Diagnosis of Retinitis Pigmentosa
A number of tests are used to diagnosis retinitis pigmentosa. Since most RP patients have no known family history of RP, a family history is often of no help in making the diagnosis. A medical history is taken to rule out systemic conditions that may affect the eyes.
An electroretinogram (ERG) measures the response of the retina to a light stimulus and can be helpful in confirming the diagnosis of RP. A corneal electrode is gently placed behind the lower eyelid, lightly touching the cornea, and neutral electrodes are placed on the skin around the eye. A light is shone in the eye and the electroretinogram records electrical changes in the retina. This provides information about the performance of rods and cones.
An exam of the retina is done using an indirect ophthalmoscope, an instrument that enables the doctor to examine the different parts of the eye through a dilated pupil. A healthy retina has an orange-red appearance when viewed during an eye examination. The retinal layer itself is actually clear and the color that is seen is due to the underlying tissue, the choroid, which is composed mostly of blood vessels.
When an eye with retinitis pigmentosa is examined, the orange area appears broken up with black or dark brown spots of pigment, caused by the breakdown of the pigment epithelial cells. These spots are called bone spicules. Other findings are a pale optic nerve and narrowed retinal blood vessels.
Visual acuity, refraction, color testing, and peripheral vision testing are all part of a complete eye examination in a patient who is suspected of having a retinal degeneration.
Visual acuity is tested using a standard eye chart, which features black letters on a white background. The chart measures how well the patient sees at various distances and may detect vision loss.
A refraction test determines visual acuity and the prescription for eyeglasses or contact lenses. While corrective lenses do not improve vision loss from retinitis pigmentosa, many patients have accompanying eye disorders such as nearsightedness that can be corrected.
The patient looks through a device called a phoropter or refractor, which resembles a large set of binoculars with multiple lenses and reads an eye chart approximately 20 feet away. The lenses are adjusted until the chart is as clear as possible.
Loss of color vision is tested by using a number of simple screening tests. The Hardy-Rand-Rittler (H-R-R) and Ishihara tests evaluate the type and the degree of color blindness.
In these tests, colored triangles, squares, and other shapes lie within a jumble of dots that vary in color and intensity. As the patient identifies the colored shapes, the eye care professional can determine the ability to differentiate colors. The Holmgren yarn-matching test and the Farnsworth-Munsell 100-hue disk-matching test evaluate the patient's ability to match up colors.
Treatment for Retinitis Pigmentosa
There is no standard treatment for retinitis pigmentosa. Recent studies report that vitamin A may slow progression of the disease.
Retinitis Pigmentosa Prevention
Retinitis pigmentosa cannot be prevented. Genetic screening can provide information about who does and does not, which can aid in family planning.