Geographic Atrophy (GA) represents an advanced form of age-related macular degeneration (AMD), a common cause of vision loss. This progressive condition impacts the macula, the central part of the retina responsible for sharp, detailed vision. Optical Coherence Tomography (OCT) is a non-invasive imaging technique frequently used in ophthalmology. This article explores the specific features, or “hallmarks,” of GA on OCT scans.
Geographic Atrophy and Optical Coherence Tomography
Geographic Atrophy is characterized by progressive degeneration of retinal cells, including photoreceptors and the retinal pigment epithelium (RPE), which supports them. This degeneration primarily affects the macula, leading to blind spots and a decline in central vision. The lesions typically begin in the parafoveal region, an area near the fovea, before expanding and possibly involving the fovea itself over time.
Optical Coherence Tomography (OCT) emits light waves into the eye, measuring reflections from different retinal layers. Similar to ultrasound, it constructs high-resolution, cross-sectional images of the retina, revealing its distinct layers and thicknesses.
The non-invasive nature and high-resolution imaging capabilities of OCT make it a valuable tool for visualizing and monitoring GA. It provides detailed structural information, allowing ophthalmologists to assess damage and track progression. OCT can detect early signs of AMD, such as drusen, which are tiny protein clumps that can precede GA.
Identifying the Hallmarks of GA on OCT
Specific structural changes on an OCT scan are key to recognizing Geographic Atrophy. These changes, referred to as “hallmarks,” provide a detailed picture of the disease’s impact on the retina. An international group of experts, known as the Classification of Atrophy Meetings (CAM) group, developed a consensus classification system for atrophy based on OCT findings.
A primary hallmark is the thinning or complete absence of the Retinal Pigment Epithelium (RPE) layer. This RPE loss is a defining characteristic of GA, correlating with the severity of the condition. The RPE band, normally distinct, appears attenuated or disrupted on OCT scans.
The absence of the RPE leads to choroidal hypertransmission, appearing as increased signal penetration into the choroid, the vascular layer beneath the retina. This creates a brighter region on the OCT scan because the RPE, which normally absorbs light, is no longer present to block the signal, creating a “window defect.” Persistent choroidal hypertransmission defects measuring 250 micrometers or larger can indicate future GA formation.
The loss of outer retinal layers, including photoreceptors, is also a feature of GA. This includes the disappearance or disruption of bands like the ellipsoid zone (EZ) and interdigitation zone (IZ), indicators of photoreceptor health. The outer nuclear layer may also show thinning.
Drusen, characteristic deposits from earlier AMD stages, often regress or disappear in GA-affected areas. This “drusen collapse” can precede atrophy development. Hyperreflective foci, bright dots or lesions within retinal layers on OCT, can also be biomarkers for disease progression.
Other associated OCT features include depression of the inner retinal layers due to outer layer loss. Changes in choroidal thickness may also be observed, with some studies indicating reduced density of choriocapillaris vessels, especially along GA lesion margins. Visibility of Bruch’s membrane and the choroidal capillaris can also increase due to overlying outer retinal atrophy.
The Role of OCT Hallmarks in Clinical Practice
Identifying these specific OCT hallmarks holds importance in the clinical management of Geographic Atrophy. These detailed structural changes, visible through OCT, play a role in confirming a GA diagnosis. The consensus definitions, such as “complete RPE and outer retinal atrophy” (cRORA) and “incomplete RPE and outer retinal atrophy” (iRORA), are specifically based on OCT criteria, providing a precise anatomical measure of the disease.
Tracking changes in these hallmarks helps clinicians monitor disease progression. For instance, expansion of RPE atrophy, choroidal hypertransmission, and outer retinal loss indicates the disease’s advancement rate. This monitoring allows ophthalmologists to better understand prognosis and anticipate future vision loss.
These precise OCT measurements are also valuable in clinical trials and research for new GA treatments. Specific OCT biomarkers, like cRORA and iRORA, define and measure GA in studies evaluating new therapies, such as complement inhibitors. Quantifying these changes accurately is important for assessing experimental drug effectiveness in slowing or halting disease progression.
Understanding these OCT hallmarks also aids patient education. When patients see specific areas of damage and thinning on their OCT scans, it can help them better comprehend their condition and the reasons for vision changes. This visual information can empower patients to engage more actively in their care and adhere to recommended monitoring schedules.