A 78-year-old man presented to an ophthalmology clinic complaining of a gritty sensation in both eyes for several months, with exacerbation in the left eye over the past 2 weeks. He had a history of diabetes mellitus treated with oral hypoglycemic drugs and chronic obstructive pulmonary disease. He was diagnosed with background diabetic retinopathy. Examination using a Snellen chart showed a visual acuity of 6/12 in both eyes, meaning that he would need to approach to a distance of 6 meters to read letters that a person with normal acuity would be able to read at 12 meters. The patient noticed pain and swelling of his left eyelid at that time, and developed headache and vomiting 1 week later. Five days before admission, he developed patchy erythema with clustered vesicles with varying sizes on his left forehead and upper eyelid. On the day of admission, he developed drooping of the left upper eyelid, and was admitted to the infectious diseases ward with a diagnosis of HZO in the distribution of V1 on the left side, complicated by superficial punctate keratitis. The patient was initially treated with intravenous acyclovir (500 mg every 8 hours), topical antiviral ointment (acyclovir 3%), and non-steroidal anti-inflammatory drugs to relieve the pain caused by severe swelling and erythema of his left upper eyelid. On day 4 of admission, the inflammation of the left upper eyelid had resolved, but he was still unable to elevate the eyelid. Physical examination showed markedly crusted skin lesions in the distribution of V1 on the left side, with a positive Hutchinson’s sign and complete levator paralysis. Examination of the extraocular movements with the eyelids held open showed near-complete ophthalmoplegia on the left side. His visual acuity was 6/12 on the right side and 3/60 on the left side. He had anisocoria with the left pupil, which was poorly reactive to light and accommodation, measuring up to 6 mm in diameter. He was diagnosed with OAS, and brain magnetic resonance imaging (MRI) on day 5 of admission showed swelling of the left periorbital tissues and left-sided exophthalmos, with no evidence of a retro-orbital space-occupying lesion. Orbital fat-suppressed MRI on day 6 of admission showed orbital myositis and enhancement of the optic nerve sheath. The patient was diagnosed with HZO complicated by OAS, and systemic steroid therapy was initiated on day 6 of admission. Adduction and supraduction of the left eye started to improve on day 3 after the initiation of steroid therapy. He received intravenous acyclovir (500 mg every 8 hours) for 15 days, and oral prednisolone (25 mg twice daily; >1 mg/kg per day) for 4 days followed by 12 weeks of prednisolone therapy that was tapered at monthly visits. The patient was discharged after completion of the acyclovir therapy. His visual acuity recovered to 6/12 on the right side and 6/30 on the left side, and his extraocular movements gradually improved, with residual limitation of abduction and paralysis of the left upper eyelid at the completion of the steroid therapy. Follow-up at 180 days after the onset of symptoms showed persistent limitation of abduction and paralysis of the left upper eyelid. Repeat orbital MRI at 180 days showed no changes to the orbital lesions compared with the MRI findings on day 6 of admission.