Double vision

Double vision is an uncommon complaint, and it nearly always means that a precision mechanism has broken somewhere: either the optics of the eye itself, or the coordinated work of the six muscles that turn the eyeballs, or the nerves and parts of the brain that command those muscles. Double vision that comes on suddenly is treated as an emergency until the cause is known. That list includes stroke, a brain aneurysm, myasthenia, inflammation of the arteries of the head and raised pressure inside the skull — conditions where hours count. The formula "usually nothing serious, get it checked when you can" does not belong here: it costs sight, and sometimes life.

The first thing to do: cover one eye

This simple check takes five seconds and splits the whole subject in two.

Cover one eye with your hand, then the other, and watch what happens to the double image.

• The doubling disappears whichever eye you cover — so each eye on its own sees normally, and the problem is that together they are pointing in slightly different directions. This is binocular double vision, and it is almost always about nerves and muscles rather than the eye itself. This is the version that can be urgent.
• The doubling stays in the uncovered eye — one eye is seeing two images by itself. This is monocular double vision, and the cause lies in that eye: the cornea, the lens, the tear film. It is almost never an emergency, but it needs an eye specialist.

Other details are worth noticing too: whether the images separate side by side, one above the other, or tilted relative to each other; whether it doubles all the time or only when looking in one particular direction, at distance or close up; whether it worsens towards evening or stays the same all day. All of that narrows the range of causes before any test is done, so make a note of it.

Double vision with both eyes: what is ruled out first

When the doubling disappears on covering either eye, the doctor looks for what has broken the coordination of movement. The range of causes is limited and well known, and the search starts with the ones where hours matter.

• Stroke or a transient ischaemic attack in the brainstem. Double vision can be the first sign, and for a few minutes the only one, with dizziness, slurred speech, arm weakness and difficulty swallowing appearing alongside it.
• An aneurysm of a brain artery. Particular alarm is warranted by the combination of double vision, a drooping eyelid and a dilated pupil on the same side, all the more so with severe pain in the eye or head. That is how an aneurysm pressing on the nerve that moves the eye announces itself, and it means going to hospital straight away.
• Giant cell arteritis, inflammation of the arteries of the head in older people. Double vision comes with pain in the temple, a tender scalp when brushing the hair, jaw pain on chewing, feeling generally unwell and, above all, the threat of sudden and irreversible loss of sight. Treatment here is started without waiting for every result.
• Raised pressure inside the skull. It gives a headache that is worse in the morning and on bending forward, nausea, brief greying-out of vision, and side-by-side double vision from strain on the nerve that turns the eye outwards.
• Injury. A fracture of the wall of the eye socket trapping a muscle, a head injury, a bleed. Double vision after a blow to the head or face calls for urgent assessment.
• Infection of the eye socket: a red, swollen, painful eye that moves badly, with a temperature. This is treated in hospital and tolerates no delay.

Causes that unfold more slowly

If nothing urgent turns up, the search shifts to illnesses that come on gradually or in waves.

• Myasthenia. Its recognisable feature is inconstancy: fine in the morning, and by evening or after exertion the doubling appears and the eyelid droops, improving again after rest. Weakness of facial expression, a nasal voice and difficulty chewing and swallowing often join in. It is not always a quiet illness: weakness of the breathing muscles is an emergency.
• Diabetes and other vascular factors. Loss of blood supply to a single nerve controlling eye movement is a common cause of sudden double vision in people with diabetes and high blood pressure. It usually hurts, the pupil is not dilated, and it settles by itself over weeks or months. But it cannot be told from an aneurysm by looking, so it still has to be investigated.
• Thyroid disease affecting the eye sockets. The eyes push forward, the lids do not close properly, the muscles thicken and move less well; the doubling builds up gradually and is usually worse on looking up.
• Multiple sclerosis, in which double vision occurs in young adults and often goes with impaired sideways eye movement, sometimes with one eye flickering.
• Tumours and inflammatory processes in the eye socket, the brainstem and the coverings of the brain, pressing on the nerve along its path.

Double vision in one eye: usually the eye itself

If the image still splits, or "doubles with a ghost", after the other eye is covered, the cause is optical. Light on its way to the retina is bent unevenly and produces two images instead of one, often a sharp one and a fainter one.

• Dry eye. The commonest and the most reversible cause: an uneven tear film distorts the image. It is noticeable towards evening, at a screen, in the wind and in air-conditioned rooms, and it clears for a second or two after blinking.
• Astigmatism, including astigmatism that glasses or contact lenses do not fully correct.
• Cataract. A clouded lens scatters light; besides the doubling there are haloes around street lights and a general sense of fog.
• Keratoconus — thinning and bulging of the cornea, which loses its proper shape. It usually begins in young adulthood, sight deteriorates quickly and glasses help less and less.
• A displaced lens, corneal scarring, the after-effects of eye surgery, and an unusually wide pupil in certain situations.

Monocular double vision is almost never an emergency, but it does not clear up on its own either: an eye specialist is needed to work out whether drops, glasses or surgery is the answer.

When to call an ambulance

The single emergency number across Europe is 112. Call, or go straight to an emergency department, if double vision has come on suddenly together with any of these:

• weakness or numbness of an arm, a leg or one side of the face; a drooping face; slurred speech;
• a severe headache unlike previous ones, especially with vomiting or a stiff neck;
• a drooping eyelid and a dilated pupil on one side, all the more so with pain in the eye;
• double vision after a head injury or a blow to the face;
• increasing weakness, difficulty swallowing, a sense of not getting enough air, or breathlessness at rest;
• pain in the temple, jaw pain on chewing and failing sight in an older person;
• a red, swollen, painful eye with a high temperature;
• sudden loss of vision or of part of the field of vision.

If the double vision came on suddenly but none of these is present, you still need to be seen the same day — not in a week's time and not "when there is a moment". Do not drive yourself.

Squint and double vision in a child

Children rarely complain of double vision: a small child simply does not know that they see differently from everyone else. Watch the behaviour instead:

• screwing up the eyes or covering one with a hand, especially in bright light;
• tilting or turning the head in order to look straight ahead;
• rubbing the eyes, complaining of headache, reading less well, missing when reaching for things;
• one eye clearly drifting outwards, inwards, up or down.

There is an important difference from adults: if a squint has been there since early childhood, the child usually does not perceive a double image, because the brain simply suppresses the picture coming from the turned eye. That spares the discomfort but creates another problem: the eye that has been switched off stops developing, and amblyopia — a lazy eye — sets in. It can only be recovered in childhood, while vision is still forming; afterwards the lost ground cannot be made up. So any persistent squint in a child should be seen by a doctor, even if the child is untroubled by it.

A squint or double vision that appears suddenly in a child, however, is an entirely different situation and a reason to seek help urgently. Behind it can be a tumour at the back of the skull, raised pressure inside the head, inflammation, injury or myasthenia.

What happens at the appointment and how it is treated

The assessment starts with questions: when it began, suddenly or gradually, whether the doubling is side by side or one above the other, which direction of gaze is worse, whether it hurts, whether it varies through the day, what came before it. Then the sharpness of vision is measured in each eye separately, along with eye movements in every direction, pupil reactions and eyelid position, and the back of the eye is examined — that shows whether the optic disc is swollen, a sign of raised pressure inside the skull. Orthoptic testing helps to pin down which particular muscle or nerve is failing.

After that it depends on the suspicion: blood tests, including inflammatory markers if arteritis is a possibility and glucose in mmol/L if diabetes is; antibody tests and electrical studies of nerve and muscle if myasthenia is suspected; MRI of the head and eye sockets; imaging of the blood vessels where an aneurysm has to be excluded.

Treatment is always aimed at the cause, and that is not an evasion: in arteritis it means steroids without delay; in myasthenia, specific drugs; in a nerve damaged by diabetes, patience along with control of blood sugar and blood pressure. While the cause is being dealt with, the double vision itself is managed with prism glasses or a temporary prism film on the lens, and with a small deviation by patching one eye for a while so that reading and walking are possible. If after some months the deviation has settled and stayed put, surgery on the eye muscles is discussed. Do not drive while the double vision persists, even if it only appears on looking to one side; ask a doctor when you can go back to driving, as the rules differ from country to country.

Online consultation

With double vision a remote appointment is above all useful for sorting: in a few questions the doctor will establish whether yours is monocular or binocular, sudden or gradual, and whether there are warning signs around it, and will tell you the thing that matters — set off now, be seen today, or book an eye appointment calmly. They will also explain which tests will be needed and in what order, help you make sense of reports you already have, and advise on dry eyes if that turns out to be the cause.

What cannot be done online: examining the back of the eye, testing eye movements and fitting prisms, all of which need hands and instruments. And most important of all — sudden double vision with a headache, a drooping eyelid, arm weakness, slurred speech, or after a head injury, needs an ambulance rather than a message thread.

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