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Common Etiologies
By incidence, the most common causes of blindness and visual impairment in the United States are:
- § Macular Degeneration
- § Glaucoma
- § Diabetic Retinopathy
- § Cataracts
- § Corneal Dystrophies
- § Retinitis Pigmentosa
These conditions can take many different forms. They may be treatable or untreatable, hereditary or non-hereditary, congenital (evidenced at birth) or adventitious (later-developing). Oftentimes, these conditions exist in combination.
Macular Degeneration
Deterioration of the macula (located in the center of the retina) can result from the aging process (i.e. age-related macular degeneration) or as the result of a congenital condition (i.e. juvenile macular degeneration or Stargardt's disease). Functional characteristics include reduced clarity, loss of detail discrimination, central “blind spots”, photophobia (light sensitivity), reduced night vision, and loss of depth perception. The most common complaint of people with this condition is that individuals cannot read standard/large print, and they cannot recognize people’s faces.
Glaucoma
There are numerous types of glaucoma (e.g. open-angle, closed-angle, acute, chronic, low-pressure), all of which result from a change in intra-ocular pressure. Glaucoma leads to loss of peripheral vision, eventually reducing the visual fields until there is no remaining vision (i.e. total blindness). Through laser technology, surgery and eye drops, this is one of the most treatable etiologies. Contrary to popular opinion, pain is not always associated with glaucoma. Therefore, the absence of pain does not mean that the condition is “cured” or non-existent; and only an ophthalmologist or optometrist can diagnosis glaucoma.
Cataracts
Clouding areas of the lens of the eye, or cataracts, are responsible for loss of clarity, diminished color vision, diminished contrast, and significant problems with glare and bright light. They may be congenital or caused by trauma to the eye or aging.
Diabetic Retinopathy
Growth of excessive or nonessential blood vessels in the retina (proliferative diabetic retinopathy) or progressive deterioration of tiny retinal vessels (non-proliferative). Both types of diabetic retinopathy result in hemorrhaging and accumulation of scar tissue which causes "blind spots" throughout the field of vision. Massive hemorrhaging may result in total blindness. Functional problems include reduced contrast, blind spots, sensitivity to light, inability to accommodate for lighting changes, loss of depth perception, diminished color perception, and loss of clarity.
Retinitis Pigmentosa (RP)
Retinitis Pigmentosa is hereditary and progressive. Initially one will notice poor or non-existent night vision. As the condition progresses, the visual field will decrease with a loss of peripheral vision, loss of lighting change accommodation, and reduced clarity and contrast. It can eventually result in total blindness. No treatment is available at this time, but much retinal cell transplantation research is currently being done with positive results. Mobility (e.g. walking around, getting from one place to another, going through doorways/narrow spaces) is one of the biggest issues for these individuals. As their visual fields are so greatly reduced they must continually scan from side to side and up and down to compensate for field loss. RP also confuses observers because the person may be able to read standard print but needs a white cane for mobility purposes.
Retinopathy of Prematurity (ROP)
Caused by an excessive amount of oxygen in the bloodstream which damages the optic nerve, ROP only occurs in premature babies who have been on oxygen in the incubator. Varying degrees of vision loss result with an overall loss of clarity and contrast. ROP may also cause some visual field deficits or blind spots that are often photophobic (light-sensitive).
Nystagmus
Usually congenital, nystagmus is characterized by a random search pattern of the eye, in which the eye attempts to find the clearest focal point. This condition is not a muscular problem, but a neurological problem that cannot be corrected by surgery. Poor/reduced visual acuity occurs due to the inability to maintain proper focus. Results in reduced clarity. Thus, most people with nystagmus have a "null point", a position in which the eye can maintain focus and relative clarity. However, this point is not found by looking straight ahead as it is for most people.
Optic Atrophy
This is a congenital condition that is caused by the underdevelopment of the optic nerve. Poor acuity, detail deficits, and in some instances, contrast deficits result. Optic Atrophy is generally not associated with visual field deficits or blind spots.
Heminopsia
Resulting from a stroke or some type of trauma to the brain, heminopsia is defined by the loss of one half of the visual field. Although currently, no treatment is available, it is typically a stable condition, and one can compensate for field loss in many ways. For example, while reading, one can turn the page at a 45 degree angle or put a colored ribbon along the side of the page to draw attention to the far edges.
Retinoblastoma
Cancer of the eye where tumors develop on the eyeball, retinoblastoma is sometimes treatable with surgery. Either partial or total blindness may result from this condition.
Myopia
Commonly known as "near-sightedness", myopia is the result of an over-elongated lens which causes the picture to focus too far forward. Prescriptive lenses refocus the image to the correct position within the eye.
Corneal Dystrophy
The deterioration of the cornea (the outer layer of the front of the eye) causes a high degree of photophobia (light sensitivity) and poor/reduced acuity, but does not generally result in field deficits or scotomas (blind spots). In some cases this condition is overcome by corneal transplantation.
Hyperopia
Also known as "far-sightedness", hyperopia is the result of an abnormally shortened lens which focuses the picture too far back. Prescriptive lenses can refocus the image in the correct position within the eye.
Astigmatism
An irregularity in the surface shape of the eye skews the point of focus inside the eye. Astigmatism can be corrected with prescriptive lenses that refocus the image by bending the light rays as they enter the eye.
Cortical Visual Impairment
This condition is not actually a problem with or in the eye or occular system. In fact, individuals with this impairment may have 20/20 visual acuity. However, they are still unable to process visual information due to cognitive damage usually resulting from brain injury or stroke. These individuals may give what appear to be very bizarre answers or responses to visual questions or appear to function as a totally blind person. While bright illumination may be distracting or disorienting for them, providing a clutter-free work environment is a simple way to accommodate employees and clients who have cortical visual impairment.
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