Herpes Zoster Ophthalmicus (HZO) is defined by involvement of the ophthalmic branch of the trigeminal nerve (CN V1). This branch supplies the forehead, the anterior scalp, the side and tip of the nose, ethmoidal sinuses, and importantly, all structures within the globe, including the retina and optic nerve. Zoster involving the maxillary division of the trigeminal nerve (CN V2) is not globe threatening, and does not require referral to an ophthalmologist.