Symptom guide
Double vision: causes, red flags and when to get it checked
Double vision (diplopia) that disappears when you cover one eye comes from a problem with eye alignment, such as a weak eye muscle or a nerve that controls it. Double vision that persists with one eye covered usually points to the eye itself, such as cataract or astigmatism. New double vision should always be assessed promptly.
Last checked: 2026-10-03
Key facts
At a glance.
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Cover test
If double vision goes when either eye is covered, the problem is alignment; if it stays with one eye covered, it is usually the eye itself.
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New onset
New double vision should be assessed within a day or so; with other neurological signs, call 999.
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Common causes
Diabetes, high blood pressure, thyroid eye disease, previous squint and age-related muscle weakness are frequent.
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Warning combination
Double vision with drooping lid, a large pupil or severe headache can signal an aneurysm pressing on a nerve.
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Who to see
Emergency eye service or GP urgently; orthoptics, ophthalmology and neurology afterwards.
What this symptom usually means
Normally the two eyes point at exactly the same spot and the brain fuses the images. Double vision occurs when the eyes are misaligned or when one eye distorts the image. Which of these is happening determines the cause and the urgency.
Possible causes, from most to least common
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Monocular causes (persists with one eye covered): astigmatism, dry eye, cataract or lens irregularities, and macular problems.
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Decompensated childhood squint, where a long-standing eye misalignment becomes noticeable in adulthood.
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Cranial nerve palsy (third, fourth or sixth nerve) from diabetes, high blood pressure or vascular disease, often resolving over weeks to months.
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Thyroid eye disease, which stiffens eye muscles and causes double vision, often with bulging or red eyes.
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Myasthenia gravis: fluctuating double vision and drooping eyelids that worsen with fatigue through the day.
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Multiple sclerosis, causing double vision from brainstem pathways.
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Stroke or transient ischaemic attack, with other symptoms such as speech or limb weakness.
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Aneurysm, tumour, head injury or raised intracranial pressure, which are less common but important.
Features that point to something more serious
Sudden onset, a drooping eyelid, a dilated or unequal pupil, severe headache, neck pain, pain on eye movement, facial or limb weakness, speech difficulty, vomiting, or double vision after head injury raise concern.
Red flags: when to seek help
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Call 999: sudden double vision with facial droop, slurred speech, limb weakness, severe headache or confusion.
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Same-day emergency eye service or A&E: sudden double vision with a drooping lid or a large pupil, or after head injury.
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Within a day or two: any new double vision in adults, particularly with diabetes, high blood pressure or age over 50, or with jaw pain and scalp tenderness (giant cell arteritis).
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Routine: long-standing double vision that has not changed.
What a doctor will ask and examine
Questions cover onset, whether it is one eye or both, horizontal or vertical, constant or fluctuating, associated headache, droopy lid, fatigue, thyroid symptoms, diabetes, blood pressure, injury and neurological symptoms. Examination includes the cover test, eye movements in all directions, pupils, lids, the retina, blood pressure, and a neurological assessment.
Tests that are commonly used and why
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Orthoptic assessment: measures misalignment and tracks recovery.
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Blood tests: glucose or HbA1c, inflammatory markers, thyroid function, and myasthenia antibodies where indicated.
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MRI or CT brain with angiography: for acute palsies with red flags or atypical features.
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Tensilon-type or ice-pack tests and nerve studies: for suspected myasthenia.
Self-care and the usual route
Do not drive with double vision. Temporarily patching one eye can relieve symptoms, as can prism glasses prescribed by an orthoptist for stable misalignment. Treat the underlying cause, such as blood pressure and diabetes control. Many nerve palsies from vascular causes improve over 3 to 6 months.
On the NHS, an eye casualty, GP or A&E assesses urgently, with orthoptics, ophthalmology or neurology to follow. Private ophthalmology and neurology consultations are available directly.
Frequently asked
Questions people ask.
Quick, plain-English answers to the questions we hear most.
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Is double vision always serious?
Not always, but it should be assessed promptly because some causes, such as stroke or an aneurysm, are urgent. Monocular double vision is more often a simple eye problem.
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What does covering one eye tell me?
If the double image disappears with either eye covered, the problem is misalignment. If it persists with one eye covered, it is usually an eye issue such as astigmatism or cataract.
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Can diabetes cause double vision?
Yes. Diabetes can damage the small vessels supplying eye-movement nerves, causing temporary double vision that usually recovers.
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Is a drooping lid with double vision urgent?
Yes. It can indicate a third nerve palsy, which in some cases is caused by an aneurysm, and needs same-day assessment.
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What is myasthenia gravis?
An autoimmune condition causing muscle weakness that varies and worsens with activity, often starting with double vision and droopy eyelids.
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Will prisms help?
Yes, for stable misalignment. Prism glasses bend light so the images line up, often relieving double vision.
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Can I drive?
No. Double vision makes driving unsafe, and the DVLA must be informed of persistent double vision.
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How long does it last?
It depends on the cause. Nerve palsies from vascular disease often improve in weeks to months, while other causes require specific treatment.
Where this comes from
Sources and checking.
This guide is for information, not medical advice.
Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.
Published 2026-10-03, next review 2027-10-03.
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NHS: Double vision (nhs.uk/conditions/double-vision/)
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NHS: Myasthenia gravis (nhs.uk/conditions/myasthenia-gravis/)
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GOV.UK: Diplopia and driving (DVLA guidance for medical professionals)