFS were included. Mean age was 59.8± 9.9 years (range, 37.0– 84.0). Of these, 54% were female and 94% were Chinese. Mean baseline IOP was significantly higher in eyes with HFS (13.9± 3.1mmHg) compared to fellow eyes (13.3± 2.8mmHg) (p=0.008). There was no significant difference in absolute or percentage change in IOP from baseline between the 2 groups at 15, 30 and 45 minutes of the WDT. Mean vertical cup–disc ratio (VCDR) on clinical examination was significantly higher in eyes with HFS (0.5± 0.2) compared to fellow eyes (0.4± 0.2) (p=0.02). There was no significant difference between the groups for visual field parameters and mean retinal nerve fiber layer thickness on OCT. Conclusion: Hemifacial spasm is associated with a small but significant difference in mean baseline IOP and VCDR between affected and fellow eyes. However, when eyes affected by HFS and fellow eyes were challenged with the WDT, both responded in similar ways. [ABSTRACT FROM AUTHOR]