Double Vision

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Are you seeing double? Double vision is the simultaneous perception of two images of a single object. The images may be displaced horizontally, vertically, diagonally, or rotationally in relation to each other.

What is double vision?

Double vision usually occurs when the eyes are misaligned, or not pointed at the same object, causing us to see two different images. Both images are sent to the brain which we process as double vision.

Most of the time, both of our eyes are pointed at the same object. This produces a clear, single image with just enough difference between the two images to give us a slightly different line of sight. This subtle difference gives us depth perception, often referred to as stereo vision. In fact, the farther apart an animals eyes are, the better depth perception it has.

Double vision can cause problems in life, including great difficulty in completing simple tasks. Luckily, the brain naturally guards against double vision by suppressing, or ignoring, one of the two images.  We have a complex set of eye muscles and nerves that communicate with each other to keep both eyes moving along together.

Causes of Double Vision

The medical term for double vision is diplopia. Diplopia can be intermittent or constant. Because the pathway for the three main nerves that control eye muscle movement is long, complex and originates in the brain, double vision can be a sign of a serious neurological problem.

A defect anywhere along these pathways can possibly cause double vision. Some defects are caused by an injury to the head, stroke, aneurysm, brain tumor or brain swelling. Diabetes, hypertension and multiple sclerosis are common diseases that can cause a temporary paralysis of the nerves that control the eye muscles, which may cause double vision.

Another cause of double vision is strabismus. Strabismus is a condition that causes the eyes to be misaligned, often referred to as an "eye turn." Most people with strabismus are born with it and have an eye that tends to move inward or outward. Strabismus can also cause one eye to be pointed up or down. In many infants, it is difficult to find a true cause. Sometimes strabismus is caused by a large refractive error (need for prescription glasses) and sometimes eye tumors.

Most young children do not suffer from double vision even though their eyes are misaligned. Our brain often compensates and prevents us from seeing double by suppressing one of the images and making it disappear. Our brain learns to ignore the extra image, known as suppression. Children seem to adapt quickly and their brains suppress one of the images quickly. However, when one image is suppressed, a child is at risk for developing amblyopia, often referred to as lazy eye, because the eye is not being used properly. When strabismus develops in adults, double vision is more likely to occur.

An adult brain has difficulty suppression one image at first because for a large part of their life, both eyes were functioning to their fullest potential.

Monocular Diplopia

For the most part, you must have two fully functioning eyes to experience double vision. However, there is one type of double vision that can occur in one eye, called monocular diplopia. The most common cause of monocular diplopia is a cataract. A cataract is a clouding and opacification of the normally, clear lens inside the human eye. Most cataracts develop as we age. Some cataracts can be caused by trauma to the eye. Occasionally, the clouding occurs in sharp junctions in the lens which causes light to split into two when it enters the eyes. Some individuals perceive that as double vision.

Astigmatism can also cause double vision out of one eye. Astigmatism, a common vision problem that distorts vision, can sometimes elongate or stretch out an image so much that it appears to be double.

Treatment of Double Vision

  • Cover It - The simplest form of treatment of double vision is to cover one eye with a patch.  This quickly eliminates one image but has the negative side effect of decreasing your dimensional vision or depth perception.
  • Fresnel “Press-on” Prism - Another useful way of treating double vision is to apply a Fresnel prism on top of your eyeglasses. Fresnel prism is a thin sheet of many little prisms that shift light in a certain direction. The prism moves the image that the misaligned eye is seeing into a position so that both eyes can fuse the image into a single, clear image.  Fresnel prism has the advantage of being temporary. The fresnel prism can be peeled off of the glasses easily as the diplopia resolves itself. Fresnel prism is useful when double vision is caused by conditions such as diabetes or hypertension. Most often, the double vision lasts less than 6-9 months and then goes away when the underlying systemic condition is being treated.  
  • Ophthalmic Prism - If double vision is more permanent, then regular, ophthalmic prism can be ground into your eyeglass prescription. It occasionally increases the thickness of the lens on one side but is better looking cosmetically than a Fresnel prism.
  • Vision Therapy - Vision Therapy (VT) encompasses a lot of different techniques to correct double vision and strabismus. Sometimes certain machines and computer models are used to give the person behavioral feedback to control the double vision. Other times, specific eye muscle exercises are done to strengthen the eye muscles.
  • Surgery - Surgery for double vision is very successful in most cases. Eye surgeons who specialize in eye muscle surgery use a variety of techniques that involve shortening or moving eye muscles. Adjustable sutures are used to fine tune the eye muscle correction directly after surgery.
  • Botox - Botulinum toxin is injected directly into one of the muscles controlling eye movement. This temporarily blocks the nerve impulse and causes the muscle to be paralyzed. This muscle relaxes and the other eye muscles take up the slack to straighten the eye. Side effects including a droopy eyelid or worsened double vision. 

What You Should Know

Double vision is a serious health concern.  However “sudden onset” double vision is of special concern. When diplopia comes on suddenly, a neurological problem in your brain should always be ruled out before investigating more common causes. If you are experiencing double vision, you should be evaluated by an optometrist, ophthalmologist, neurologist or other medical professional immediately.

Upon evaluation, your doctor will ask you the following questions:

  • Did the double vision appear suddenly, or has it been there for some time?
  • Is it constant, or does it seem to come and go?
  • Does the double vision go away when you cover one eye?
  • Do you see double out of one eye, or both eyes?

Source: Neuro-Ophthalmology - Textbook of Ophthalmology. Slamovits, Thomas L. and Ronald Burd. Copyright 1994, Mosby-Year Book Europe, Ltd.

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