Retinitis pigmentosa (RP) is a group of inherited diseases that affect the retina, the light-sensitive tissue at the back of the eye. These genetic conditions cause the progressive breakdown of photoreceptor cells, which are responsible for detecting light, leading to vision loss. While many forms of RP follow a predictable pattern, some cases present differently. These variations are known as atypical retinitis pigmentosa and deviate from the classic symptoms and progression, often creating diagnostic challenges.
Differentiating Typical from Atypical Retinitis Pigmentosa
The classic presentation of retinitis pigmentosa begins with difficulty seeing in dim light or at night, a symptom known as nyctalopia. This is followed by a gradual loss of peripheral vision, which slowly narrows the field of view, creating “tunnel vision.” During an eye examination, an ophthalmologist often observes dark pigment deposits in the retina known as bone spicules, a waxy, pale appearance of the optic disc, and thinning of the retinal blood vessels.
A diagnosis of atypical retinitis pigmentosa is considered when the disease deviates from this established pattern. The variation can manifest in the age when symptoms first appear or the specific pattern of vision loss. For example, some individuals may experience a decline in their central vision before their peripheral sight is affected. In other cases, the retina may appear normal or lack the characteristic bone spicule pigmentation, especially in the early stages of the disease.
These variations make diagnosing atypical RP more complex than its typical counterpart. Because the classic signs may be absent, the condition can be mistaken for other retinal diseases. The absence of pigmentary changes, for instance, can obscure the connection to RP, leading to initial misdiagnoses.
Non-Syndromic Atypical Presentations
When atypical RP occurs without affecting other body systems, it is classified as non-syndromic. Some distinct presentations include:
- Retinitis pigmentosa sine pigmento: Translating to “RP without pigment,” individuals experience the functional symptoms of RP, including night blindness and visual field loss, but the hallmark bone spicule deposits in the retina are absent. This lack of pigmentation can make diagnosis difficult, relying more on functional tests.
- Sector retinitis pigmentosa: The disease is confined to a specific quadrant or section of the retina. The photoreceptor cells in the affected area degenerate, leading to a localized patch of vision loss, while the remainder of the retina can stay healthy for an extended period. This form often progresses much more slowly.
- Unilateral retinitis pigmentosa: An exceptionally rare form where the condition develops in only one eye, while the other remains completely unaffected. The affected eye shows the characteristic signs of photoreceptor degeneration and progressive vision loss, but the disease does not spread.
- Pericentral retinitis pigmentosa: In this variant, photoreceptor degeneration begins in a ring-like pattern around the central part of the retina (the macula). This creates a donut-shaped blind spot, sparing the far peripheral vision and the very center of sight initially.
Syndromic Retinitis Pigmentosa
In some instances, retinitis pigmentosa is one component of a broader genetic syndrome that affects multiple parts of the body. When RP is associated with other health conditions, it is referred to as syndromic. These forms highlight the complex role that certain genes play in both retinal function and other tissues. Identifying the syndromic nature of the disease is important for comprehensive patient care.
Usher syndrome is the most common condition that combines retinitis pigmentosa with hearing loss. Individuals with Usher syndrome are often born with hearing impairment or deafness, and they later develop the vision loss characteristic of RP during adolescence or young adulthood. The dual sensory impairment presents unique challenges, and management requires a coordinated approach that addresses both hearing and vision needs.
Bardet-Biedl syndrome (BBS) is another multi-system disorder where RP is a primary feature. Alongside vision loss, BBS is characterized by a range of other signs, including obesity that begins in childhood, the presence of extra fingers or toes (polydactyly), and kidney abnormalities. Learning difficulties and other developmental issues can also be part of the syndrome.
Refsum disease is a metabolic disorder where the body is unable to process a substance called phytanic acid, a fatty acid found in certain foods. The buildup of phytanic acid becomes toxic to various cells, leading to a combination of symptoms including retinitis pigmentosa, hearing loss, a reduced sense of smell, and neurological problems. Treatment for this form involves a strict diet low in phytanic acid.
Diagnostic Process and Management Strategies
Confirming a diagnosis of atypical retinitis pigmentosa involves a series of specialized eye examinations. A visual field test is used to measure the full extent of a person’s peripheral and central vision, identifying the pattern of any blind spots. An electroretinogram (ERG) is performed to assess the function of the retina by measuring the electrical responses of photoreceptor cells to light. A fundus examination allows for inspection of the retina and optic nerve, though pigmentary changes may be absent in some atypical cases.
Given the genetic nature of RP, genetic testing is an important part of the diagnostic process. By analyzing a blood or saliva sample, labs can identify the specific gene mutation responsible for the disease. This information can confirm a diagnosis, help predict the likely course of the condition, and determine if the RP is an isolated issue or part of a larger syndrome. For example, identifying a mutation in the RPE65 gene can make a patient eligible for specific gene therapies.
Currently, there is no single cure for all forms of retinitis pigmentosa, so management focuses on slowing disease progression and maximizing remaining vision. Low-vision aids and rehabilitation services can help individuals adapt to vision loss. For some patients, high-dose Vitamin A palmitate may be prescribed under medical supervision to slow the decline of retinal function. Emerging treatments include gene therapy like voretigene neparvovec-rzyl (Luxturna), which is approved for patients with mutations in the RPE65 gene. For syndromic forms, management requires a multidisciplinary team to address the associated health conditions.