Hemianopsia, or hemianopia, is a decreased vision or blindness (anopsia) in half the visual field, usually on one side of the vertical midline. The most common causes of this damage are stroke, brain tumor, and trauma.
This article deals only with permanent hemianopsia, and not with transitory or temporary hemianopsia, as identified by William Wollaston PRS in 1824. Temporary hemianopsia can occur in the aura phase of migraine.
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Etymology
The word hemianopsia is from Greek origins, where:
- hemi means "half",
- an means "without", and
- opsia means "seeing".
Types of hemianopsia
When the pathology involves both eyes, it is either homonymous or Heteronymous.
Homonymous hemianopsia
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.
Heteronymous hemianopsia
A Heteronymous hemianopsia is the loss of half of the visual field on different sides in both eyes. It is separated into two categories:
- Binasal hemianopsia - the loss of the fields surrounding the nose;
- Bitemporal hemianopsia - the loss of the fields closest to the temples.
Other forms
- Superior hemianopsia - the upper half of the field of vision is affected;
- Inferior hemianopsia - the lower half of the field of vision is affected.
Quadrantanopia
Quadrantanopia (quadrantanopsia or quadrantic hemianopsia) 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, and the side of the lesion. For example, a lesion to the right temporal lobe with damage specifically to Meyer's loop will give rise to a left upper (superior) quadrantanopsia, while a lesion to the right parietal radiation with damage specifically to Baum's loop will result in a left lower (inferior) quadrantanopsia.
Visual neglect
Visual neglect (also called hemispatial neglect or unilateral spatial neglect) differs from hemianopsia in that it is a perceptual deficit rather than a visual one. Unlike patients with hemianopsia who actually don't see, those with visual neglect have no trouble seeing but are impaired in attending to and processing the visual information they receive. Whereas hemianopsia can be assuaged by allowing patients to move their eyes around a visual scene (ensuring that the entire scene makes it into their intact visual field), neglect cannot. Neglect can also apply to auditory or tactile stimuli and can even leave a patient unaware of one side of his or her own body.
Ellis and Young (1998) showed that neglect can also affect patients' mental maps such that if they are asked to picture themselves standing in a familiar location and name the buildings around them, they will neglect to name the buildings on their impaired side but will be able to name them when asked to mentally face the opposite direction.
Some patients with neglect also have hemianopsia, however the two often occur independent of one another.
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Treatment
Many different types of treatments are available to patients with hemianopsia, depending on patient preference, age, type of hemianopsia, 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 hemianopsia, 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 hemianopsia (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 hemianopsia 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
Audiovisual stimulation training has been developed as an effective treatment for patients suffering from Homonymous hemianopsia 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.
How it works
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.
Results
Many patients have seen lasting results from Audio-visual Stimulation Training, allowing them some recovery. This training affects different parts of the patients' saccades, including spatial and temporal aspects (Passamonti et al., 2009). Patients with Right hemianopsia 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 hemianopsia 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 while 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
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.
How it works
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 hemianopsia, computerized trainers, and even puzzles (Roth et al., 2009).
Results
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 hemianopsia 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 hemianopsia 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
Optical Visual Span Expanders are yet another option for patients with hemianopsia. 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.).
Operation
There are three main varieties of expanders, which have all been derived from Fresnel prisms: the Gottlieb Visual Field Awareness System, the Chadwick hemianopsia lens, and the EP horizontal lens.
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 hemianopsia lens
Chadwick hemianopsia lens (Figure 2) 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 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 effect.
EP horizontal lens
The EP (Peli) horizontal lens (Figure 3) was 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 or one below the line of sight, 12 mm 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 the visual field, and the brain fills in the rest of the perception (Bowers et al., 2008).
Results
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 hemianopsia 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
Visual Restoration Therapy is another option for treatment for hemianopsia 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).
How it works
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.
Results
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). After completing the therapy, 70% of patients report an increase in visual field (Romano et al., 2008).
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