Neuro-Ophthalmology · Patient Guide
Seeing double?
Cover one eye.
The answer splits in two.
Double vision, or diplopia, is one symptom with two completely different families of cause. If the second image vanishes when you cover either eye, your eyes are out of alignment — a muscle, nerve or orbit problem. If it stays with one eye open, the doubling is inside that eye — often a cataract, dry eye or an irregular cornea. Here is how the cover test sorts it, what each cause looks like, and which signs mean go today.
then the other
each eye
that drive them
Section 01 — Two Kinds of Double
One eye, or both?
That question comes first.
Normally each eye sends the brain its own image, and because both eyes point at exactly the same spot, the brain fuses them into one. Double vision appears when that system breaks — and it can break in two places: inside the optics of a single eye, or in the alignment between the two.
Clinicians call these monocular and binocular diplopia. The distinction matters more than any other detail, because the causes barely overlap. Monocular doubling is almost always an optical problem of the eye itself and is rarely dangerous. Binocular doubling comes from the muscles, nerves or orbit that aim the eyes, and some of its causes are neurological emergencies.
Double vision that disappears when either eye is covered is binocular: each eye is seeing clearly, but they are pointed at different places, so the cause lies in the eye muscles, the cranial nerves that control them (III, IV and VI), the neuromuscular junction, or the orbit. Double vision that persists when one eye is open on its own is monocular: light is being split inside that eye, most often by cataract, dry eye, uncorrected astigmatism or an irregular cornea. New binocular diplopia deserves same-day assessment; monocular diplopia deserves an eye examination, but rarely an emergency department.
Section 02 — Interactive
Try the cover test
on a model pair of eyes.
Below, the gold sphere is the object both eyes should be looking at. Turn up the misalignment and the right eye drifts inward, as in a sixth nerve palsy. Then cover each eye and watch which image survives. Switch to the monocular case to see why covering the other eye does not help when the problem is inside one eye.
The model makes one point that surprises many patients: in binocular diplopia neither eye is “seeing double”. Each eye produces one sharp image. The doubling exists only when both are open, because the misaligned eye places the object on a part of its retina that the brain interprets as a different direction. That is why patching one eye is an effective — if blunt — temporary fix, and why prisms work: they bend light so the image lands back where the brain expects it.
Section 03 — Monocular Causes
When the doubling
lives inside one eye.
Monocular diplopia is an optics problem. Somewhere between the tear film and the retina, light is being split or scattered into more than one image. It is common, usually benign, and typically worse in dim light when the pupil widens and exposes more of the irregular optics.
| Cause | Typical clue | Usual fix |
|---|---|---|
| Cataract | Older adult, ghosting in one eye, glare at night, slowly worsening | Cataract surgery with an intraocular lens |
| Dry eye / unstable tear film | Varies with blinking, worse with screens or wind | Lubricants, lid hygiene, treating the cause |
| Uncorrected astigmatism | Smearing or ghost edges, improves with correct glasses | Updated spectacles or contact lenses |
| Keratoconus / corneal scar | Multiple ghost images, progressive in younger patients | Rigid or scleral contact lenses; cross-linking if progressive |
| Decentred or tilted IOL, capsule opacity | After cataract surgery, ghosting that was not there before | YAG capsulotomy, refraction, rarely IOL repositioning |
Very rarely, monocular doubling is not optical at all but arises in the brain’s visual areas — and then it is usually present in each eye separately. That pattern, or monocular diplopia with no optical explanation on examination, is a reason for further assessment.
Section 04 — Binocular Causes
Six muscles, three nerves,
one fragile alignment.
Each eye is steered by six extraocular muscles, driven by three cranial nerves: the oculomotor (III), the trochlear (IV) and the abducens (VI). A weakness anywhere along that chain — brainstem, nerve, neuromuscular junction, muscle or the orbit around it — breaks alignment and produces binocular diplopia.
| Cause | Pattern | What it can mean |
|---|---|---|
| Sixth nerve palsy | Horizontal, side-by-side images, worse looking toward the affected side and at distance | Often microvascular in older adults with diabetes or hypertension; also raised intracranial pressure, tumour, trauma |
| Fourth nerve palsy | Vertical or tilted images, worse looking down (reading, stairs); head tilt to compensate | Head trauma, microvascular, or a long-standing congenital palsy that has decompensated |
| Third nerve palsy | Eye turned down and out, droopy lid, sometimes a large pupil | With a dilated pupil, an aneurysm must be excluded urgently. Pupil-sparing forms are often microvascular |
| Thyroid eye disease | Vertical or horizontal, often worse in the morning; bulging, gritty eyes | Inflamed, stiff orbital muscles — inferior and medial recti most often |
| Myasthenia gravis | Variable, fatigable diplopia with drooping lids, worse later in the day | Autoimmune neuromuscular junction disease; can generalise to swallowing and breathing muscles |
| Decompensated squint | Gradual, intermittent, often with a childhood history of a turn or patching | A lifelong deviation the brain can no longer suppress or fuse |
| Orbital fracture or mass | After facial trauma, or with bulging and restricted movement | Trapped muscle or space-occupying lesion in the orbit |
per eye
controlling them
per prism dioptre
In a prospective multicentre study of adults aged 50 and over with an isolated third, fourth or sixth nerve palsy, Tamhankar and colleagues found that most were microvascular, but roughly one in six had another cause on MRI — including tumours, stroke and aneurysm. That finding is the main reason many neuro-ophthalmologists no longer simply watch and wait on a presumed “diabetic” palsy without imaging.
Section 05 — Red Flags
When double vision
means go today.
Most double vision is not an emergency, but new binocular diplopia can be the first sign of a problem that is time-critical. Go to an emergency department, not a routine clinic, if double vision comes with any of the following.
Section 06 — Treatment
Fix the cause first,
then the image.
Treatment depends entirely on which family the diplopia belongs to. The aim is two-fold: treat whatever is driving it, and meanwhile let the patient function with one clear image.
Section 07 — After Cataract Surgery
Double vision after
cataract surgery.
Cataract surgery usually cures monocular ghosting. Occasionally, new double vision appears afterwards, and the cover test again sorts it.
If it is binocular, common explanations include a long-standing small squint that a poor-seeing eye had been hiding, a large change in prescription between the two eyes (anisometropia, which gives each eye a different image size), or, uncommonly, muscle effects related to the local anaesthetic block. If it is monocular, the usual suspects are posterior capsule opacification, residual astigmatism, dry eye or, rarely, a decentred lens. Most are treatable, which is why new doubling after surgery is worth reporting rather than waiting out.
A 68-year-old notices that car headlights at night each have a faint twin, and that street signs look “shadowed”. Covering the left eye does nothing; covering the right eye removes the twin. The doubling is monocular and from the right eye. Through a pinhole it fades. Examination finds a cortical cataract in that eye, and after uncomplicated cataract surgery the ghosting resolves.
Section 08 — FAQ
Frequently asked questions
about double vision.
Monocular double vision persists when the good eye is covered: the doubling comes from inside one eye, usually the cornea, lens or tear film, so a cataract, dry eye, astigmatism or keratoconus is the typical cause. Binocular double vision disappears when either eye is covered: each eye sees a single clear image, but the two eyes are not pointing at the same place, so the brain receives two images it cannot fuse. That points to the eye muscles, the nerves that drive them, or the orbit.
Look at a distant object and cover one eye, then the other. If the double image vanishes when either eye is covered, the diplopia is binocular. If it persists when one particular eye is open on its own, it is monocular and coming from that eye. Looking through a pinhole often reduces or removes monocular doubling caused by the cornea or lens. This is a sorting test, not a diagnosis, and it never replaces an examination.
New binocular double vision should be assessed urgently, the same day, especially with a droopy eyelid, a large pupil on one side, severe headache, weakness, numbness, slurred speech, trouble swallowing, unsteadiness, jaw pain on chewing in someone over 50, or after a head injury. A third nerve palsy with a dilated pupil can signal an expanding brain aneurysm, and sudden diplopia can be a sign of stroke. If these features are present, go to an emergency department rather than waiting for a clinic appointment.
Yes. Cataract is one of the commonest causes of monocular double vision in older adults, because uneven clouding inside the lens splits light into more than one image. It typically shows up as ghosting or a faint second image in one eye that does not go away when the other eye is covered. Cataract surgery usually resolves this kind of doubling, because the cloudy lens is replaced with a clear intraocular lens.
Often, if the cause is microvascular, meaning a small-vessel problem linked to diabetes, high blood pressure or high cholesterol. Many such fourth and sixth nerve palsies, and pupil-sparing third nerve palsies, improve substantially over weeks to a few months. But a meaningful minority of presumed microvascular palsies turn out to have another cause, so recovery is something to monitor, not assume, and many clinicians have a low threshold for brain imaging.
Treatment follows the cause. Monocular diplopia is treated at the source: lubrication for dry eye, glasses, contact lenses for irregular corneas, or cataract surgery. Binocular diplopia may be managed with temporary press-on or ground-in prisms that shift one image onto the other, patching or a frosted lens while a palsy recovers, botulinum toxin injection for some muscle imbalances, and strabismus surgery for stable deviations. Underlying conditions such as thyroid eye disease or myasthenia gravis need their own treatment.
References & Evidence Base
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