Diplopia (Double Vision) - The Eye Care Advocate

Diplopia, universally known as double vision, is the simultaneous perception of two images of a single object. It is a clinical symptom that requires immediate investigation to determine its underlying cause.

Linguistic Context:
Type: Noun. UK Usage: Spelt “Diplopia.”

Diplopia (double vision) is a disturbing visual symptom where you can see two of everything. This that can be categorised as either monocular or binocular.

Monocular diplopia persists even when one eye is covered and is typically optical in origin, often caused by uncorrected astigmatism, early cataracts, or dry eye syndrome.

Conversely, binocular diplopia resolves completely when either eye is covered. This indicates a misalignment of the visual axes (strabismus), meaning the fovea of each eye is targeting a different location in space.

Binocular diplopia can arise from decompensating muscle imbalances, trauma, or severe neurological conditions such as cranial nerve palsies. The College of Optometrists advises that any sudden onset of binocular double vision requires an urgent, comprehensive evaluation to rule out acute neurological or systemic pathology.

Common tests to investigate diplopia include the Cover Test and Ocular Motility. Optometrists and optometry students wishing to practice these skills will benefit from our cover test and ocular motility simulators, available to all members in The Eye Care Advocate Community.

Frequently Asked Questions About Diplopia (Double Vision)

What is diplopia and what causes it?

Diplopia is the medical term for seeing two images of a single object instead of one. It happens when your eyes are not working perfectly as a team, or when the light entering your eyes isn’t focusing correctly on the retina.

The two images might be side-by-side, one on top of the other, diagnonally separated, or slightly overlapping.

It can be a temporary issue caused by something like dry eyes or tiredness, or it can be caused by more significant issues with your eye muscles or nerves.

Any new double vision requires urgent investigation to rule out significant underlying disease.

Why does my double vision go away when I close one eye?

This is a very important distinction. If the double vision disappears when you cover either eye, it is called binocular diplopia. This means your eyes are physically misaligned and are pointing in slightly different directions.

If you still see double even with one eye closed, it is monocular diplopia, which is usually caused by an issue within the eye itself, such as a cataract, severe astigmatism, or a dry eye.

Can being tired or stressed make me see double?

Yes. Many people have a tiny “hidden” squint (called a heterophoria) that their brain normally works hard to keep under control.

When you are very tired, unwell, or under intense stress, your brain might struggle to maintain this alignment. This causes your eyes to drift slightly, leading to intermittent double vision.

However, because it can be hard to tell the difference between “tiredness” and a more serious problem, any new double vision should always be checked by an eye care professional.

What is the clinical difference between comitant and incomitant diplopia?

This is a foundational concept in binocular vision. Comitant diplopia means the angle of the eye turn (and the distance between the two images) remains the same regardless of which direction the patient looks. This is common in long-standing childhood squints.

Incomitant diplopia is much more concerning; it means the double vision gets significantly worse or the images move further apart when the patient looks in a specific direction. This is a classic sign of a “paralytic” squint, usually caused by a cranial nerve palsy or a muscle restriction.

How do you use the ‘Cover Test’ to investigate diplopia?

The Cover Test is your most powerful tool for objective assessment. By covering one eye and watching for any corrective movement in the other, you can determine if a “heterotropia” (a manifest squint) is present.

If you then remove the cover and see the eye move back into position, you are looking at a “heterophoria” (a latent squint).

For more information on the cover test, please see my article on The Cover Test, or use my cover test simulator as part of The Eye Care Advocate Community membership.

Medical Disclaimer: This glossary entry is intended for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment. If you are experiencing changes in your vision or ocular discomfort, please consult a qualified optometrist or your primary care provider immediately.

UK Authority Resources & Clinical Guidelines:
NHS England | NICE Guidelines | The College of Optometrists | Association of Optometrists (AOP) | General Optical Council (GOC)