Double vision is seeing two images of an object instead of one. Also called diplopia , it can occur suddenly and have serious causes.

Sometimes, the term "ghost image" is used to describe the less dominant image seen during diplopia.

Types of diplopia include:

• Horizontal diplopia : Double vision where the two images are separated laterally (horizontally).

• Vertical diplopia : Double vision where one image is higher than the other.

• Monocular diplopia : Double vision that persists in one eye when the other eye is closed.

READ MORE: Types of diplopia

Diplopia can be a symptom of very serious health problems. See a physician or an eye doctor near you immediately if you start experiencing double vision.

Diplopia can be considered a condition itself or a symptom of an underlying condition.

It also can occur separately or combined with other symptoms. When experiencing double vision, it’s possible to also experience:

• Headache

• Migraine

• Nausea

• Dizziness

• Droopy eyelids (ptosis)

• Loss of balance

• Eye weakness

• Eye misalignment ( strabismus ) that affects one or both eyes

• Pain in or around eyes triggered by eye movement

Double vision and headaches or migraines can sometimes be a “chicken or the egg” situation — meaning it’s difficult to determine whether the diplopia caused the headache or the headache caused the diplopia.

If you suddenly experience double vision — with or without any of the other symptoms above — seek medical attention or see your eye doctor as soon as possible.

Temporary episodes of double vision can happen for many reasons, including drinking too much alcohol or being overly tired. This type of short-term double vision is usually not cause for worry.

But if diplopia is long-lasting or keeps coming back, causes can include:

• Stroke, head injury, brain tumor, brain swelling or brain aneurysm. A head or brain injury , tumor, stroke or related condition can cause diplopia. [Read more about double vision after a stroke .]

• Eye problems. Eye conditions such as keratoconus , cataracts and even dry eyes can cause double vision. (Most cases of monocular diplopia are caused by eye problems.)

• Refractive surgery. If you have had LASIK, PRK or any refractive surgery to help you see better without glasses or contacts, you may experience some minor double vision or ghost images because of changes to your corneas . An irregular corneal surface, caused by the surgery itself or by dryness, may cause light rays to scatter instead of focusing properly. Diplopia from refractive surgery usually clears up within weeks or months. But in some cases, a second laser vision correction procedure might be necessary.

• Cranial nerve palsies. Double vision also can be caused by paralysis or loss of coordination of one or more muscles that control the position and teaming of the eyes due to a cranial nerve palsy. Cranial nerve palsies can be caused by diabetes, head injury , tumor, multiple sclerosis , meningitis, high blood pressure, blockage in an artery or an aneurysm.

READ MORE: Causes of double vision

Diplopia and strabismus

The ability to see a single image with two eyes involves a complex system of muscles, nerves and other eye parts.

When two eyes correctly and accurately point and focus at the same time, we see only one image of the world. When two eyes point and focus differently from each other, diplopia may occur.

Some people are born with eyes that are not properly aligned — a condition called strabismus . Eyes can be crossed inward or turn outward. One eye can even go up while the other goes down.

If you have strabismus, you will see double if your brain allows it, because each eye sees a different thing at the same time. But the brain usually adapts by shutting down or ignoring the information from one eye. This is called suppression , which can lead to amblyopia (lazy eye). Surgery or vision therapy can help many people with strabismus.

Generally, treatments for double vision can include surgery, vision therapy, prism in the glasses prescription or medications.

Double vision can occur when the eyes look in separate directions because of cranial nerve palsies, strabismus or other reasons.

Again, it's important to see an eye doctor immediately if you experience diplopia. Depending on its cause, your eye doctor may be able to immediately treat the double vision or will recommend a specialist (such as a neurologist or neurosurgeon).

If you have sudden double vision that you ignore and then it goes away over a long time period, this may mean your brain has tuned out one of the images (suppressed it). Although this is certainly more comfortable and bearable for you, it's not a good sign. Suppression could be masking a serious problem that needs treatment.

Some conditions causing double vision are difficult, if not impossible, to fix. Some strokes and nerve palsies cause fluctuating double vision that can't be measured accurately enough to correct.

In these cases, you may need a period of adjustment so that you can learn to live with the symptoms. Your eye doctor can help by prescribing glasses that have special prisms in the lenses to reduce the diplopia.

Other possible temporary treatments for diplopia include patching one eye for periods of time or prescribing special contact lenses.

When to see an eye doctor

Remember: The sudden onset of diplopia could signal a condition that may be a matter of life and death, such as a brain tumor or aneurysm. See an eye doctor immediately if you experience double vision unexpectedly.

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