Vision loss

Sight can vanish in a second, or it can fade so slowly that the loss is only noticed when faces on the other side of the street stop being recognisable. These are two entirely different situations and confusing them is dangerous. Sudden loss of vision is an emergency in the literal sense: the retina and the optic nerve have very little time, and what is not saved in the first few hours is usually not saved at all. Gradual loss is dangerous in another way: it does not hurt, it does not frighten, and so it goes unattended for years — precisely during the period when something could still be done. What follows is how to tell the two apart and what each picture points to.

Sudden loss of vision is an emergency

The rule is simple and has no exceptions: if sight has gone suddenly — completely, in part of the field, or as a patch in the centre, in one eye or both — help is needed within the hour, not in the morning. Whether it hurt or not makes no difference: the most dangerous causes are the painless ones.

One point is misread more often than any other. If the vision came back on its own, after a minute or after ten, that is not the all-clear. A brief loss of sight in one eye, with a dark curtain coming down from above and lifting again a few minutes later, means the eye was left without a blood supply for a while. It is essentially the same event as a transient ischaemic attack, only happening in the eye, and it can be followed within days by a stroke or by blindness that is already permanent. It needs investigating urgently, not when it "happens again".

Call an ambulance or go straight to a hospital with an eye department if sight goes suddenly, if part of the visual field disappears, if double vision appears, if the eye becomes sharply painful, or if weakness in an arm or a leg, a drooping face or slurred speech is added to the visual problem. Spain, Italy, Portugal, Poland and Ukraine all use the single emergency number 112; if the ambulance number in your country is different, use that one. Do not drive in this state — ask somebody to take you.

One eye, suddenly and without pain

Vision in one eye goes out within seconds, painlessly — "as though the light had been switched off" — or a dense shadow covers part of the field. The commonest cause is a blockage in a vessel supplying the retina. When the artery closes, the retina is left without oxygen and the clock runs in hours: this is an event of the same order as a stroke and has to be handled with the same urgency. When the vein blocks, sight usually falls less abruptly and is often noticed on waking, but here too there is nothing to wait for.

In an older person, this picture can be caused by giant cell arteritis, inflammation of the wall of the temporal and other arteries. This is the case that makes texts like this one worth reading. The warning signs are a new headache in the temple, unlike previous headaches; tenderness of the scalp that makes even combing the hair unpleasant; aching, tiring jaw muscles while eating; weakness, weight loss, a raised temperature, and stiff, painful shoulders and hips in the morning. Here the time really is counted in hours, and treatment with high-dose steroid medicines is started at once, without waiting for blood results or a biopsy — because untreated, the second eye can go blind within days, and that cannot be undone.

A curtain, flashes and floaters

A different picture: a dark curtain appears at one side of the visual field and creeps towards the centre, and either before this or at the same time the person sees flashes of light in the dark and a sudden shower of floating specks, cobwebs or what looks like black soot. This is how retinal detachment announces itself.

Individual floaters that somebody has had for years and that do not change are ordinary and not worrying in themselves. What matters is the change: a sudden crop of many more, flashes added to them, part of the field starting to go missing. The risk is higher in people with strong short sight, after an eye injury, after cataract surgery, and in anyone who has already had a detachment in the other eye. An ophthalmologist should be seen the same day: while the detachment has not reached the central part of the retina, surgery has a far better chance of saving the sight.

A painful red eye with haloes around lights

This picture resembles nothing else. The eye becomes sharply and severely painful, the pain spreads to the temple and half the head, the eye goes red, vision clouds over, and rainbow haloes appear around lamps and street lights. Nausea and vomiting are often added, and because of them people are not infrequently treated for food poisoning or migraine while time is lost. The eye feels hard to the touch and the pupil is wide and barely reacts to light.

This is an acute attack of closed-angle glaucoma: the drainage of fluid from the eye has been blocked and the pressure inside has shot up. The optic nerve is damaged quickly, so the pressure has to be brought down within hours. The attack often begins in the evening, in dim light, at the cinema, or after drops that widen the pupil. It is an emergency: it needs an ambulance or an immediate ophthalmology appointment, not a painkiller and a wait until morning.

Half the visual field has gone, or things have started to double

If the same half of the visual field suddenly disappears in both eyes — the person stops seeing everything on the left or everything on the right, walks into door frames, misses half of what is on the plate — the problem is usually not in the eyes but in the brain. This is what a stroke at the back of the brain looks like. The person is almost always convinced that "one eye has gone bad", and checking is easy: cover one eye, then the other. If the missing part is still missing with the right eye and with the left, it is not an eye problem. Call an ambulance immediately, even if there is nothing else wrong.

There is one more worrying combination: sudden double vision, a drooping eyelid and a headache. Behind it can lie pressure on the nerve that moves the eye from inside the skull, including pressure from an aneurysm of a brain artery. That too calls for immediate attention rather than watchful waiting. Any double vision appearing for the first time, even without pain, is worth showing to a doctor the same day.

When sight goes without being noticed

Slow deterioration is not frightening, and that is exactly where its danger lies. The brain adapts, the person turns up more light, holds the phone further away, stops driving at night — and spends years assuming their eyes are simply tired.

• Cataract. The lens clouds over: the world looks as though it were behind dirty glass or in a haze, colours lose their strength, headlights and street lamps dazzle and smear at night, and glasses need changing more and more often. Surgery restores the sight, and there is no need to wait until vision is almost gone before considering it.
• Open-angle glaucoma. Pressure inside the eye slowly damages the optic nerve and the field of vision narrows from the edges — so gradually that nothing is noticed until only a narrow corridor is left. Nerve fibres that die do not come back, so the whole point of treatment is to begin before there are any symptoms at all.
• Age-related macular degeneration. The centre of the field is affected: reading and recognising faces become difficult, and straight lines — a door frame, a tile joint, a line of text — start to look wavy or broken. If the distortion has appeared or worsened within days, an ophthalmologist is needed urgently: the "wet" form can be treated, and the treatment works far better the earlier it starts.
• Diabetic retinopathy. High blood sugar damages the small vessels of the retina over years. Up to a certain point nothing whatever is felt, and then sight can drop suddenly because of a bleed inside the eye. That is exactly why how you feel is no guide here.

Who should have their retina examined before symptoms appear

The general rule is this: the diseases that take sight away permanently stay silent for a long time. So the only way to catch them in time is to have the eyes checked while nothing is troubling you. An examination with the pupil dilated and a measurement of the pressure inside the eye take little time and need no preparation, apart from one thing: after the pupil has been dilated you cannot drive for a few hours.

Regular checks are needed by people with diabetes, from the moment the diagnosis is made and then at the interval their doctor sets; by anyone with glaucoma in the family; by people with high blood pressure, strong short sight or previous eye injuries; and by everybody from a certain point in adult life, a threshold that differs from country to country and is worth confirming with an ophthalmologist or family doctor.

Vision loss in children is uncommon, but a child almost never complains of it: they do not know it is possible to see differently. Watch the behaviour instead — a squint, a habit of bringing the face very close to a book or a screen, tilting the head to one side, screwing up one eye, headaches after school. Routine sight checks in childhood exist for precisely this reason: many of these problems correct well, but only while the visual system is still developing.

What not to do

First, do not try to sit it out. The commonest and most expensive mistake in sudden vision loss sounds like this: "I'll lie down and it will pass by this evening." Those are exactly the hours in which it is decided whether sight comes back, and time lost then cannot be bought back afterwards.

Second, do not use somebody else's eye drops. Drops left over from a relative or found in the medicine cupboard are not a harmless thing. Drops that widen the pupil can trigger an acute glaucoma attack in a person with a narrow angle. Steroid drops raise the pressure inside the eye and make a herpes infection of the cornea sharply worse. Drops that shrink the blood vessels "for redness" simply mask a picture that ought to be shown to a doctor.

And a few more things. Do not rub the eye or press on it. If the injury is a penetrating one, or something has gone in and lodged there, do not try to remove it and do not rinse the eye: cover it without pressing on anything and travel to hospital. With a chemical burn the opposite applies — rinse at once with clean water, generously and for a long time, and only then set off. Contact lenses should be taken out if the eye is red or painful and left out until it has been examined. And when going to the doctor, take your glasses, the boxes your eye drops came in and a list of every medicine you take: it speeds up the assessment considerably.

Online consultation

A conversation at a distance does one job well: working out quickly which of three situations you are in — the one that means leaving the house now, the one that means an appointment this week, or the one that means a routine check. The doctor will ask exactly how it started, in seconds or over months, whether one eye or both are affected, whether there is pain, flashes, floaters, double vision or a missing part of the field; and will ask about diabetes, blood pressure, short sight, previous eye surgery and injuries, and about medication. It helps to test each eye separately beforehand, covering the other with a hand — that simple step tells the doctor a great deal.

Plenty of other questions are convenient to settle online: how to read an ophthalmologist's report, why particular drops have been prescribed and why they must not be dropped in glaucoma, how to prepare for cataract surgery, how often to have the retina checked with diabetes, what to do about floaters that have appeared out of nowhere. The limits are clear: neither the pressure inside the eye nor the retina can be examined remotely, and a consultation cannot replace being seen in person. And with sudden loss of vision, severe eye pain or a missing visual field, there is no consultation to wait for — that is an ambulance.

This material is for information only and does not replace medical advice.