Quick answer. Double vision (diplopia) can be caused by problems in the eye itself (such as cataract or refractive error) or by issues affecting the nerves and muscles — including cranial nerve palsy, thyroid eye disease, myasthenia gravis or a stroke. The biggest red flag is sudden double vision, which can signal a stroke and needs 995 immediately.
Double vision, known medically as diplopia, means seeing two images of a single object — they may appear side by side, one above the other, or overlapping. It is an important symptom because, while some causes are relatively benign, sudden double vision can be a warning sign of a stroke or another serious neurological problem.
A key first step is working out whether the double vision is monocular (present even when one eye is covered) or binocular (only present with both eyes open). This distinction helps point to the cause. In Singapore, anyone with sudden-onset double vision, especially with other neurological symptoms, should treat it as an emergency. This guide explains the main causes and which specialist can help.
At a glance
| Possible cause | Typical features | How urgent |
|---|---|---|
| Stroke | Sudden double vision, weakness, slurred speech, facial droop | Emergency — call 995 |
| Cranial nerve palsy | Eye misalignment, sometimes with headache or eye pain | Urgent assessment needed |
| Thyroid eye disease | Bulging eyes, swelling, often with thyroid problems | See a specialist soon |
| Myasthenia gravis | Double vision and drooping eyelids, worse when tired | See a specialist soon |
| Cataract / refractive error | Monocular double vision, blurring, glare | Routine eye review |
Red flags: when to seek urgent care
Sudden double vision can be a sign of a stroke or other serious condition. Call 995 immediately if double vision comes on suddenly, especially with any of the following stroke warning signs:
- Face drooping on one side.
- Arm or leg weakness or numbness, usually on one side of the body.
- Slurred or difficult speech, or trouble understanding others.
- Sudden severe headache, dizziness, loss of balance or difficulty walking.
Also seek urgent medical care if double vision is accompanied by severe eye pain, a severe headache, drooping of an eyelid, a dilated pupil, or recent significant head injury. When in doubt with sudden double vision, treat it as an emergency — time is critical in stroke.
Monocular vs binocular double vision
An important step is to check whether the double vision is monocular or binocular by covering one eye at a time:
- Monocular double vision: persists in the affected eye even when the other eye is covered. It is usually caused by a problem within the eye itself, such as cataract, refractive error or dry eye, and is generally less urgent.
- Binocular double vision: disappears when either eye is covered and only occurs with both eyes open. It is caused by the eyes not aligning properly, often due to a problem with the nerves or muscles that move the eyes — and is more likely to need urgent assessment.
This simple test gives doctors valuable clues about the cause.
Cranial nerve palsy and stroke
The movement of each eye is controlled by muscles supplied by cranial nerves. A cranial nerve palsy occurs when one of these nerves is not working properly, causing the eyes to become misaligned and producing binocular double vision. Causes range from poor blood supply (more common with diabetes or high blood pressure) to pressure from a swelling or, rarely, an aneurysm.
A stroke affecting the parts of the brain that control eye movement or vision can also cause sudden double vision. Because a stroke is a time-critical emergency, any sudden double vision — particularly with weakness, slurred speech or facial droop — means you should call 995 at once.
Thyroid eye disease
Thyroid eye disease is an autoimmune condition often associated with an overactive thyroid gland (Graves’ disease). It causes inflammation and swelling of the muscles and tissues around the eyes.
This swelling can push the eyes forward (bulging eyes) and restrict the eye muscles, leading to double vision, particularly when looking in certain directions. Other features include red, gritty, watery eyes and swelling of the eyelids. If you have a known thyroid problem and develop double vision, you should be assessed promptly by an eye specialist.
Myasthenia gravis and eye-related causes
Myasthenia gravis is a condition where the signals between nerves and muscles are disrupted, causing muscle weakness that typically worsens with activity and improves with rest. It often affects the eye muscles first, causing double vision and drooping eyelids that are usually worse later in the day or when tired.
Other eye-related causes of double vision include cataract (clouding of the lens) and refractive errors, which tend to cause monocular double vision and are usually correctable with glasses or surgery. Because the causes range from minor to serious, double vision should always be properly assessed.
Which specialist should you see?
For double vision, an eye specialist (ophthalmologist) can examine your eyes, check alignment and identify causes such as cataract, refractive error or thyroid eye disease. You can compare and find a doctor here: best eye specialists in Singapore.
If a nerve or brain problem is suspected — for example a cranial nerve palsy, myasthenia gravis or stroke — a neurologist may be needed: best neurologists in Singapore. Remember that sudden double vision, particularly with weakness, slurred speech or facial droop, is a possible stroke — call 995 or go straight to A&E rather than waiting for an appointment.
What to expect when you are assessed
An eye or neurology assessment for double vision is usually painless and methodical. The doctor will first ask whether the double vision is present with one eye covered (monocular) or only with both eyes open (binocular), as you can check at home, and how it began. They will examine your eye movements, alignment, pupils and eyelids, and may test your vision and the back of the eye. Depending on the findings, brain or eye imaging and blood tests may be arranged.
How to prepare and stay safe
- Note when the double vision started, whether it came on suddenly, and whether the two images are side by side, one above the other, or tilted.
- Mention any headache, eyelid drooping, eye pain, weakness, slurred speech or recent head injury.
- Bring your current glasses and a list of your medical conditions and medicines.
- Do not drive while you have double vision, as your depth perception and judgement of distance are affected; arrange another way to travel.
Common questions
Does covering one eye really help work out the cause?
Yes. If the double vision disappears when either eye is covered, it is binocular and usually points to a problem with eye alignment, nerves or muscles, which often needs prompter assessment. If it remains with one eye covered, it is monocular and more often due to a problem within that eye, such as cataract or refractive error.
Can tiredness or alcohol cause temporary double vision?
Brief double vision can occur with extreme tiredness or alcohol and settles once you rest or sober up. New, persistent or sudden double vision should never be assumed to be due to tiredness and needs medical assessment.
Related guides
- Best Eye Clinics in Singapore — our editorial shortlist
- eye clinics directory
- Coughing up blood
- Vomiting blood
- Wheezing
Sources
- NHS — Double vision (diplopia)
- HealthHub Singapore — Stroke
- SingHealth — Thyroid Eye Disease
- Mayo Clinic — Double vision
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