A dense painless red area appearing rapidly and obscuring the sclera of one eye is likely to be a sub-conjunctival haemorrhage.
Patchy or diffuse prominence of the blood vessels overlying the sclera usually of one eye only may indicate episcleritis (mild pain) or scleritis (usually severe pain) If long-standing and painless there may be a pterygium.
Red eyes that are sore and/or gritty are often due to blepharitis or dry eye or to rolling outwards of the lower lid (ectropion) or inability to close the lids, usually due to facial nerve palsy. Additional discharge points to conjunctivitis.
Redness with pain and aversion to bright light (photophobia) usually indicates keratitis or iritis. An additional sensation of scratching or something being in the eye suggests foreign body or corneal abrasion or entropion
A red eye with severe pain, haloes in the vision and feeling unwell or vomiting suggests acute glaucoma.
Inflammation of the eyelids with localised swelling is seen with infection (cellulitis) or an inflamed cyst. More generalised swelling occurs with allergic dermatitis and severe blepharitis.
