Testing for Retinitis Pigmentosa

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You may wonder how RP is diagnosed, and why so many tests are
needed to diagnose it properly. It is important to understand that a
combination of many tests is often needed to separate RP from other
retinal conditions. It is not uncommon for a person to visit several
doctors before arriving at the diagnosis of RP. The tests also help
your doctor understand how well your retina functions. Some of
the tests are:

  • Visual Acuity Testing — Visual acuity is another term for visual clarity. Most people are familiar with this test, in which they read letters from a chart while seated at a certain distance. A person with normal visual acuity is said to have 20/20 vision. A person with 20/40 vision can see at a distance of 20 feet what a person with “normal” vision can see at 40 feet.
  • Visual Field Testing — This test measures a person’s field of vision.A light is brought in from the side on ascreen,and slowly moved to the center of vision. Patients press a button as soon as they see the light. For individuals with RP, the field of vision gradually decreases over time. The area of vision will become smaller and smaller until it is like looking through a straw. At left you can compare the visual field of a person with healthy vision to that of a person with RP.
  • Electroretinogram (ERG) — This tests rod and cone function,and is important for confirming a diagnosis of RP. In some cases, the ERG shows signs of RP even before the patient is aware of symptoms or before the doctor can see signs of RP in the retina. This specialized test is performed in only a small number of centers nationwide. For ERG testing, a numbing drop is put in the eye and a special type of contact lens electrode is placed on the eye. Flashes of light are used to stimulate the retina. Electrodes measure the electrical response of the rods and cones to the flashing lights. This test is usually performed in a darkened room. The test is not painful, but some find it to be uncomfortable.
  • Fundus photographs — Using a special camera, your doctor can photograph the fundus, or back of the eye. The testing is relativelyfast, but requires that the eyes be dilated. The images at the right show the fundus images of a person with healthy vision and a person with RP.
  • Optical Coherence Tomography — This test captures cross-sectional images of the retina. It measures the thickness of the retina and can identify retinal abnormalities. The device scans the retina surface with light to obtain images.
  • Fluorescein Angiogram — This test may or may not be used at your visit. It involves a special dye (fluorescein) that allows your doctor to see the blood vessels at the back of the eye. The eyes are dilated and the dye is injected into a vein in the arm. A special camera records the dye as it passes through the blood vessels in the eye. The resulting photos allow your doctor to identify retinal problems.