The ability to see is one of our most amazing and complicated senses. An image enters the front of the eye through a clear membrane (cornea), goes through a focusing lens, and then strikes a light sensitive surface (optic retina) that converts the light to nerve impulses (figure 1). These impulses go to the optical center in the brain (occipital cortex) where they are converted into the images we see.

Figure 1

Muscles play a very important role in the process of vision. A muscle around the lens (ciliary muscle) contracts or relaxes, thereby changing the shape of the lens. This is the focusing mechanism of the eye. When the muscle is contracted, the lens is rounder and objects that are nearer are in focus. When the muscle is relaxed, the lens is flatter and objects that are farther away are in focus (figure 1).

Muscles are also involved in the movement of the eyes (figure 2). These muscles (extra-ocular muscles) let you look up, down and sideways. Responding to orders from the brain, they act to coordinate the eyes with each other, so that both eyes look at the same place at the same time (binocular vision), thereby creating a single, clear picture in the brain (fusion).

Given such a complicated visual system, it’s not surprising that there are a number of ways in which this finely balanced system can be disturbed. Anything that makes it difficult for the eyes to work together to keep objects in clear focus can cause dizziness, headaches, reading difficulty and blurred vision. Some patients have one of these symptoms, while others may have two or more symptoms. With the wide array of symptoms, it is not unusual for these people to have sought help from a number of different types of doctors, usually without adequate relief. Most people would not think that the visual system could be the cause of these symptoms and as a result have not sought help from a vision specialist. The most common conditions that can cause these symptoms include Vertcal Heterophoria, nearsightedness (myopia), farsightedness (hyperopia) and incorrect eye glass prescription.

Figure 2 Figure 3

Specific Problems

Vertical Heterophoria and Vertical Heterophoria

Vertical Heterophoria has as its root cause vertical heterophoria, which is a condition where the two eyes have difficulty keeping in vertical alignment. This can lead to double vision, which the brain does not tolerate. Sometimes one eye is physically higher than the other (figure 3). Sometimes the eyes are aligned correctly but muscle or nerve abnormalities cause the problem. In some instances, this condition may be caused by head trauma, stroke, or neurological disorders. However, most often this is a condition you are born with (congenital). It may take years before symptoms occur, as the body will do the best it can to try and compensate for these problems.

In order for the brain to create one image from the two eyes (fusion) and avoid double vision, the brain forces the eyes to look at the exact same spot. To accomplish this, the eye muscles will strain to make one eye look up a little more, and the other eye to look down a little more. Over a long period of time, the stress and strain on the visual system precipitate the symptoms of Vertical Heterophoria (VH). These include headaches, usually in the front of the face or in the temples, as well as a feeling of being disoriented, lightheaded or dizzy. As the muscles strain, they become fatigued, causing the image seen by one eye to not continuously overlap the image seen by the other eye. In other words, fusion is no longer continuously maintained. This moving in and out of fusion creates the feeling of dizziness, lightheadedness and a sense of imbalance. Those who suffer from Vertical Heterophoria may also have other symptoms in addition to those of headaches and dizziness:

• additional pain symptoms symptoms such as face ache, eye pain or pain with eye movement (symptoms similar to sinus problems, migraines, TMJ problems); neck ache and upper back pain due to a head tilt (symptoms similar to spinal misalignment problems);
• additional vestibular symptoms such as motion sickness, nausea, poor depth perception, unsteadiness while walking or drifting to one side while walking (“I’ve always been clumsy”), lack of coordination (symptoms are similar to those seen in patients with MS, sequela of a stroke, an inner ear disorder or Meniere’s Disease);
• reading symptoms such as difficulty with concentration (symptoms are similar to those experienced with ADHD), difficulty with reading and comprehension, skipping lines while reading, losing one’s place while reading, words running together while reading (symptoms similar to those seen with a learning disability);
• vision symptoms such as blurred vision, double or overlapping vision, shadowed vision (symptoms similar to those seen in patients with MS); light sensitivity, difficulty with glare or reflection;
• psychological symptoms such as feeling overwhelmed or anxious when in large contained spaces like malls or big box stores, feeling overwhelmed or anxious in crowds (symptoms similar to those seen in patients with anxiety or agoraphobia).

Vertical Heterophoria tends to run in families.

To correct this problem, the optometrist will add prism to your glasses. Prism is a way of making the glasses such that the image seen by the eye is moved up or down – whatever is needed to allow for fusion to occur without straining the extraocular muscles. Once this occurs, the headaches, dizziness, blurred vision and other symptoms resolve.

Hyperopia (Far sighted)

Hyperopia is a condition where far vision is clear, but near vision is not. In hyperopic individuals, the length of the eyeball is too short (figure 4), making it harder to see objects close-up.

Figure 4

The eye compensates for this by changing the shape of the lens using the muscle around it (ciliary muscle). This muscle has to work very hard to keep the lens in the correct position. This causes the muscle to get tired (fatigue) and ache. When the muscle is no longer able to keep up, the vision becomes blurry close-up. To correct this problem, glasses are made such that the lens in your glasses takes over the job that the lens in your eye is doing. This allows the lens in your eye to be flat, which means the ciliary muscle can relax, and when that happens, the pain goes away and near vision becomes clearer.

Incorrect prescription

It is absolutely critical that your eyeglass prescription be exactly correct. If your prescription is off, you can get blurred vision, headaches and dizziness. If there is any doubt as to the correctness of your prescription, a second opinion should be obtained.

In Conclusion

Vision is a very complicated and finely balanced process. There are many areas along the way in which this process can be disturbed. When this occurs, many symptoms can arise, some of which can be quite severe. If your visual system is indeed the cause of your symptoms, then the appropriate evaluation and subsequent eyeglass prescription should reduce your symptoms significantly, if not completely.