The red eye

Look at where the redness is worst before you put any drops in. Conjunctival injection is brightest in the fornices and towards the lids, fades towards the cornea, and the vessels move when the conjunctiva is pushed with a cotton bud through the lid. Ciliary injection, or circumcorneal flush, is a violet ring of deep vessels around the limbus that fades towards the fornices, and it means the cornea or the inside of the eye. Scleral injection is deep, bluish red, tender, often in one sector, and the vessels do not move or blanch. A single flat sheet of bright red with a sharp edge and white conjunctiva around it is a subconjunctival haemorrhage, and it is painless.

A drop of phenylephrine settles the harder cases: conjunctival and episcleral vessels blanch within minutes, scleral vessels do not.

Look at the lid margins for crusting, redness and blocked gland openings, which is blepharitis, the commonest cause of chronically gritty red eyes and easily missed because the eye itself looks nearly normal. Look at the discharge: watery in viral conjunctivitis, mucopurulent and sticking the lids together in the morning in bacterial, stringy and white in allergy, and a copious purulent discharge in a sexually active adult is gonococcal until proved otherwise and can perforate the cornea. Evert the upper lid and look at the tarsal conjunctiva: follicles, small pale bumps like grains of rice, in viral and chlamydial disease; papillae, a red cobbled surface, in allergy and contact lens intolerance; and a foreign body caught under the lid, which is the commonest reason for a corneal scratch that will not heal. Chemosis, the conjunctiva swollen into a glassy jelly, is allergy, or a sign of orbital disease if the eye is also proptosed.

Feel in front of the ear: a tender preauricular lymph node means viral or chlamydial conjunctivitis.

Look at the cornea in white light for clarity, then put fluorescein in and switch to cobalt blue. A bright green stain is an epithelial defect: a sharp-edged patch after trauma is an abrasion, a branching line with knobbed ends is a herpes simplex dendrite, a scatter of fine dots across the lower cornea is exposure or dry eye, and dots across the upper cornea suggest a foreign body under the upper lid. Then look for an infiltrate, a white or grey opacity in the stroma. An infiltrate with an overlying defect in a contact lens wearer is bacterial keratitis; a small peripheral infiltrate with an intact epithelium and a clear gap from the limbus is usually marginal keratitis from staphylococcal blepharitis. A hazy, steamy cornea all over with a hard eye is angle closure. Vesicles on the lid and the tip of the nose with a dendrite that stains poorly is herpes zoster.

Test corneal sensation with a wisp of cotton wool before any anaesthetic drop: reduced sensation points to herpes. If a topical anaesthetic takes the pain away completely, the cause is on the surface; if the deep ache remains, look inside the eye.

Narrow the slit, angle the arm and focus into the chamber against the dark pupil. Cells drifting through the beam and a smoky flare mean anterior uveitis, and with it come a small, sometimes irregular pupil, photophobia and a deep ache. A white level at the bottom of the chamber is a hypopyon and means severe inflammation: a corneal ulcer, endophthalmitis after surgery, or a violent uveitis. A red level is a hyphaema after trauma. Then judge the depth of the chamber at the edge with a narrow beam or a torch from the side, because a shallow chamber, a fixed mid-dilated pupil, a hazy cornea and a rock-hard eye in a patient with headache and vomiting is acute angle closure, and it needs pressure-lowering treatment within the hour.

The pupil sums up the inside of the eye: small and sluggish in uveitis, fixed and mid-dilated in angle closure, peaked after trauma when the iris has plugged a wound.

Six diagnoses account for almost every red eye that loses vision, and each has a signature. Bacterial keratitis: contact lenses, pain, an infiltrate that stains. Anterior uveitis: ache, photophobia, small pupil, cells. Acute angle closure: headache, vomiting, hazy cornea, fixed pupil, hard eye. Scleritis: boring pain that wakes the patient, deep injection, tenderness. Endophthalmitis: recent surgery or injection, pain, falling vision, hypopyon. Orbital cellulitis: a swollen hot lid with proptosis, restricted eye movement and fever, especially in a child, and it is a CT scan and intravenous antibiotics today. Herpes zoster ophthalmicus with the nasociliary branch involved, vesicles on the side of the nose, threatens the eye and needs an ophthalmologist.

The rest, conjunctivitis, blepharitis, episcleritis, subconjunctival haemorrhage and dry eye, are managed with reassurance, lubricants and hygiene, and reviewed if they do not settle in a week or if any of the three questions turns to yes.

The red eye pathway Examine cells and a hypopyon on the model eye Emergencies