There are several different types of diplopia (double vision), each classifying a slightly different form of the condition. Depending on the diagnosis, a patient’s double vision may be classified using more than one type.
 

Monocular vs. binocular

When a patient experiences double vision, the first thing a doctor will determine is whether it’s monocular or binocular diplopia. This can help indicate if the cause of double visionis related to a problem with the brain or within the eye itself and whether the underlying condition may be life-threatening.

Monocular double vision

Monocular diplopia is when a person experiences double vision — often as a ghost image — even if one eye is closed. There are a number of things that can cause monocular diplopia, including:

• Astigmatism— Refractive error (like nearsightedness or farsightedness) that affects how your eyes bend light. This can result in blurred or double vision at any distance.
• Cataract— Clouding of the eye’s lens caused by age, smoking, diabetes and/or steroid medication use. If the hazy film covers the lens of only one eye, monocular diplopia can result.
• Keratoconus— Abnormal thinning of the corneacauses the front of the eye to bulge forward, which leads to double vision. Keratoconus can affect one or both eyes and is typically caused by heredity or eye rubbing.
• Dry eye— If your eyes don’t provide enough tears to keep your eyes moist, moving and blinking the eyes can make them itch or sting, as well as cause blurred or double vision.
• Pterygium— When the mucous membrane that lines the whites of the eye thickens, it can create a bump or growth on the eyeball. If the bump spreads to the cornea, double vision can result.
• Lens dislocation — Eye trauma can cause ligaments that hold the lens of the eye in place to break. This allows the lens to move around and get out of place, resulting in double vision.

Binocular double vision

Unlike monocular diplopia, binocular or bilateral double vision affects both eyes and only occurs when both eyes are open. Misalignment of the eyes, the root cause of binocular diplopia, occurs when:

• The muscles around the eye (the extraorbital muscles) are weak or underdeveloped. The cranial nerves controlling the extraorbital muscles are affected by an underlying condition or disorder.
• The extraocular muscles control the direction of the eyes’ gaze, so when the muscles are weak or the nerves that control them are damaged, the eyes have difficulty focusing and tracking simultaneously.

Conditions that can cause binocular diplopia include:

• Strabismus— A very common cause of double vision, strabismus ( crossed eyes) typically begins in childhood as a result of weakened eye muscles. This causes misaligned or crossed eyes, which can result in double vision and lead to serious vision problems if medical treatment isn’t sought.
• Diabetes — The source of a number of vision problems, diabetic eye diseasecan cause nerve damage that leads to double vision and potential sight loss.
• Extraocular nerve damage — The nerves that control extraocular muscles can be affected by head trauma, infection, brain or eye tumor, stroke, multiple sclerosis and other neurological conditions.
• Graves’ disease— Linked to hyperthyroidism, Graves’ disease (also called thyroid eye disease) can cause the muscles within the eye socket to swell or thicken, which can lead to blurred or double vision in both eyes.
• Myasthenia gravis— This autoimmune disease hinders the nerves’ ability to communicate with the muscles, causing the muscles to tire and weaken. This makes it difficult for the muscles that control the eyes to work properly.

Horizontal, vertical or oblique

The terms horizontal, vertical and oblique diplopia describe the orientation of the two images, or how they are separated. However, it’s important to note that people rarely see a full, completely separate “double” of an object — there is typically overlap between the two images.

Horizontal double vision creates a side-by-side effect of the two images. Because they’re separated horizontally, if someone with horizontal diplopia is looking at a dog, they would see a faint, hazy duplicate or “double” of the dog to the right or left of the actual dog.

Vertical double vision presents a second image above or below the actual object. The images are separated vertically, causing one to look higher than the other.

Oblique double vision (also called torsional or diagonal diplopia), though less common, describes when the extra image is seen diagonally from the actual object. Because the images are separated diagonally, one appears kitty-cornered from the other.

Temporary, intermittent or constant

Diplopia can also be classified by the frequency and duration in which it’s experienced. Double vision can occur suddenlyand unexpectedly, and last for different lengths of time and to varying degrees of severity depending on the person and situation.

Temporary double vision is typically not a cause for alarm, as it can result from stress, fatigue and even intoxication. Neurotransmitters work to send messages from the brain to the body, and when you get overly tired or your alcohol level gets too high, that communication is delayed.

The delayed communication between the brain and the eye muscles can cause the muscles to weaken, resulting in temporary double vision.

Intermittent (reoccurring) double vision can be a symptom of an underlying condition with other fluctuating symptoms, including migraine headache, high blood pressure and dry eye syndrome. It can also be the side effect of certain medications.

Constant (ongoing) double vision may be the most concerning because it can be a sign of a neurological and/or potentially life-threatening issue, such as a head injury, brain tumor, aneurysm, stroke or multiple sclerosis.

Pathological vs. physiological

Though they aren’t mentioned as frequently as other classifications, the two overarching types of diplopia are pathological and physiological.

Pathological diplopia occurs when there is an abnormality in the visual system. It’s a symptom of another issue, which may be caused by problems in the brain, eye, or the muscles in nerves that control the eye. Causes of pathological diplopia include strabismus, cataracts, stroke, dry eye syndrome and brain aneurysm.

Physiological diplopia is a fundamental part of normal binocular vision, where objects outside a person’s focus area are seen as double. This phenomenon is actually used by eye doctors to test that the eyes are working together properly, and it can be used in vision therapy to treat strabismus.

When to see a doctor

If you experience double vision or any other dramatic change in vision, it’s important to contact your eye doctor as soon as possible for proper testing, diagnosis, and, depending on what’s most appropriate, diplopia treatmentor referral to another type of doctor for evaluation.

While short-term cases of diplopia — due to stress, fatigue or intoxication for example — usually aren’t cause for worry, the sudden onset of double vision should never be ignored . It may be signaling a life-threatening condition that requires urgent medical attention.