March 1, 2014

What Is Low Vision?


What Is Low Vision?

By Dan Roberts
An estimated fourteen million Americans have low vision, also called visual impairment. It is generally any interference with sight that hinders the performance of daily activities.

More specifically, low vision describes varying degrees of sight loss caused by disease, trauma, or a congenital disorder.

Low vision may appear as one or more of six general conditions:

1. Blurriness, where visual acuity with best spectacle correction is no better than 20/70:

2. Narrowing of peripheral or side vision:

3. Defects within the field of vision, such as distortion or blind spots:

4. Loss of contrast:

5. Sensitivity to glare or light:

6. Loss of color perception:

“Low vision” should not be confused with “legally blind”. Legal blindness is defined as acuity of 20/200 or worse in the better eye with correction, or a visual field of 20° or less in the better eye.

A person who meets one of these criteria might still have usable vision but would not be able to read without training on an assistive device. Also, the person would not be able to meet the requirements for obtaining a driver’s license. This person would be described as having low vision, but another person who has low vision may not necessarily be legally blind.

In children, low vision can be the result of conditions such as albinism, cataracts, glaucoma, retinopathy of prematurity, optic nerve dystrophy, nystagmus, and several types of juvenile macular degeneration.

In adults, low vision may be caused by progression of any of the above, plus diseases like age-related macular degeneration (AMD), stroke, retinitis pigmentosa, diabetic retinopathy, and degenerative myopia.

Low vision can also result from events such as eye injury and retinal detachment.

Visual Assessment
Assessment  of low vision is accomplished by a specialist trained in the use of appropriate tests and techniques. Visual acuity and refractive status are tested using larger-than-usual testing charts, control of illumination, the use of trial frame refraction, and techniques that allow for eccentric, or off-center, viewing. The examiner may also use:

  • An Amsler grid, a simple way to identify defects in my central visual field; 
  • A contrast sensitivity test to determine my ability to discriminate subtle changes in vision that occur in the real world, rather than the absolute black-on-white contrast of visual acuity charts;
  • A brightness acuity tester, or BAT, to look at the impact of glare on my retina; 
  • A color vision test for possible functional implications.

Ocular motility may also be evaluated. This is to determine if there are any problems with impaired eye movement.

Visual field will be analyzed in order to predict how the individual might function in day-to-day activities and respond to various rehabilitative approaches. Four types of visual field tests are available for this purpose:

  • A tangent screen test, during which the patient identifies a spot of light moving into his peripheral field.
  • A confrontations test, wherein the doctor introduces objects such as fingers or lights from the side to see if there are any large field defects like loss of side vision to one side due to stroke.  This test is not sensitive to small or slight changes in the vision.
  • A perimeters test, wherein lights or other targets are presented to the patient in various positions.  In a manual perimeters test, the doctor controls the target movement and speed of the target and maps out the field loss areas.
  • A computerized perimeters test, wherein a computer program presents a specific pattern of lights and then prints out the results with analysis.
  • Scanning Laser Ophthalmoscope (SLO) microperimetry and Preferential Hyperacuity Perimetry (PHP), more technologically advanced visual field tests used by some doctors.

Finally, to ensure that there are no other ocular diseases or complications, an external eye health evaluation will be made. This will include testing for intraocular pressures and also dilation of the pupils for an internal examination of the eye.

In special cases, additional testing may include an electroretinogram (ERG) and a visual evoked potential (VEP) study. These are usually administered in a hospital or clinic and may require sedation.

Treatment Strategies
Strategies for individuals with total blindness must be non-visual. Many times, however, low vision individuals will also benefit from non-visual solutions for improved accuracy and efficiency with a variety of tasks. The worse the vision, the greater will be the reliance on non-visual techniques and devices.

For people with low vision, glasses and/or contact lenses may first be prescribed to maximize vision. After that, optical devices like magnifiers, binoculars, telescopes, or tinted lenses may be recommended. Devices such as desktop electronic magnifiers, large print books, and many types of adaptive technology are available.

A child below the age of three should be referred to an Early Intervention program. Older children should be assigned to a teacher of the visually impaired within the school setting. Such intervention may also include physical, occupational and speech therapy.

Rehabilitation and Support
At least one study has indicated that health may actually improve as a result of access to information and by participating in a support group.

Participation in a good rehabilitation program and sharing information and experiences with similarly-affected people are the best treatments for the practical and emotional challenges of low vision. Referral to such programs should be as integral as testing and diagnosis to professional eye care.
Reviewed by Roy Cole, O.D., F.A.A.O.

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