Glaucoma. Ophthalmoscope view of the retina of an eye showing glaucoma, an increased pressure of the fluid inside the eye. It causes the blood vessels (red) to have a hooked appearance as they emerge from the optic disc (yellow). Glaucoma is usually caused by a gradual blockage of the outflow of aqueous humour, the liquid that fills the front chamber of the eye. This type of glaucoma tends to run in families. Alternatively, the outflow may be blocked suddenly, for instance by an injury to the eye. It causes gradual reduction of sight, which is often unnoticed until the damage is permanent. The sudden form may cause pain and nausea. Drugs and surgery can correct the condition.