RPE dropout refers to the loss of cells within the retinal pigment epithelium (RPE), a single layer of pigmented cells situated directly behind the retina. This cellular loss creates gaps in what is normally a uniform and highly organized layer. The term “dropout” describes these areas where RPE cells are absent, indicating a compromised region.
The Role of the Retinal Pigment Epithelium
The retinal pigment epithelium (RPE) serves as a support system for the light-sensing photoreceptor cells of the retina, including rods and cones. This single layer of tightly joined cells forms a barrier, regulating the exchange of substances between the retina and the underlying choroid, a layer rich in blood vessels. The RPE transports essential nutrients from the blood supply to the photoreceptors, ensuring their proper function.
A second function of the RPE involves the removal of waste products. Photoreceptors continuously shed their outer segments, and the RPE engulfs this debris through a process called phagocytosis. This prevents the accumulation of waste materials that could impair retinal health. The RPE also contains melanin, a dark pigment that absorbs stray light, reducing light scatter and enhancing visual clarity.
Causes of RPE Cell Loss
RPE dropout is not a disease itself but a sign of underlying conditions affecting the retina. Age-related macular degeneration (AMD), particularly its “dry” form, is a common cause of RPE cell loss. In dry AMD, progressive atrophy of RPE cells can lead to geographic atrophy, characterized by well-defined areas of RPE and photoreceptor cell death.
High myopia, or severe nearsightedness, can also contribute to RPE cell loss. In highly elongated eyes, the retina and RPE become stretched, potentially leading to thinning and degeneration of these cells in the macula. Inherited retinal dystrophies, such as Stargardt disease and pattern dystrophies, are another group of genetic conditions that cause RPE dropout. Stargardt disease involves the accumulation of waste products in the RPE due to a genetic mutation, leading to RPE and photoreceptor dysfunction and eventual cell death. Pattern dystrophies are characterized by various patterns of pigment deposition and RPE changes, often due to genetic mutations.
Past inflammation, such as uveitis, or trauma to the eye can also result in localized RPE cell damage and subsequent dropout. These events can disrupt the RPE layer, leading to areas of cellular atrophy. The presence and pattern of RPE dropout can provide clues for diagnosing the underlying eye condition.
Impact on Vision and Associated Symptoms
The loss of retinal pigment epithelium cells directly impacts vision because the RPE provides support for the overlying photoreceptor cells. When RPE cells are lost, photoreceptors in that area are deprived of nutrients and waste removal, leading to their dysfunction and eventual death. This results in a scotoma, which is a blind spot in the field of vision that directly corresponds to the area of RPE dropout.
The location and size of the RPE dropout determine the severity and type of visual symptoms. If the dropout occurs in the macula, the central part of the retina responsible for sharp, detailed vision, patients may experience blurry central vision. Another common symptom is metamorphopsia, where straight lines appear warped or distorted. This occurs because the loss of RPE cells can alter the normal smooth surface of the retina, causing light to be perceived incorrectly.
Symptoms can be subtle or even unnoticeable if the area of RPE dropout is small or located in the peripheral retina, outside the central visual field. However, if the dropout affects the macula, especially the fovea, the most sensitive part of the macula, the impact on reading, recognizing faces, and other detailed tasks can be significant. The visual disturbance directly reflects the compromised support for the photoreceptors and their subsequent inability to process light accurately.
Diagnosis and Monitoring
Eye doctors identify RPE dropout through specialized imaging techniques. Optical Coherence Tomography (OCT) is a non-invasive scan that provides cross-sectional images of the retina, allowing clinicians to visualize distinct layers and detect areas where the RPE layer is missing or thinned. On OCT, RPE loss is often seen as a complete absence of the RPE in combination with increased signal transmission to the underlying choroid.
Another diagnostic tool is Fundus Autofluorescence (FAF), a type of photograph that highlights RPE cells based on their natural autofluorescence. Areas of RPE dropout appear as hypoautofluorescent, or dark, regions because the RPE cells are absent. This imaging modality is useful for mapping the extent of RPE atrophy and monitoring its progression.
Currently, there is no treatment that can regenerate or replace lost RPE cells in areas of dropout. Therefore, management of RPE dropout focuses on monitoring atrophy progression and addressing the underlying cause. For example, in cases related to Age-Related Macular Degeneration, nutritional supplements like AREDS2 may be recommended to slow disease progression. Regular eye exams track any changes in RPE dropout and manage associated complications.