Double Vision

Understanding Double Vision
Have you ever wondered why we normally see only one object when we are using two eyes? Or how flat and featureless things look when we use only one eye? The two eyes really do get different points of view, but our brain controls the movements of the two eyes so that the focused light falls upon very special areas in the retinas (the nervous tissue at the back of each eye) called the foveas. The foveas are corresponding points to our brain, so whenever images fall upon our foveas, our brain tries to make the two images into a single one. When successful, the brain takes advantage of the differing views from either eye to give a 3-dimensional (or stereoscopic) picture. Sometimes this system breaks down, and the two eyes give two pictures that are too different for the brain to fuse into one; the result is double vision, or diplopia.

Binocular Diplopia
If you enjoy good vision, you can perform a simple experiment on yourself. Try looking at your finger held out in front of you and then pay attention to something out in the distance WHILE STILL LOOKING AT YOUR FINGER! You will notice that the distant image is doubled. This is called physiologic diplopia and it occurs because you are using your foveas to examine your finger, leaving non-corresponding portions of your retinas to view the distant object. This type of diplopia is normal and your brain can work with it. But what happens when things that you are looking at with your two foveas are doubled? When a patient has this abnormal diplopia, it means the patient cannot make their eyes look together in the same direction. There are many diseases that cause this problem. Sometimes, a nerve controlling one or more of the muscles moving the eyes is damaged. Patients will often adopt a head-tilt to try to use what motility remains to see a fused image. This type of damage occurs frequently in diabetes (from a small stroke), trauma, and brain tumors. In other patients, the muscles themselves are diseased. Weak muscles are seen with certain poisonings, such as botulism or alcohol intoxication, or with certain progressive disorders, such as myasthenia gravis. Muscles can also become inflamed, as seen with infections in the eye socket (often starting in the adjacent sinuses), with thyroid disease, and after trauma. Patients with diplopia, especially of new onset, must see a physician immediately; many of the diseases causing sudden diplopia are extremely deadly. The ophthalmologist has instruments to measure the eye movements and determine which muscle or muscle groups are defective. The Ophthalmologist will then determine the cause of the deficit and help the patient understand when and how therapy should be commenced. Although many cases of diplopia will resolve with time, the ophthalmologist can often help initiate therapy that will decrease the length and severity of the diplopic interval. In some cases, the diplopia will not resolve spontaneously and surgery is required to help the eyes straighten.

Monocular Diplopia
One eye can see two images simultaneously! If you wear bifocals or can borrow a pair, try looking through them as you line up your eye with the edge joining the two lenses. Your eye will perceive two images because it is getting two disparately focused rays of light into the pupil. The eye can do this to itself in pathological states. For example, when the cornea is irregular (high astigmatism) or scarred, it can focus light differentially and supply the retina with two images. If the iris (the colored part of the eye surrounding the pupil) has another opening in addition to the normal pupil, this aperture may transmit sufficient light to give a second image. Patients who have undergone eye surgery or have sustained eye trauma, or who suffer with certain diseases that cause thinning of the iris can experience this phenomenon. Finally, a cataract (lens opacity) can split the light entering the lens, causing monocular diplopia. When a patient comes to an ophthalmologist complaining of doubled vision in only one eye, the ophthalmologist will first try to determine whether the vision is really doubled or just blurred (many patients use the term interchangeably), and then determine whether the diplopia is really monocular or binocular. Monocular diplopia will usually have a fairly obvious cause and the ophthalmologist can often offer the patient a cure, usually through surgery.