The two commonest causes of avoidable blindness produce no symptoms at all until they are advanced — and the brain politely paints over the missing parts so you never notice the gap.

How fast does this need you?

Tick what actually changed. The tool routes to one of four windows and names the condition each window is protecting against — and it tells you what a wrong call costs in both directions, because the fear of wasting somebody's time is exactly what makes people sit on a detaching retina overnight.

The part that is silent

• Glaucoma takes the periphery first, and the brain hides it. The visual system fills gaps from the surrounding image and from the other eye, so a substantial field loss can feel like nothing at all. By the time somebody notices bumping into door frames or missing steps, a lot of nerve has already gone, and nerve does not come back.
• It is found on a test, not by a symptom. Pressure, optic-nerve appearance and a visual-field test are the whole detection strategy, which is why a routine eye examination every one to two years after forty is the single most useful thing on this page. Treatment — usually drops — slows or halts it, so what is caught is kept.
• The risk factors are worth knowing: age, a parent or sibling with glaucoma (which raises risk several-fold), African or Caribbean ancestry for open-angle, East Asian ancestry for angle-closure, high short-sightedness, diabetes, and long-term steroids of any kind.
• Diabetic retinopathy is the same story with a different mechanism. Leaking and blocked retinal vessels do their damage without symptoms, and vision loss appears when the centre is already involved or a bleed happens. Annual screening exists precisely because the eye gives no warning, and laser or injections work far better before symptoms than after.
• Blood pressure and glucose are eye treatments, unglamorously. Tight-ish control slows retinopathy more reliably than anything done at the eye itself, and stopping smoking roughly halves the trajectory toward macular degeneration.
• “My vision is fine” is not a field test. Reading a phone perfectly says nothing about the outer field or the optic nerve, because central acuity is the last thing glaucoma touches.

The emergencies, in order of how fast

• Sudden painless loss of vision in one eye — minutes. A blocked retinal artery is a stroke of the eye: the retina survives only a short time without blood, and the event also means a stroke risk elsewhere that needs investigating the same day. Even if the vision comes back within minutes, that is a warning shot, not an all-clear.
• A curtain, shadow or veil spreading across the vision, usually with new floaters and flashes — same day. A retinal detachment is repairable, and the results are far better before the central retina lifts. This one gets slept on constantly because it does not hurt.
• Severe eye pain with haloes around lights, a hard red eye, blurred vision, nausea or vomiting — minutes. Acute angle-closure glaucoma is a pressure emergency and one of the few painful ones. People often present to a doctor for the vomiting and the headache while the eye goes unexamined.
• Over 50, with new temple headache, tenderness combing the hair, jaw ache while chewing, and any visual loss — minutes. Giant cell arteritis blinds the second eye within days if the steroids wait; treatment is started on suspicion, not after the biopsy.
• New double vision, a drooping lid, an unequal pupil or eye pain with movement — minutes to hours. These are neurological until proven otherwise, and some causes are aneurysms.
• A contact-lens wearer with a red, painful, light-sensitive eye — same day, lenses out. Microbial keratitis can scar the cornea in days, and it is the reason sleeping in lenses matters.
• New floaters and flashes without any curtain — within a day or two. Most are the harmless jelly pulling away with age, but a small proportion sit next to a retinal tear that can be sealed before it becomes a detachment, so it is a phone call and an appointment, not an ambulance.
• Something hit the eye at speed, or it was cut — minutes, and do not rub it. Fragments from grinding, strimming, hammering metal or a champagne cork can penetrate with surprisingly little pain and normal vision at first. Do not rub, do not press anything onto the eye, do not try to remove anything embedded: cover it loosely with something rigid if you can, do not eat or drink in case theatre is needed, and go in. Most of these injuries are prevented entirely by protection that costs almost nothing.
• Straight lines bending, or a new distorted patch in central vision — days. Wet macular degeneration responds to injections, and the response is much better started early; this is an urgent referral rather than a routine one.

Glaucoma, treated as a plumbing problem

• The pressure inside the eye is a balance between fluid made and fluid drained, and when drainage fails the pressure rises and the optic nerve dies slowly. There is no such thing as feeling your eye pressure — that is the entire clinical problem.
• Normal-tension glaucoma exists, which is why a normal pressure reading alone does not exclude it. The nerve and the field matter as much as the number.
• Drops work if they are used, and adherence is the weak link: they are for life, they do nothing you can feel, and roughly half of people drift off them. Attaching them to teeth-brushing, using a phone alarm and asking about preservative-free or combination drops if they sting all measurably help.
• Technique matters more than people think. One drop is enough, the eye holds no more; close the eye gently for a minute rather than blinking hard, press lightly at the inner corner to stop it draining into the nose, and leave five minutes between different drops.
• Laser is a first-line option now, not a last resort, and for many people it replaces drops for years. It is worth asking about explicitly rather than waiting to be offered it.
• Tell your relatives. First-degree relatives of somebody with glaucoma carry several times the risk and in many countries qualify for free or earlier screening — passing on the diagnosis is one of the most useful things a patient can do.

Contact lenses, where most avoidable damage happens

• Sleeping in lenses is the big one. It raises the risk of a sight-threatening corneal infection several-fold, and “just once” on a sofa is how a large share of cases start.
• Water and lenses do not mix. Tap water, showers, swimming pools and hot tubs carry organisms including an amoeba that causes a rare, exceptionally painful and hard-to-treat infection. Rinse with solution, never water, and take lenses out before swimming.
• A red eye in a lens wearer is not conjunctivitis until proven otherwise. Take the lens out, keep it in its case for the clinic, and be seen the same day rather than trying drops.
• Replace as instructed and replace the case monthly, because the case is where the biofilm lives. Topping up old solution rather than emptying and drying the case is a common shortcut with a real cost.
• Decorative or costume lenses bought without a fitting cause a disproportionate share of injuries, and coloured lenses ordered online are one of the few genuinely dangerous fashion items.
• Pain, light sensitivity and blurring are the three warning signs. Grittiness that clears when the lens comes out is different from pain that stays — the second one is the emergency.

Children, where timing is everything

• A squint or a lazy eye has a window. The visual system is only trainable for a limited period in childhood, so a treatable difference caught at three does well and the same thing found at ten often cannot be fully recovered. Any constant turn of an eye after about four months needs checking, not waiting out.
• A white reflection in a photograph, instead of the usual red-eye, needs urgent assessment. Most causes are benign, but this is how retinoblastoma is found, and it is curable when caught early.
• Short-sightedness in children is rising and is not just an inconvenience, because high myopia carries lifelong risks of detachment and glaucoma. Time outdoors slows progression, and there are treatments — specific spectacle lenses, soft multifocal contact lenses, low-dose atropine drops — that reduce it. Ask, rather than assuming stronger glasses each year is the only option.
• Screens do not damage eyes, but near work without breaks provokes eyestrain and headaches, and long stretches indoors are associated with more myopia. The useful message is time outside and regular breaks, not banning devices.
• Children rarely complain, because they have nothing to compare with. Sitting close to the television, screwing up one eye, head tilting, losing place while reading, or a sudden dislike of homework are all worth a sight test.
• School screening is not comprehensive. It is a coarse net, and a child who passes it can still have a significant problem; a proper examination is free for children in many countries and is worth using.

Ageing eyes, and what is actually fixable

• Cataract is the good news story. It is a clouding of the lens, it is fixable with a short operation that is one of the most effective in medicine, and it does not need to be “ripe”. The threshold is when it interferes with what you want to do — driving at night, reading, recognising faces.
• Cataract surgery also cuts fall risk, which matters more than the vision itself for some people: better contrast and depth perception mean fewer fractures, so it is worth pushing for rather than postponing.
• Macular degeneration is central, glaucoma is peripheral, and knowing which is which changes what you monitor. Dry macular degeneration is slow and has no treatment beyond not smoking, diet and possibly specific supplements once it is established; wet needs injections urgently.
• Use an Amsler grid or a doorframe. Looking at a straight line with one eye covered, once a week, is how wet degeneration gets caught early enough for injections to work well.
• Dry eye is common, miserable and treatable, and it is not a warning of anything worse: lubricants, lid hygiene, warm compresses, blinking on purpose at screens, and reviewing medicines that dry you out.
• Sudden difficulty with reading is usually not the eye at all after fifty — it is normal lens stiffening. New glasses fix it. What is not normal is losing part of the field, seeing distortion, or one eye differing from the other.

What does not matter, and what does

The drill: 16 decisions

Sixteen ordinary moments — a shadow noticed while shaving, a splash of oven cleaner, a lens slept in, a grandmother who has stopped using her drops. Most have an instinctive answer that either wastes a night in a waiting room or costs an eye. Pick your move; every answer explains why.

The card

Print it for the fridge, or for whoever would be driving.

🖨️ Print the card