What is retinitis pigmentosa?

Risk Factors

Males who have family members with RP are at greatest risk for the disease. Females may also inherit RP, though at a lower rate than males, and usually with less severe symptoms.

Etiology and Genetics

Retinitis pigmentosa is an inherited disorder, yet the identification of the genes involved has proven to be an extraordinary challenge. Since retinal cells are so highly specialized, they depend on a large number of specific genes and their protein products to create vision. Mutations that can cause retinitis pigmentosa have been identified in more than sixty different genes.

In the majority of cases—those linked to at least thirty-five of the known genes—the disease is inherited in an autosomal recessive fashion, which means that both copies of a particular gene must be deficient in order for the individual to be afflicted. Typically, an affected child is born to two unaffected parents, both of whom are carriers of the recessive mutant allele. The probable outcomes for children whose parents are both carriers are 75 percent unaffected and 25 percent affected.

At least twenty known retinitis pigmentosa mutations are inherited in an autosomal dominant manner, meaning that a single copy of the mutation is sufficient to cause full expression of the disease. An affected individual has a 50 percent chance of transmitting the mutation to each of his or her children. However, many cases of dominant retinitis pigmentosa result from a spontaneous new mutation, so in these instances affected individuals will have unaffected parents.

Mutations in two genes on the X chromosome—RP2 and RPGR, at locations Xp11.3 and Xp11.4 (or Xp21.1), respectively—are known to cause retinitis pigmentosa, and these show a sex-linked recessive pattern of inheritance. Mutations in at least four other genes are also thought to cause X-linked RP. Mothers who carry the mutated gene on one of their two X chromosomes have a 50 percent chance of transmitting the disorder to each of their male children. Female children have a 50 percent chance of inheriting the gene and becoming carriers like their mothers. Affected males will pass the mutation on to all of their daughters but none of their sons.

Finally, one rare form of retinitis pigmentosa, known as neuropathy, ataxia, and retinitis pigmentosa (NARP), results from mutations in the mitochondrial gene
MT-ATP6. Each retinal cell contains anywhere from several to more than one hundred copies of mitochondrial DNA (deoxyribonucleic acid) eligible for testing, and each mitochondrial DNA molecule contains thirteen structural genes that encode protein components of respiratory chain complexes. Inheritance of mitochondrial DNA follows a pattern of strict maternal inheritance, since all of the mitochondria in a fertilized egg (zygote) come from the egg cell. Thus affected females will transmit the disease to all of their offspring, but affected males produce unaffected children.

Symptoms

Loss of vision is usually first noted in childhood or early adulthood. The disease gradually worsens, so that after a number of years, vision loss may become severe. Symptoms vary depending on the type of retinal cell that is affected. Both eyes often experience similar vision loss.

It should be noted that RP is a slowly progressive disease, advancing over many years, and that most patients never become completely blind. In fact, even though many people with RP are considered “legally blind,” it is only because they have poor or nonexistent peripheral vision, resulting in a constricted visual field. Some still maintain excellent central visual acuity.

Overall, symptoms may include night blindness (the most common symptom); eyes taking longer to adjust to dim lighting; trouble seeing in foggy or rainy weather; eyes being slow to make the adjustment from bright sun to indoor lighting; and decreased peripheral vision and a narrowing field of vision, often called tunnel vision. Additional symptoms include difficulty seeing colors, especially blue; vision loss, partial or complete, usually gradually progressive; and clumsiness due to lack of sight, especially in narrow spaces, such as doorways. Blurry vision from cataracts may complicate RP later in the disease.

Screening and Diagnosis

The doctor will ask about a patient’s symptoms and medical history and will perform an eye exam. The patient may be referred to an eye specialist, such as an ophthalmologist.

Vision tests may include visual field testing to check peripheral vision, which is how well a patient sees off to his or her side, rather than directly ahead, without moving his or her eyes.

Visual acuity tests check how well a patient can see progressively smaller objects, usually a row of letters or numbers. Additional tests may include dark adaptometry, which tests how a patient’s vision adapts to darkness; color testing, which determines how well a patient can differentiate colors; and an electroretinogram (ERG), a test to measure electrical activity in the eye. An ERG identifies any loss of cell function in the retina and can be used to track the progression of the disease.

Treatment and Therapy

There is no effective cure for retinitis pigmentosa. Treatment is designed to help patients function with impaired vision. Doctors can counsel patients about expected patterns of vision loss based on the type of RP they have.

Recommendations include the use of vitamin A. One study implied that large doses of vitamin A palmitate—as high as 15,000 IU per day—can slow the progression of RP by approximately 2 percent per year. However, such high doses of vitamin A may cause liver problems and osteoporosis, and women who are or plan to become pregnant should avoid them due to an increased risk of birth defects. A more recent study, published in the Archives of Ophthalmology in 2012, concluded that among patients taking the recommended vitamin A supplements, those with a diet high in omega-3 fatty acids showed a 40 percent slower average annual decline than those who consumed low levels of omega-3. Patients should always talk to their doctors before taking any supplements.

Other recommendations include avoiding exposure to ultraviolet (UV) light, which can increase the rate of retinal degeneration. It is generally recommended that everyone, especially patients with disorders such as RP, wear dark UV-protected sunglasses and a wide-brimmed hat in bright, sunny conditions, such as while skiing or at the beach.

Aids for low vision may include magnifying glasses; electronic magnifiers, which project an enlarged image onto a screen; night-vision scopes, which enlarge distant objects under conditions of low light; and lenses for distant vision, such as eyeglasses or contacts.

Some community organizations offer classes to help people with vision loss adjust and learn how to use vision aids. If a patient is considered legally blind, he or she is entitled to many low-vision services at no cost.

Prevention and Outcomes

Once RP has been inherited, there are no known ways to prevent the disorder from occurring. Individuals who have RP or a family history of the disorder can talk to a genetic counselor when deciding whether to have children.

Bibliography

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