Drusen are yellowish deposits that accumulate beneath the retina, the light-sensitive tissue at the back of the eye. Optical Coherence Tomography (OCT) is an advanced imaging technology that provides detailed, cross-sectional views of these deposits. OCT visualization of drusen is important for understanding and monitoring eye health, especially in age-related macular degeneration.
What Are Drusen?
Drusen are extracellular accumulations primarily composed of lipids, proteins, and carbohydrates. These deposits form specifically between the retinal pigment epithelium (RPE) and Bruch’s membrane beneath the retina. While small drusen can be a normal part of aging, their presence, size, number, and location are closely linked to age-related macular degeneration (AMD), a leading cause of vision loss.
The exact mechanism of drusen formation is still under investigation, but they are thought to result from the extrusion of damaged proteins and lipids by RPE cells. Components of the complement system, along with minerals like zinc, calcium, and iron, also contribute to their composition and enlargement. Drusen can potentially disrupt the flow of oxygen and nutrients to the RPE and photoreceptor cells.
How OCT Visualizes the Eye
Optical Coherence Tomography (OCT) is a non-invasive imaging technique that generates high-resolution, cross-sectional images of the eye’s internal structures. It operates by emitting a beam of near-infrared light towards the tissue and measuring the echoes of the light that scatter back. This process is analogous to how ultrasound uses sound waves, but OCT uses light, allowing for much finer detail.
The system then constructs a detailed layered image of the retina and the underlying tissues, including the choroid. OCT provides high axial resolution, surpassing the resolution of other medical imaging methods like ultrasound or MRI. This capability enables clinicians to visualize the intricate layers of the retina and detect subtle changes without direct contact with the eye.
Detailed View of Drusen with OCT
OCT provides a detailed view of drusen, allowing for their precise localization and characterization. Hard drusen appear as small, well-defined hyperreflective deposits located beneath the retinal pigment epithelium (RPE). Soft drusen, which are generally larger, present as dome-shaped elevations of the RPE with varying internal reflectivity.
Another type, reticular pseudodrusen (also known as subretinal drusenoid deposits), are distinct from typical drusen as they are located above the RPE, appearing as granular hyperreflective deposits between the RPE and the ellipsoid zone. OCT can further reveal specific internal features within drusen, known as OCT-reflective drusen substructures (ODS). These include low reflective cores, high reflective cores, conical debris, and split drusen, each representing different compositions within the deposit. For example, low reflective cores might be primarily lipids, while high reflective cores could contain proteins and hydroxyapatite. Drusenoid pigment epithelial detachments (PEDs), large elevations of the RPE, are also clearly visible and are often composed of confluent soft drusen.
Implications for Age-Related Macular Degeneration
The characteristics of drusen observed on OCT scans provide important information about the risk and progression of age-related macular degeneration (AMD). The size and volume of drusen are important, with larger drusen or an increased drusen volume indicating a higher risk of advancing to late-stage AMD.
Specific OCT-reflective drusen substructures (ODS) are also considered biomarkers for disease progression. The presence of ODS, such as conical debris or high reflective cores, is associated with an increased risk of developing geographic atrophy (GA), a severe form of dry AMD characterized by retinal tissue loss. Other OCT biomarkers, including retinal pigment epithelium (RPE) abnormal thinning, RPE disruption, hyperreflective foci, and choroidal hypertransmission, also correlate with the risk and rate of AMD progression. These detailed findings help predict whether AMD will advance to geographic atrophy or choroidal neovascularization (CNV), a wet form of AMD involving abnormal blood vessel growth.
Monitoring and Management
Optical Coherence Tomography plays a role in the diagnosis and ongoing management of patients with drusen and age-related macular degeneration. OCT enables clinicians to detect early signs of AMD, such as the presence and characteristics of drusen, allowing for timely intervention. Regular OCT scans are used to monitor changes in drusen size, volume, and the development of associated retinal changes over time.
This ongoing monitoring helps track disease progression and assess the effectiveness of any treatments. The detailed structural information provided by OCT aids in differentiating between dry and wet forms of AMD, which guides treatment decisions. While OCT offers benefits, interpreting the images can be complex due to factors like patient cooperation and other ocular conditions, requiring specialized expertise for accurate assessment.