What Causes Double Vision, and When Should an Eye Specialist Check It?

You are reading a document, and suddenly some text appears to split into two. Or perhaps a familiar face seems to have a faint “ghost” beside it. Double vision can be unsettling, especially because it may appear suddenly and interfere with driving and reading. While some causes are minor and treatable, others can point to nerve, muscle or brain-related problems that need prompt attention.

This blog explains what double vision is, its main causes and when you should consult an eye specialist.

What Is Double Vision?

Double vision, medically called diplopia, means seeing two images of a single object. It may be constant, intermittent, vertical, horizontal, or diagonal, and it can affect one or both eyes. An eye specialist can accurately assess the pattern of these visual changes.

The key distinction is whether the double image disappears when one eye is covered:

Type	What it means	Common source
Monocular diplopia	Doubling continues when one eye is covered	Problem within that eye, such as dry eye, astigmatism, cataract, or retinal surface changes
Binocular diplopia	Doubling disappears when either eye is covered	Misalignment between the two eyes, often involving eye muscles or the nerves controlling them

Common Causes of Double Vision

Double vision is a symptom which can be caused by an easily corrected refractive error or a neurological cause. There are many causes of double vision such as:

Eye-related causes

• Uncorrected refractive error: Astigmatism can cause ghosting/double vision, especially in one eye.
• Dry eye: Uneven tear film causing distortion.
• Cataract: Clouding of the natural lens causing images to be double or blurred.
• Distorted cornea: Scarring, keratoconus, or other corneal disease can cause distorted vision.
• Retinal membrane disease: Can occasionally cause double vision.

Muscle/nerve related causes

• Strabismus: Longstanding eye deviation might become apparent when the brain can no longer compensate.
• Cranial nerve palsy: Diabetes, poor circulation, inflammation or pressure on nerves can cause weakness in eye muscles.
• Thyroid eye disease: Inflammatory changes in the thyroid can cause swelling around the eye, limiting eye movement.
• Myasthenia gravis: An autoimmune disease which can cause variable double vision.
• Trauma: Eye or head trauma can cause damage to muscles, nerves or the bony eye socket.

Neurological and Systemic Causes

Less commonly, double vision may be linked with migraine, stroke, brain tumours or multiple sclerosis. They may increase pressure inside the skull or cause giant cell arteritis. Because these causes can affect the brain, nerves or blood vessels, double vision requires prompt assessment from an eye doctor.

When Should You See an Eye Specialist?

Any new, persistent or recurring double vision needs a professional eye assessment, even if it improves on its own. Seek emergency care from an eye specialist immediately if double vision occurs with any of the following:

• Sudden severe headache
• Eye pain or pain when moving the eye
• Drooping eyelid or a change in pupil size
• Weakness, numbness, or paralysis on one side of the body
• Trouble speaking, swallowing, walking, or maintaining balance
• Confusion, dizziness, nausea or vomiting
• Recent head or eye injury
• Bulging of one or both eyes
• Sudden vision loss or a dark curtain across vision

What an Eye Doctor May Check?

An eye doctor will usually begin with a detailed history: when the doubling started, whether it is constant or intermittent, whether it affects one or both eyes and whether it is associated with headaches, trauma, diabetes, thyroid disease or muscle weakness.

A typical eye assessment may include:

• Visual acuity and refraction testing
• Cover test to detect eye misalignment
• Assessment of eye movements in different directions
• Pupil examination
• Slit-lamp examination of the cornea, lens, and front of the eye
• Retinal and optic nerve evaluation
• Prism testing, where appropriate

Final Thoughts

Double vision should be assessed cautiously because it can stem from something as simple as astigmatism or as significant as a nerve- or brain-related condition. A useful first clue is whether the doubling remains when one eye is closed, but only a comprehensive examination can identify the true cause. If the symptom is new, persistent, or accompanied by headache, pain, drooping eyelid, weakness or difficulty speaking, seek urgent medical care from an eye specialist.