Reviewed by Jonathan Miu, B Optom / B Sci, Kellyville Optical
Double vision (diplopia) is when you see two images of a single object, side by side, one above the other, or at an angle. It most often happens when the eyes aren't lined up, so each eye sends the brain a slightly different picture. Double vision that comes on suddenly needs urgent medical care. Long-standing double vision from a stable eye misalignment can often be eased with prism glasses.
This guide explains what causes double vision, when it's an emergency, and how prism glasses can help your eyes work together again.
Double vision at a glance
- What it is: seeing two images of one thing, all the time or only some of the time.
- One eye or both: if the double image stays when you cover one eye, it’s usually an eye problem such as cataract, dry eye or astigmatism. If it disappears when you cover either eye, the eyes aren’t lining up.
- Common causes: an eye turn (strabismus), eyes that drift when tired, convergence problems, cataract, or uncorrected astigmatism.
- Serious causes: stroke, nerve problems, head injury, diabetes or thyroid eye disease.
- Get urgent care if: double vision starts suddenly, or comes with headache, weakness, numbness, a drooping eyelid or trouble speaking. Call 000 or go to an emergency department.
- Prism glasses: glasses with a prism built into the lenses that move the image so both eyes can line up. They can ease double vision from a stable misalignment.
- Getting checked: an eye test with your optometrist can find the cause and the right treatment, or a referral if you need one.
Frequently asked questions
Double vision is most often caused by the eyes not lining up, for example from an eye turn (strabismus), eyes that drift when tired, or weak convergence for close work. Other causes include cataract, dry eye and uncorrected astigmatism. Less commonly, it comes from a nerve, muscle or brain problem such as stroke, diabetes, thyroid eye disease or a head injury.
Sudden double vision can be caused by a stroke, bleeding or swelling in the brain, a nerve palsy, a head injury, or conditions such as diabetes or myasthenia gravis. Because some of these are serious, sudden double vision needs urgent medical care. Call 000 or go to a hospital emergency department, especially if you also have a headache, weakness, numbness, a drooping eyelid or trouble speaking.
Double vision in one eye (monocular diplopia) stays even when you cover the other eye. It’s usually caused by a problem in the eye itself, such as cataract, dry eye, uncorrected astigmatism or an irregular cornea like keratoconus. It’s rarely a sign of a brain problem, but it should still be checked by an optometrist.
In older adults, double vision is often caused by cataract, dry eye, or eye muscles that become weaker or less coordinated with age. Diabetes, high blood pressure and thyroid eye disease can also affect the nerves and muscles that move the eyes. Stroke is a serious cause, so any sudden double vision needs urgent care.
Tiredness can bring on double vision in people whose eyes tend to drift apart (a phoria). When you’re rested, the eye muscles keep the eyes together, but after long days, screens or reading they may not manage. If you notice double vision when tired, an eye test can check how well your eyes work as a team.
Glasses can often help double vision. If it’s caused by the eyes being out of line, prism glasses can move the images so the brain can join them into one. If it’s caused by astigmatism or an out-of-date prescription, updated glasses may be enough. Double vision from cataract or a medical condition needs treatment of that cause.
Prism glasses can relieve double vision caused by the eyes being slightly out of line, by moving the images so the brain can join them into one. They don’t change the eye muscles or cure the cause, so the double vision usually returns without the glasses. Prisms work best for smaller, stable misalignments.
When you first wear prism glasses, you may notice mild dizziness, a feeling that the floor is tilted, eye strain or headaches. These generally settle down within a few days as your brain adjusts, but may take up to two weeks for more sensitive people. If discomfort continues beyond that, or your vision feels worse, see your optometrist so the prescription and fit can be checked.
Prism glasses can help some people whose symptoms come from a measurable eye alignment problem, such as double vision or eye strain when reading. Binocular vision dysfunction is a broad term, and symptoms like dizziness or headaches have many possible causes. A thorough binocular vision assessment helps work out whether prisms are likely to help you. Read more about binocular vision.
Getting used to prism glasses generally takes a few days, but may take up to two weeks for more sensitive people. Wear them as your optometrist advises, usually full time, so your brain can adapt. Taking them on and off often can make adjusting slower.
Not always. Some people need prism long term, while others need it for a period, for example while double vision from a nerve or muscle problem settles. Your optometrist will recheck your eyes to see whether the prism still needs changing.
Yes. Prism can be added to single vision, bifocal and progressive (multifocal) lenses, and to prescription sunglasses. Accurate measurements and a well-fitted frame are especially important, because small errors in where the lenses sit can change the prism effect.
Contact lenses generally can’t correct prism for eye alignment, because they move with the eye. If you need prism, glasses are usually the best option. Some people wear contact lenses for their main prescription and glasses with prism when they need them.
What is double vision?
Double vision (diplopia) is seeing two images of a single object. The second image may sit beside, above or at an angle to the first, and it may be there all the time or come and go, for example when you’re tired or looking in one direction.
There are two main types, and a simple test tells them apart. Cover one eye, then the other:
- Binocular double vision disappears when you cover either eye. It means the eyes aren’t lining up, so each eye sees the object in a slightly different place. This is the type prism glasses can help.
- Monocular double vision stays in one eye even when the other is covered. It usually comes from the eye itself, such as cataract, dry eye, astigmatism or keratoconus.
What causes double vision?
Eye alignment problems are the most common cause of binocular double vision:
- Strabismus: one eye turns in, out, up or down
- Phoria: the eyes drift apart when tired, causing double vision or strain
- Convergence insufficiency: the eyes struggle to turn in together for reading and close work
- Vertical imbalance: one eye aims slightly higher than the other
Eye conditions are the usual cause of monocular double vision:
- Cataract, which scatters light inside the eye
- Dry eye, which makes the front surface of the eye uneven
- Uncorrected astigmatism or keratoconus
Medical conditions can affect the nerves and muscles that move the eyes:
- Stroke or transient ischaemic attack (TIA)
- Diabetes or high blood pressure affecting the eye nerves (nerve palsy)
- Thyroid eye disease
- Head injury
- Myasthenia gravis or multiple sclerosis
When is double vision an emergency?
Double vision is an emergency when it comes on suddenly, or with other symptoms that can signal a stroke or another serious problem. Call 000 or go to a hospital emergency department straight away if you have new double vision with:
- A sudden severe headache
- Weakness or numbness in the face, arm or leg
- Trouble speaking or understanding
- A drooping eyelid or a large pupil
- Dizziness, loss of balance or vomiting
- Pain around the eye, or after a head injury
If your double vision is new but mild and you feel otherwise well, see an optometrist promptly so the cause can be checked.
How do prism glasses work?
A prism is a wedge-shaped piece of lens that is thicker at one edge (the base) than the other (the apex). Light passing through a prism bends towards the base, so the image you see shifts towards the apex. By placing the right amount of prism in the right direction, your optometrist can move the image to where your eye is actually pointing.
Normally, both eyes aim at the same point and the brain blends the two images into one. If the eyes are slightly out of line, the brain gets two images that don’t match. That causes double vision, or the eye muscles work extra hard to keep them together, which causes strain. Prism glasses move the images so the eyes can line up with less effort.
Who needs prism glasses?
Prism glasses may help people with:
- Double vision (diplopia) from a long-standing or stable eye misalignment
- Strabismus, where one eye turns in, out, up or down
- Phoria, where the eyes tend to drift apart when tired or when one eye is covered, causing strain
- Convergence insufficiency, where the eyes struggle to turn inwards together for close work
- Vertical imbalance, where one eye sits slightly higher than the other in how it aims
- Double vision after a stroke, head injury or nerve palsy, once it has been medically assessed. Temporary stick-on (Fresnel) prisms are sometimes used while things are still changing, and we can order these in if needed
Not every alignment problem needs prism. Some respond better to updated glasses, or need referral to an ophthalmologist.
What are the signs you might need prism glasses?
Signs that your eyes may not be working well together include:
- Seeing double, all the time or when tired
- Eye strain or headaches, especially after reading or screen work
- Words moving, swimming or overlapping on the page
- Losing your place or skipping lines when reading
- Closing or covering one eye to see more comfortably
- Tilting your head to see clearly
- Feeling dizzy or off-balance in busy places, such as supermarkets
These symptoms can have other causes too, so an eye test is the best way to find out what’s going on. Read more about digital eye strain.
How is prism measured and tested?
Your optometrist checks how your eyes work together as part of a thorough eye test. This may include:
- A cover test, watching how each eye moves when the other is covered
- Measuring how much your eyes drift and in which direction, using prism bars or other tests
- Checking how well your eyes turn in together for close work, and how easily they can recover
- Trialling prisms in front of your eyes to see whether they make vision clearer or more comfortable
If prism is likely to help, your optometrist prescribes the smallest amount that gives relief.
How to read prism on your glasses prescription
Prism is written on your prescription in prism dioptres (Δ), with a direction called the base:
- Base in (BI): base towards the nose
- Base out (BO): base towards the ear
- Base up (BU) and base down (BD): for vertical alignment
For example, “1Δ BI” in each eye means one prism dioptre, base in, for each lens. Prism is often split between both eyes so the lenses look balanced. Learn more in our guide to reading your eye prescription.
What to expect when you get prism glasses
Prism glasses look like ordinary glasses. Small prescriptions are built into the lens so no one can tell. Larger amounts can make one edge of each lens thicker, and a high-index lens or a smaller frame can help with this.
Accurate measurements matter. Your pupil distance and the height of the lenses are measured carefully, because a lens sitting even slightly off can change the prism effect. When you first wear them, you may notice the floor or walls look a little tilted, or feel slightly dizzy. This generally settles down within a few days, but may take up to two weeks for more sensitive people.
See your optometrist again if symptoms don’t improve, or if your double vision changes. Explore our specialty prescription lenses.
