Preferential hyperacuity perimetry
Encyclopedia
Preferential hyperacuity perimetry is a psychophysical test used to identify and quantify visual abnormalities such as metamorphopsia and scotoma.
are symptoms of retinal diseases such as Macular degeneration
. In advanced stages of the disease, photoreceptor cells may be irreversibly damaged. Hence,if not treated Macular degeneration
may lead to blindness. Awareness to early changes in vision, especially in high risk patients, leads to early treatment (such as intravitreal injection of anti-VEGF factors, e.g. bevacizumab
or ranibizumab
) and prevents loss of vision. Because of complex brain mechanisms such as Filling-in
, patients with small and peripheral defects in their vision are often unaware of such changes until late stages in the disease. Another problem is that minute visual aberrations can be normal and therefore should be distinguished from genuine visual abnormalities. Preferential hyperacuity perimetry (PHP) is a technology that bypasses Filling-in
and quantifies the extent of visual abnormalities.
, the ability to separate between distinct objects. Therefore, in retinal diseases such as Macular degeneration
responses to hyperacuity stimuli may be abnormal long before any changes in Visual acuity
are observed. Another key element in PHP technology relies on the competition principle. According to this principle, visual attention is preferably attracted to the larger object. When two stimuli, which are different in size, are simultaneously and briefly displayed, the observer is more likely to perceive the larger of the two, while missing the other.
(central area of the retina) is scanned with a succession of stimuli, each stimulus consisting of a series of dots arranged along a vertical or horizontal axis. In each stimulus, a small number of dots are misaligned, thereby creating an artificial distortion (bump or wave). The examinee's task is to perceive these artificial distortions and mark their locations on the visual field. When a stimulus is projected on a healthy portion of the retina, the examinee identifies the artificial distortion and is likely to mark a correct location. If the stimulus is projected on a damaged region of the retina, a pathological distortion may be perceived instead of the artificial distortion, especially if the pathological distortion is more prominent than the artificial distortion. The examinee may then mark a location that is distant from the artificial distortion, indicating that a pathological distortion may have been perceived. By manipulating the amplitude of artificial distortions, the amplitude of the pathology in the area of interest can be quantified. At the end of test, comparison of the set of erroneous responses against a normative data base is used to determine if test results are within normal limits.
Background
Vision abnormalities such as metamorphopsia (distortions) and scotomaScotoma
A scotoma is an area of partial alteration in one's field of vision consisting of a partially diminished or entirely degenerated visual acuity which is surrounded by a field of normal - or relatively well-preserved - vision.Every normal mammalian eye has a scotoma in its field of vision, usually...
are symptoms of retinal diseases such as Macular degeneration
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...
. In advanced stages of the disease, photoreceptor cells may be irreversibly damaged. Hence,if not treated Macular degeneration
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...
may lead to blindness. Awareness to early changes in vision, especially in high risk patients, leads to early treatment (such as intravitreal injection of anti-VEGF factors, e.g. bevacizumab
Bevacizumab
Bevacizumab is a drug that blocks angiogenesis, the growth of new blood vessels. It is commonly used to treat various cancers, including colorectal, lung, breast, kidney, and glioblastomas....
or ranibizumab
Ranibizumab
Ranibizumab is a monoclonal antibody fragment derived from the same parent mouse antibody as bevacizumab . It is much smaller than the parent molecule and has been affinity matured to provide stronger binding to VEGF-A...
) and prevents loss of vision. Because of complex brain mechanisms such as Filling-in
Filling-in
In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in normal visual analysis...
, patients with small and peripheral defects in their vision are often unaware of such changes until late stages in the disease. Another problem is that minute visual aberrations can be normal and therefore should be distinguished from genuine visual abnormalities. Preferential hyperacuity perimetry (PHP) is a technology that bypasses Filling-in
Filling-in
In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in normal visual analysis...
and quantifies the extent of visual abnormalities.
PHP Technology
PHP takes advantage of visual hyperacuity, also known as Vernier acuity- the ability to identify the misalignment of visual objects. Visual hyperacuity is at least 10-times more sensitive than Visual acuityVisual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
, the ability to separate between distinct objects. Therefore, in retinal diseases such as Macular degeneration
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...
responses to hyperacuity stimuli may be abnormal long before any changes in Visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
are observed. Another key element in PHP technology relies on the competition principle. According to this principle, visual attention is preferably attracted to the larger object. When two stimuli, which are different in size, are simultaneously and briefly displayed, the observer is more likely to perceive the larger of the two, while missing the other.
PHP Test
In a PHP test, the maculaMacula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
(central area of the retina) is scanned with a succession of stimuli, each stimulus consisting of a series of dots arranged along a vertical or horizontal axis. In each stimulus, a small number of dots are misaligned, thereby creating an artificial distortion (bump or wave). The examinee's task is to perceive these artificial distortions and mark their locations on the visual field. When a stimulus is projected on a healthy portion of the retina, the examinee identifies the artificial distortion and is likely to mark a correct location. If the stimulus is projected on a damaged region of the retina, a pathological distortion may be perceived instead of the artificial distortion, especially if the pathological distortion is more prominent than the artificial distortion. The examinee may then mark a location that is distant from the artificial distortion, indicating that a pathological distortion may have been perceived. By manipulating the amplitude of artificial distortions, the amplitude of the pathology in the area of interest can be quantified. At the end of test, comparison of the set of erroneous responses against a normative data base is used to determine if test results are within normal limits.