| Hemianopsia | |
|---|---|
| Classification and external resources | |
A picture as seen with a hemianopsia on the left eye. |
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| ICD-10 | H53.4 |
| MeSH | D006423 |
Hemianopia, or hemianopsia, is a type of anopsia where the decreased vision or blindness takes place in half the visual field of one or both eyes. In most cases, the visual field loss respects the vertical midline. The most common causes of this damage include stroke, brain tumor and trauma.[1]
Contents |
The word Hemianopsia is from Greek origins, where:
When the pathology involves both eyes, it is either Homonymous or Heteronymous.
A homonymous hemianopsia is the loss of half of the visual field on the same side in both eyes. The visual images that we see to the right side travel from both eyes to the left side of the brain, while the visual images we see to the left side in each eye travel to the right side of the brain. Therefore, damage to the right side of the posterior portion of the brain or right optic tract can cause a loss of the left field of view in both eyes. Likewise, damage to the left posterior brain or left optic radiation can cause a loss of the right field of vision.[2]
A Heteronymous hemianopsia is the loss of half of the visual field on different sides in both eyes. It is separated into two categories:
Quadrantanopia (quadrantanopsia/quadrantic hemianopia) is decreased vision or blindness in one quarter of the visual field. The particular quarter of vision missing depends on whether the location of the brain damage is temporal or parietal.[3] For example, a lesion to the right temporal lobe with damage specifically to Meyer's loop will give rise to a left upper quadrantanopsia, while a lesion to the right parietal radiation will result in a left lower quadrantanopsia.[4]
Many different types of treatments are available to patients with Hemianopia, depending on patient preference, age, type of Hemianopia, and how long they have been diagnosed. Some of these treatments include Audiovisual Stimulation Training, Explorative Saccade Training, Optical Visual Span Expanders, and Visual Restoration Therapy. None of these treatments will cure Hemianopia, but they will allow patients to have better perception of the world around them. Most doctors recommend the uses of a combination of these treatments, after a complete vision examination (Koons et. al., 2010). Research is still being done on which of these treatments is most effective, and which patients will benefit the most from different combinations of treatments. The Department of Veteran Affairs medical center currently uses Explorative Saccade Training as their ‘go to’ form of treatment for patients with Hemianopia (Koons et. al., 2010). Also, a treatment outcome model has been developed for Visual Restoration Therapy in order to better understand which patients this specific therapy will help (Guenther et. al., 2009). More studies need to be done in order to develop a similar model for more of the Hemianopia treatments and different combinations of them. Currently, advantages and disadvantages are known for most treatments, which is discussed below along with descriptions of each treatment.
Audiovisual stimulation training has been developed as an effective treatment for patients suffering from Homonymous Hemianopia which uses multi-sensory stimulation to improve vision. Patients can go through an intensive program of up to 4 hours a day for 2 weeks, in which areas of their visual field, both intact and affected, are stimulated using sound and light.
A semicircular structure should be set up with marks at azimuths of 8°, 24°, 40°, and 56° on both right and left of the central fixation point (Passamonti et al., 2009).At each point a light and a sound speaker should be fixed. For one round of training, a light will be illuminated along with a shout sound, and the patient is asked to move their gaze toward the stimulus which just has occurred. As trials continue, the sound and the visual cue will randomly alternate between being located at the same azimuth, or different azimuths. The stimulation should be most focused on the patients’ blind hemi-field.
Many patients have seen lasting results from Audio-visual Stimulation Training, allowing the some recovery. This training affects different parts of the patients’ saccades, including spatial and temporal aspects (Passamonti et al., 2009). Patients with Right Hemianopia tend to show fewer progressive and regressive saccades, larger amplitude of these saccades, and reduced duration of fixation time (Passamonti et al., 2009). They also show improved accuracy on reading performance tasks; however performance is still impaired (Passamonti et al., 2009). Left Hemianopia patients show even greater improvements, showing a significantly smaller number of saccades, and following reading tasks their ocular responses are comparable to ‘normal’ vision patients (Passamonti et al.,2009).. There are also some negative aspects of Audio-visual stimulation Therapy, being that the field of vision is not improved (Windsor et. al., n.d.). Studies have shown that sensitivity of perception showed no significant difference when the patient was told to keep their eye stationary from the original treatment which allowed the patient to move their eye will adjusting their gaze(Passamonti et al.,2009).. If visual field had been increased, the sensitivity should have increased when the eye was held in one position.
Explorative Saccade Training was developed to train patients to make exploratory saccades without head movement in the area of the visual field which has been lost, and sometimes may be referred to as Scanning Therapy (Koons et. al., 2010). Many methods have been developed for performing this type of therapy, most of which are performed by an occupational therapists, and the individual patient. These patients learn to apply these search strategies to everyday tasks.
Explorative Saccade Training begins with the development of large saccades, followed by the development of smaller saccades. The patient should start to try to improve the speed of these saccades in order to develop a natural flow to their visual perception. Once this has been accomplished, the patient should try and attempt to integrate this system into everyday life. Some activities which may help develop large saccades include head and eye shifts, descriptive walking, different search strategies, large table cars, and even games such as WII Tennis (Roth et. al., 2009). Small saccades can be improved upon by pen and paper search, last letter cancellation for Right Hemianopia, computerized trainers, and even puzzles (Roth et. al., 2009).
Explorative Saccade Training improves exploratory behavior, and enhances performance of digit search tasks (Roth et. al., 2009). Lasting effects are shown for improvement of natural search performance on the patients’ blind side after only about six weeks of training (Windsor et. al., n.d); with scanning improvements of up to 35° into the affected side of vision (Koons et. al., 2010). An advantage Explorative Saccade Training is that it has been shown to be affective for patients who have been living with Hemianopia for many years and have had the time to come up with their own adaptive strategies to deal with their vision (Roth et. al., 2009). Even with the benefits to patients who have had Hemianopia for a long time, Explorative Saccade Training is most effective with younger patients who have been recently diagnosed (Roth et. al., 2009). A downside to this therapy is that reading speed is not improved (Roth et. al., 2009). Also, since Explorative Saccade Training uses activation eye-movement exploration, asymmetry has been seen in patients on their blind side (Roth et. al., 2009). Overall patients should experience significant improvements in performing activities of daily life.
Optical Visual Span Expanders are yet another option for patients with Hemianopia. The main idea behind this treatment is that the patients’ glasses have a special prism mounted on them to help increase the visual field (Windsor et. al., n.d.). The patient must combine the image from the prism with the image they receive from the rest of the visual field in order to perceive a larger visual field (Windsor et. al., n.d.).
There are three main varieties of expanders, which have all been derived from Fresnel prisms. These include the Gottlieb Visual Field Awareness System, the Chadwick Hemianopia Lens, and the EP Horizontal Lens.
The Gottlieb Visual Field Awareness System: The Gottlieb System (Figure 1.) uses a circular prism mounted on the lens of the affected eye with the base pointing in the direction of the affected visual field. Ideally, small wedge prisms are used at 18.5 prism diopters (Gottlieb, 1988). This allows for improved optics without severe spatial distortion or reduced resolution (Gottlieb, 1988). The prisms can be coated to suit the prescribed power of the patients’ glasses.
Chadwick Hemianopia Lens:The Chadwick lens (Figure 2.) is the second type of visual expander available, which was developed by Michael Onufryuk of Rochester, New York (Windsor et. al., n.d.). Along with the Gottlieb system, this is a monocular system. The prism used here is mounted on the periphery of the affected side, taking up the entire outer twenty percent (roughly) of the glasses lens (Windsor et. al., n.d.). The glasses used to mount these lenses should have relatively small frames, in order to control the thickness of the glasses (Windsor et. al., n.d.). The larger the frame, the thicker the Chadwick lens must be in order to achieve the same affect.
EP (Peli) Horizontal Lens: The third is called the EP Horizontal Lens (Figure 3.), developed by Eli Peli a professor at Harvard in 1999 (Windsor et. al., n.d.). These lenses are mounted in the center of one lens of the patients’ glasses, either the left or right lens to coincide with the side of vision which is affected. The patient may be prescribed with one or two EP lenses at 57 diopters ("Chadwick optical inc.," 2011), mounting one above the line of sight and/or one below the line of sight, 12mm apart from one another (Bowers et. al., 2008). Due to placement, the eye can freely scan the horizontal. The EP segments shift the image into the patient’s’ unimpaired side of their visual field, and the brain fills in the rest of the perception (Bowers et. al., 2008).
All three lenses have shown improvement of the perception of the visual field. Overall, experience has shown that younger patients tend to adapt to the expanders better than older patients (Windsor et. al., n.d). Not all patients will benefit from Optical Visual Span expanders, as some patients are so well adapted to their visual field loss that the expanders are not necessary for them to be high functioning (Windsor et. al., n.d).. The Gottlieb Visual Field Awareness System and the Chadwick Hemianopia Lens have significantly higher contrast and brightness than the EP lens (Windsor et. al., n.d). The Gottlieb Visual Field Awareness System may not be ideal for patients with severe cognitive impairments, since the patient must alternate viewing between the prism and the rest of the lens. Also, due to the alternation between the prism and the rest of the lens, perception can be quite jumpy (Windsor et. al., n.d). The innovation of EP Horizontal Lens has taken away this problem, and allows the patient to no longer have to scan to the edge of the visual field to jump between the two perceptions (Bowers et. al., 2008). However, as stated above, the EP lens does sacrifice contrast so that the images from the expander and visual field can be integrated better together (Windsor et. al., n.d). EP lenses also use a higher power, giving the patient about 30° of visual expansion, more than the other types of expanders (Figure 4.) ("Chadwick optical inc.," 2011).
Visual Restoration Therapy is another option for treatment for Hemianopia patients, which is available from NovaVision and can be performed on the patient’s home computer. Patients must obtain a prescription from a physician following a complete assessment about what their impairment exactly is, and where it is located. The therapy takes about a half hour, and it is suggested that the patient completes this training twice a day for two months ("Novavision," 2007).
During sessions, the patient focuses on a central fixation point. Light stimuli will appear at different positions, and at such time the patient should respond. The stimuli are tailored to each individual patient, and after six months NovaVision will analyze the data and make any necessary adjustments.
During the therapy, the brain is being stimulated and begins to heal by neuroplasticity, reorganizing the tissue in order to produce visual field recovery (Windsor et. al., n.d). Neuroplasticity is activated due to repetition, and stimulation of specific nerves. Visual Restoration Therapy has been shown to increase stimulus detection, and decrease the patient’s blind areas by shifting the edge of blindness up to 8.877° (Romano et. al., 2008). Neuroimaging has also shown changes in cortical activation after completion of the therapy (Romano et. al., 2008). Many patients have reported up to 70 percent increase visual field (Romano et. al., 2008).