Bright Sight

Oliver Backhouse, Consultant Eye Surgeon www.cataract.org.uk

Visual Field Loss

Homonymous Hemianopia refers to a loss of vision towards one side in each eye. The damage that causes this problem is in the brain and not in the eyes. An injury to the left half of the brain produces visual field loss in the right half of the visual field of each eye, or an injury to the right brain produces visual loss in the left half of the visual field of each eye.

It is difficult to explain the sensation of a homonymous hemianopia. Affected people often “feel like” the problem is in the right eye when they have a right homonymous hemianopia, but checking each eye by itself shows that the right side of each eye is not seeing. People may bump into things on the side of the visual field defect. Actions such as crossing the street may be dangerous as patients fail to appreciate oncoming vehicles on that side. Driving may be particularly problematic as they change lanes where there is an oncoming car. Objects on a desk or table may not be seen when located to the side of the visual field loss and sometimes even the food on that side of a plate is left uneaten.

Reading has its own special set of difficulties. People with a left homonymous hemianopia have difficulty finding the correct line when the finish one line and try to move their eyes back to the start of the next line. When reading, the eyes make a series of small quick movements from one word or group of words to the next. Since we read from left to right, people with a right homonymous hemianopia have difficulty with this and make more, smaller movements, often only getting from the beginning to the end of a single word. This makes reading slow and frustrating to many patients.

Visual hallucinations are common with homonymous hemianopia, especially if it develops suddenly as it would from a stroke. These hallucinations may be “unformed” – lights, shapes, geometric figures – or “formed”, meaning an image of a recognizable object. Sometimes an object in the normal field is mirrored in the blind field. For example, when moving the arm on the normal side toward a plate on the table, the patient may “see” an arm making the same movement on the other side. Affected people are often reluctant to mention this symptom but should be reassured by their family members and doctors of the cause of the problem. Unlike auditory (hearing) hallucinations, visual hallucinations usually are not caused by a psychiatric disorder, but are the result of some medical problem in the visual system. In the case of a stroke, the brain adjusts and the visual hallucinations usually resolve after a few weeks. Individual hallucinations may be transient but when they are more persistent it may be helpful to look at them rather than looking away.

Imaging of the brain by an MRI scan is the most commonly used test to diagnose the location and cause of the problem.

Will the visual field get better?

Recovery of a homonymous hemianopia depends on the underlying cause and the severity of the injury to the ‘seeing part of the brain’ (occipital lobe). There is often poor recovery if the cause is a stroke, especially if the lesion is as dense and severe. Time is the best healer with the visual field improving over the first 6 months but it can continue even up to 2 years later.

Reading may be improved by using a straight edge to direct the eyes to the target line of text and working consciously to increase the size of the small eye movements made as one moves along a line of text. Some have had luck holding the text at a 90 degree angle to the normal direction, so that it is read vertically. People with right homonymous hemianopia should read down, thereby keeping the next line of text in the intact left visual field. On the other hand, people with left homonymous hemianopia should read up for the same reason. While this sounds rather strange, many reading teachers sit across from their students and read along with them even though they are viewing the text upside down.

Moving through the environment can be accomplished by directing the eyes toward the hemianopic visual field. Looking for something in the blind field requires a different strategy. Research has shown that people with homonymous hemianopias usually make a series of small eye movements into the blind field when looking for something there. It is more effective if the person consciously makes a very large eye movement into the blind field and then lets the eyes come back to the object. When walking it may be helpful to let the person’s partner walk on the bad side and have the patient take their arm. When in groups the family or visitors should be situated on the side of the affected person’s normal visual field as much as possible.

Prisms or mirrors have been used on glasses to compensate for the hemianopia. These may attempt to shift or relocate the visual field toward the defect to attract attention to objects there but still require an active movement of the eyes in that direction to focus on the object. More formal attempts to induce visual field recovery using computer assisted programs have generated controversy. Ongoing research into the utility of such programs should settle the issue of their ability to induce meaningful improvements in the visual function of people with hemianopia. Until then, caution should be used concerning the investment of large amounts of effort and funds into any unproven treatment program.

Low vision specialists can be consulted to work with patients with these techniques. In general none of these techniques has produced measurable improvement in activities of daily living scales. Affected people are often enthusiastic about them but this usually is true in rehabilitation studies, and telling the difference between placebo effect and real benefit is often difficult.

Unfortunately it is not legal to drive with this type of visual field defect. An Esterman driving visual field can be done to better understand if your visual field has recovered to an acceptable level for the DVLA. You should notify the DVLA of you eye sight problem should you wish to continue to try and drive. Occasionally the DVLA do allow people with established small areas of visual field loss to drive.

Disclaimer: Adapted from Nanos