Common Visual Disorders

Visual Field Loss:  Certainly one of the most common problems experienced by the visual system of the stroke and head injured patient is loss of the side or peripheral vision. The most common field loss is hemianopsia, a condition in which half of the person’s visual field is missing. “Hemi” stands for half and “anopsia” means lack of vision. This is usually a definite and complete loss of one side of the patient’s visual field. It often obeys the patient’s midline and is often complete.

Figure 3. Hemianopsia

 

The Visual Field:  Even though this is not an anatomical part of the eye, it is a critical part of the visual system. The visual field can be thought of as an extension of the eyes into space. Think of it as a flashlight shining out from the eyes. Each eye’s visual field can be essentially divided into four quarters or quadrants. Depending on the area of the brain that is traumatized, it can affect a part of a quadrant or any combination of quadrants.

 

Patients with a field loss can also experience a condition known as neglect. This is to say that a patient may not acknowledge that anything even exists on the side of the field loss. The two can occur separately or together. In other words a patient can have a field loss and no neglect, neglect and no field loss, or both a field loss and neglect. To diagnose a field loss, the neuro-optometrist performs a test called confrontational visual fields. Essentially the practitioner stands in front of the patient and has the patient identify the number of fingers the practitioner is holding up in the patient’s side vision. More specific computerized tests can be performed in the vision clinic, but confrontational fields are fast and fairly reliable.

 

Field losses are usually caused from trauma to one side of the brain’s occipital lobe. This can occur from a bleed into the brain tissue (hemorrhage) or a lack of blood that occurs during an ischemic stroke.. Field losses can be permanent or temporary depending on the underlying cause of the damage. As mentioned above, neural plasticity and decreased pressure on the brain can lead to visual field improvement. Neglect tends to be temporary. Certainly there are no hard and fast rules in predicting the prognosis of either type of field defect. Treatment interventions are discussed in the next section.

 

Figure 4. Visual Field Pathways

 

It should be pointed out that most of the time a right field loss is not a loss of vision in just the right eye but rather a vision loss in both eyes. More specifically the patient loses vision on the outside part (temporal) of the left eye’s retina and the inside part (nasal) of the right eye’s retina. This can be seen in numbers 4 and 5 in the visual field pathway illustration above.

 

Diplopia (double vision):  Another visually debilitating and common post traumatic visual problem is diplopia. Double vision occurs when the eyes do not properly align on the target. The underlying cause is usually damage to the area of the brain responsible for eye movements. Another common cause can be a physical impairment of one or more of the extra ocular muscles as can be seen in facial trauma from automobile or sporting accidents. Diplopia can be a very distressing problem for the patient; seeing two doors, two TVs or two of anything that should be one makes it difficult to negotiate and communicate normally.

 

Figure 5. Ocular Alignment and Misalignment

Most of the time the diagnosis of diplopia is made by asking the patient if he sees double. A cover test is also necessary, in which the practitioner covers alternate eyes and determines whether the eye when uncovered swings in or out. A cover test is done at near and far distances. It is not uncommon for the patient to have double vision for near objects and not for distance objects. It is also common for the patient to have greater degrees of double vision in different gazes.

 

It is important to mention here that many diplopic patients stop reading. The visual skills necessary to read include the ability for both eyes to cross inward (converge). If the patient experiences diplopia for near objects, it is virtually impossible to read comfortably or efficiently. Many patients resort to patching an eye, which is certainly a viable option but is not always the best solution. Treatment options will be discussed in the next section.

 

If an eye is patched too long, a problem known as suppression can occur. Suppression is when a person shuts off the information to the brain without using a physical occluder such as a patch. This occurs more frequently in younger patients as their visual system is more adept at rewiring and adapting. As we get older, the visual system becomes more ingrained and thus more difficult to spontaneously change its original function. Suppression can lead to problems such as a loss of correctable vision or even blindness.

 

The neuro-optometrist can determine the amount of diplopia and the severity of suppression. Treatment options can then be outlined to minimize damage and loss of vision the patient might experience. Much of the conventional thinking was simply to patch an eye to eliminate the double vision and learn to live with field losses. This approach may be the best option for some patients. Other treatments should be investigated, though, because different vision problems such as eye movement (tracking) disorders, can develop. These disorders are discussed in detail in the following sections.

 

Spatial Awareness Disorder:  Problems resulting in the loss of balance or loss of awareness of one’s surroundings are common visual disorders, but often involve more than just the visual system. Treatments become dependent on many facets of the rehabilitation team. As mentioned, specific treatment modalities are discussed in the next section. Intervention in this area is exciting and rewarding, but is the most controversial too.

 

It is necessary to discuss two basic visual processes that exist. The first is the central visual processing system. This system is responsible for our recognition of detail. Central processing originates at the macula, or the center of our retina. People with central visual processing problems often miss the details in their environments. They see the forest, but miss the deer. The second system is referred to as the peripheral. This provides the patient with stimulus on the sides of our vision. This system is critical when maintaining balance, especially when moving.

 

Patients with defects in their central or peripheral visual systems often break down when presented with stimulus overloads. A person whose visual system is working efficiently can walk down a crowded hall and identify a specific object while being aware of changes in the surroundings. A person with a central visual processing problem may be able to react to the surroundings, but finds it difficult if not impossible to identify a specific object. This person can find jobs that require details difficult to perform. Another person whose peripheral processing system has been damaged may find the object of concern, but be completely unable to react to changes in the surroundings. These people may find crowded places like a shopping mall an impossible environment in which to function.

 

Patients who have peripheral or central vision problems secondary to some sort of Traumatic Brain Injury (TBI) often are referred to as having Post Traumatic Vision Syndrome (PTVS). This syndrome is certainly not mutually exclusive from spatial awareness disorders, but it is widely accepted that patients who exhibit balance and concentration problems secondary to a TBI could certainly have PTVS. Early detection and rehabilitation of these problems will aid the patient in an effective recovery and earlier return to normal function.

Eye Movement Disorders (tracking objects):  Eye movement problems are common occurrences after brain trauma. Most any patient with a significant head injury finds it difficult to follow an object through space. For example, when a physician is performing a physical he holds up a finger and moves it through the air as the patient follows it visually. Almost inevitably the physician will find some significant problems with tracking.
Eye movements can be broken down into two basic processes. The first is pursuits, or the ability of the eyes to follow a moving object smoothly and accurately through space. The next is saccades. This is the ability of the eyes to jump quickly and accurately from one point to another. Both pursuits and saccades are necessary for a person to read. As mentioned before, convergence is another eye skill used when reading. Many traumatic brain injured and stroke patients lack the necessary eye skills needed to read efficiently. When observing patients during rehabilitation, most of the patients report that they have discontinued reading. This is often due to the fact that they do not possess the eye skills needed to read.

 

Nystagmus is the involuntary shaking of the eyes. This is another unfortunate visual motor disorder that some brain trauma patients experience. It is necessary to evaluate the nystagmus in all gazes (to determine if the shaking is slower in a particular gaze) to facilitate effective rehabilitation.

 

Photophobia (light sensitivity) and Dry Eyes:  Many patients complain of light sensitivity after a traumatic brain injury. These patients can report an extreme problem with fluorescent lights, flashing lights and direct sunlight. Some patients also report severe dry eye problems. Symptoms of dry eye include redness, a sandy or gritty feeling and often light sensitivity. While the two are not mutually exclusive, they often run together. Thus, the treatment of one may very well positively affect the other.

 

The underlying cause for the dryness and light sensitivity may stem back to the glands in the lids that help secrete the tears. Many patients after LASIK refractive surgery experience dry eyes. This may be due to the nerves in the cornea being cut; thus the feedback loop needed to stimulate tearing becomes dysfunctional. The same reasoning may be used with brain trauma patients. Many critical feedback loops thought to help with normal and necessary body functions can be disrupted. These systems can and do affect corneal sensitivity and lid lacrimation (tearing). A dry eye can undergo many changes, including a dry cornea. Anytime the corneal surface is compromised, one of the side effects is photophobia.

 

Pupil defects, inability to blink, inflammation of the iris and other visual problems can also contribute to dry eye and photophobia. A careful work-up is necessary to ensure the proper diagnosis and treatment.