Retinitis pigmentosa causes the degeneration of photoreceptor cells in the retina. As these cells degenerate and die, patients experience progressive vision loss.
Photoreceptor cells capture and process light helping us to see. There are types of photoreceptor cells: rod cells and cone cells. Rod cells are concentrated along the outer perimeter of the retina. Rod cells help us to see images that come into our peripheral or side vision. They also help us to see in dark and dimly lit environments. Cone cells are concentrated in the macula, the center of the retina, and allow us to see fine visual detail in the center of our vision. Cone cells also allow us to perceive color.
Most forms of RP begin with the degeneration of rod cells, which causes night blindness. Patients with Retinitis pigmentosa cannot adjust well to dark and dimly lit environments. As the disease progresses and more rod cells degenerate, patients lose their peripheral vision. Patients with Retinitis Pigmentosa often experience a ring of vision loss in their mid-periphery with small islands of vision in their very far periphery. Others report the sensation of tunnel vision, as though they see the world through a straw. Many patients with Retinitis Pigmentosa retain a small degree of central vision throughout their life.
Other forms of Retinitis Pigmentosa, sometimes called cone-rod dystrophy, first affect central vision. Patients first experience a loss of central vision that cannot be corrected with glasses or contact lenses. With the loss of cone cells also comes disturbances in color perception. As the disease progresses, rod cells degenerate causing night blindness and peripheral vision.
Nyctalopia (night blindness) is a decreased ability to see in reduced illumination that is seen in patients with impaired rod function; often associated with a deficiency of vitamin A. Patients may report difficulty driving in low light, at dusk, or in fog. Dark stairwells, movie theaters, and restaurants are situations where the transition between bright illumination and semidarkness may also present difficulties to some patients.
Ring scotoma or loss of peripheral vision may be profound before the patient explicitly recognizes it. Symptoms may include bumping into furniture or door-frames or difficulty in playing games (eg, tennis, basketball, baseball).
Retinal cells are among the most specialized cells in the human body and depend on a number of unique genes to create vision. A disease-causing mutation in any one of these genes can lead to vision loss. To date, Foundation researchers have discovered over 100 genes that can contain mutations leading to Retinitis Pigmentosa.
Other inherited diseases share some of the clinical symptoms of RP, including: Usher syndrome, which causes both hearing and vision loss, Bardet-Biedl (Laurence-Moon) syndrome, Best disease, choroideremia, gyrate-atrophy, Leber congenital amaurosis, and Stargardt disease.
Mucopolysaccharide disorders may be associated with RP. These include: Hurler syndrome, Scheie syndrome, Sanfilippo syndrome.
Mitochondrial disorders may also be associated. Kearns-Sayre syndrome manifests as ptosis, external ophthalmoplegia, and heart block.
Abetalipoproteinemia is an autosomal recessive disorder in which apolipoprotein B is not synthesized, leading to fat malabsorption, fat-soluble vitamin deficiencies, spinocerebellar degeneration, and retinal degeneration. The red blood cells show acanthocytosis.
The Electroretinogram (ERG) is the most critical diagnostic test for RP because it provides an objective measure of rod and cone function across the retina.
Formal visual field measurements are useful for ongoing follow-up care of patients with RP because a progressive loss of side vision is often the major symptom along with visual acuity changes.
The following tests are useful in excluding masquerading diseases or detecting conditions that are associated with RP:
VDRL and FTA-ABS tests
Serum phytanic acid when other neurologic abnormalities are present
Ornithine levels in patients where a diagnosis of gyrate atrophy may be confused with RP
ECG to rule out heart block in patients with suspected Kearns-Sayre syndrome
Lipid profile with possible protein electrophoresis in patients with suspected abetalipoproteinemia
Antiretinal antibodies, particularly antirecoverin antibodies may be present
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