What effect does a central retinal artery occlusion (CRAO) have on the electroretinogram (ERG)?

A central retinal artery occlusion (CRAO) primarily affects the inner layers of the retina, specifically the inner retinal cells responsible for generating the b-wave of the electroretinogram (ERG). In a CRAO situation, the outer retinal layers, including the photoreceptors that contribute to the a-wave, might still be functioning initially, especially if the occlusion is not longstanding.

Therefore, in the case of CRAO, the a-wave, which reflects the activity of the photoreceptors (rods and cones), may still be present as these cells can remain viable for a certain period even in the absence of adequate blood supply. However, the b-wave, which reflects the activity of the bipolar and Müller cells, will typically show a significant reduction or complete absence due to the lack of blood flow and nourishment that these inner retina cells require.

Thus, the observation that the a-wave remains while the b-wave disappears aligns with the pathophysiological changes occurring in CRAO, making this understanding crucial for interpreting ERG results in the context of retinal vascular occlusions.