Retinitis pigmentosa

Retinitis Pigmentosa (RP) is the name given to a group of inherited eye diseases that cause progressive vision loss. These disorders affect the retina, the light-sensitive layer of tissue at the back of the eye. In RP, there is a gradual breakdown and loss of photoreceptor cells—the cells that detect light. As these cells die, vision deteriorates over time.

Despite its name, RP is not an inflammatory disease. The term "retinitis" is a historical misnomer; the underlying process is a primary degeneration of the retinal cells, not inflammation. The term "pigmentosa" refers to the characteristic clumps of pigment that can deposit in the retina as the disease progresses.

RP is typically bilateral (affecting both eyes) and is one of the most common causes of inherited blindness. Its onset can be in childhood, with progression leading to significant visual impairment, and often legal blindness, by middle age. However, the severity and speed of vision loss vary widely depending on the specific genetic cause.

Signs and Symptoms

The progression of symptoms in Retinitis Pigmentosa typically follows a distinct pattern, beginning with the degeneration of rod cells, which are responsible for vision in dim light.

Nyctalopia (Night Blindness): This is the most common and earliest symptom. Individuals find it increasingly difficult to see in dimly lit environments, such as at dusk or in a darkened room. Adjusting from bright to dim light also takes much longer.
Loss of Peripheral Vision: After the onset of night blindness, individuals experience a slow but progressive loss of their side vision. This creates the sensation of seeing through a narrow tunnel, often referred to as "tunnel vision." This makes tasks like navigating crowded spaces or driving difficult and eventually unsafe.
Loss of Central Vision: While central vision is often preserved until later stages, it can eventually be affected. This loss can be due to the eventual degeneration of cone cells (responsible for central and color vision) or from complications like cystoid macular edema (swelling in the central retina). Loss of central vision impacts the ability to read, recognize faces, and perform detailed tasks.
Photopsia and Photophobia: Some individuals may experience seeing flashes of shimmering or flickering light (photopsia). Many also develop sensitivity to bright light (photophobia).
Impaired Color Vision: As cone cells begin to degenerate in later stages, the ability to distinguish between colors can be diminished.

Cause and Genetics

Retinitis Pigmentosa is a genetic disorder caused by mutations in any one of more than 100 different genes. These genes are responsible for carrying the instructions to create proteins needed for the normal function and survival of photoreceptor cells (rods and cones) or the retinal pigment epithelium (RPE), a cell layer that nourishes the photoreceptors.

RP can be inherited in several ways:

Autosomal Recessive: This is the most common pattern, where an individual inherits a non-working gene from both parents.
Autosomal Dominant: An individual inherits one non-working gene from one parent.
X-Linked: The mutated gene is located on the X chromosome. This form typically affects males more severely than females.

In some cases, RP is part of a broader syndrome that affects other parts of the body. The most common example is Usher syndrome, characterized by both RP and hearing loss.

Diagnosis

Diagnosis of RP is made through a combination of patient history, clinical examination, and specialized testing:

Dilated Eye Exam: An ophthalmologist will examine the retina to look for characteristic signs, most notably dark pigment deposits called bone spicules, a thinning (attenuation) of retinal blood vessels, and a pale, waxy appearance of the optic nerve.
Electroretinography (ERG): This is the most critical test for confirming an RP diagnosis. It measures the electrical activity of the photoreceptor cells in response to flashes of light. In RP, the electrical responses are significantly reduced or absent.
Visual Field Testing: This test maps a patient's peripheral vision, documenting the extent of tunnel vision and tracking its progression over time.
Genetic Testing: Blood tests can identify the specific gene mutation responsible for the disease. This is crucial for confirming the diagnosis, providing prognostic information, and determining eligibility for gene-specific treatments.

Treatment and Management

While there is currently no cure for most forms of Retinitis Pigmentosa, a variety of strategies can help manage the condition and slow its progression.

Supportive Care:
- Low-vision aids, such as magnifiers, special lighting, and electronic devices, can help maximize remaining vision.
- Orientation and mobility training can help patients navigate safely as their visual field narrows.
- Sunglasses are recommended to reduce light sensitivity and to protect the retina from potentially harmful UV light.
Medical Therapies:
- Vitamin A Palmitate: High-dose supplementation may slow the progression of ERG decline in some types of RP, but it must be medically supervised due to the risk of liver toxicity.
- Treatment of Complications: Eye drops or injections may be used to treat cystoid macular edema if it develops.
Emerging and Advanced Therapies:
- Gene Therapy: In 2017, the FDA approved Luxturna (voretigene neparvovec-rzyl), a gene therapy for patients with RP caused by mutations in the RPE65 gene. It works by delivering a correct copy of the gene to retinal cells.
- Retinal Prostheses: Devices like the Argus II Retinal Prosthesis System (often called a "bionic eye") can provide a form of artificial vision for individuals with profound vision loss from RP. It uses a camera mounted on glasses to send electrical signals to the retina, allowing the user to perceive patterns of light.
- Research: Active research continues in areas like stem cell therapy, optogenetics, and neuroprotective drugs to preserve retinal cells.