Low Vision Facts - Vision Assessment

Vision loss can occur in varying degrees in the following areas: (1) impairment to central vision resulting in blind spots, poor acuity and difficulty or inability to see fine detail and perceive color, (2) impairment of the cornea or lens resulting in blurred vision and sensitivity to glare, (3) impairment of peripheral vision leaving only the ability to see the center of the normal visual field (tunnel vision), thereby limiting the ability to perceive motion, see an entire picture at one time, and see in low light, (4) loss of vision in other parts of the visual field, including the left, right , upper or lower parts of the visual field, or (5) a combination of any of these condition.

Since vision changes can occur without notice, eye examinations are recommended on a regular basis. The minimum recommended frequency of examination for those at low risk for vision loss, as recommended by the Canadian Association of Optometrists (2003), is as follows:

Infants and toddlers (birth to 24 months) – initial exam at or before 6 months of age
Preschool (2 to 5 years) - at age 3, and prior to entering elementary school
School age (6 to 19 years) - annually
Adult (20 to 64 years) - every one to two years
Older adult (65 years and older) - annually

A comprehensive eye examination evaluates general eye health and may result in a corrective lens prescription, if appropriate. A number of tests may be conducted to examine the external and internal parts of the vision system including the eyelashes, eyelids, conjunctiva, iris, lens, cornea, pupils, extra-ocular muscles, blood vessels, optic nerve and the retina. Vision problems including glaucoma, cataracts and corneal diseases, as well as ocular changes associated with diseases such as diabetes, high blood pressure and neurological disorders will be assessed (EyeHealth Northwest, 2003). The following is a description of some common eye tests and procedures.

Glaucoma Tests

Fluid pressure inside the eye is called intraocular pressure (IOP). This is a balance, called tension, between the production and the drainage of the aqueous fluid inside the anterior chamber of the eye. It is measured with a procedure called tonometry. In the non-contact procedure, a puff of air is blown onto the eye and an instrument calculates pressure from the change in the light reflected off the corneas as the air puff is blown (Douglas, 2001). Elevated IOP can cause pressure within the eye to increase and damage the optic nerve. As abnormal pressures are often asymptomatic it is important to have the pressure checked regularly.
The visual field test is used to examine peripheral vision. It provides information related to neurological function of the retina, optic nerve, and brain. It is often used to monitor eye diseases such as glaucoma, and also as a screening prior to surgery. Visual field tests vary, but most have a white bowl with a small fixation light in the center and are often computerized. Patients are asked to stare straight ahead as tiny lights flash in the periphery and press a button each time a light appears. The computer analyzes the data and prints a chart that indicates where in the visual field the lights were seen (St. Luke’s, 2003).
Gonioscopy is an eye examination used to evaluate the angle between the cornea and the iris using mirrors and a special lens called a”gonio lens” (Anard, 2003). This test indicates whether the area where fluid drains out of the eye is blocked or damaged in some way. Gonioscopy is often done as part of a complete glaucoma exam, and may also be routinely done if the patient is at risk for developing glaucoma (Spengler, 2002).
Over time, increased pressure in the eye can cause damage to the optic nerve (Saine, 1996). Fundus photography, a highly specialized form of medical imaging using a customized camera mounted to a microscope with mirrors and high-powered lenses, is used to document the health of the optic nerve, vitreous humor, macula, retina and its blood vessels (St. Luke’s, 2003).

Retinal Tests

An ophthalmoscope is an illuminated instrument used to examine the internal structures of the eye including the retina and vitreous humor (Cassin and Solomon, 1997). Ophthalmoscopes can be direct or indirect. Direct ophthalmoscopes are hand-held instruments, about the size of a flashlight, with a battery powered light source. Rotating lenses incorporated in the instrument are used to focus the examiner’s field of view on the retina. The direct ophthalmoscope is useful for examining the central retina. With the indirect ophthalmoscope technique, the examiner wears an instrument on the head that resembles a miner's light. A larger area of the retina can be viewed with an indirect ophthalmoscope (Medline Plus, 2003).
The Amsler Grid is a screening test used for detecting central visual field distortions that occur in macular diseases such as macular degeneration. It consists of evenly spaced horizontal and vertical lines printed on black or white paper that form a grid. In the center of the grid is a dot that the patient focuses on, one eye at a time. If the lines of the grid do not appear straight and parallel or areas appear to be missing, it is an indication that further examination of the macula is needed (St. Luke’s, 2003).
Fluorescein angiography (FA) is a test used to determine if there is proper circulation in the retinal blood vessels. Fluorescein dye is injected, usually in a vein in the arm, and then a series of rapid photographs of the back of the eye are taken as the dye circulates (Cassin and Solomon, 1997). This test is useful for evaluating many eye diseases that affect the retina.
An Indocyanine Green study (ICG) is a special dye test used to evaluate the circulatory system of the choroid, the layer just behind the retina. The test is administrated in a manner similar to the Fluorescein angiography, however, it differs in that ICG allows the examiner to view circulatory “leaks” under a layer of blood vessels which is not possible with fluorescein angiography (Cassin and Solomon, 1997).
Ophthalmic ultrasound is the transmission of high frequency sound waves into the eye which are reflected by the tissue and displayed on a screen (Cassin and Solomon, 1997). This non-invasive test is used to estimate eye structure dimensions, document pathology such as tumors, and examine the inside of the eye. The sound frequency emitted from the probe determines the type of image formed on the screen.

Corneal Tests

Corneal topography is used to examine the curvature of the cornea. A device called a “corneal topographer” projects a series of illuminated rings onto the corneal surface. The rings are then reflected back and analyzed by a computer which generates a colored topographical map of the cornea. This map is useful in evaluating astigmatism, monitoring corneal disease and as a diagnostic procedure for patients being considered for refractive surgical procedures, such as Laser-Assisted In Situ Keratomileusis (LASIK) (EyeMDLink.com, 2003).
Keratometry also measures the corneal curvature. However, rather than mapping the entire surface as corneal topography does, it measures only the flattest and the steepest curves, called the “flattest meridian” and the “steepest meridian.” Unequal meridional powers indicate astigmatism (Cassin and Solomon, 1997). These two measurements provide the examiner with information related to distortions in the cornea’s focusing power and are helpful for fitting contact lenses and monitoring the curvature of the cornea following surgery (St. Luke’s, 2003).
The pachymeter is a device that measures the thickness of the cornea using ultrasound (WebEyeMd.com, 2003). It is useful in monitoring the progression of certain disorders that cause the cornea to become thickened resulting in vision loss. Pachymetry is also an essential measurement prior to certain refractive surgical procedures, such as LASIK which remove tissue from the cornea (St. Luke’s, 2003).
Corneal and conjunctival dryness can be caused by low tear production The Schirmer Test is one test that is used for measuring the production of tears. Small paper strips are placed in the lower lids and removed after a few minutes. The moist parts of the strips are then measured (Cassin and Solomon, 1997).
Specular endothelial microscopy is a test that uses a microscope and a camera to examine and photograph the size and regularity of endothelial cells that line the back of the cornea. It is useful in calculating the risk to the cornea during cataract removal (Cassin and Solomon, 1997).

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