Retinitis pigmentosa (RP) is a heterodegenerative disorder that typically starts with midperipheral vision loss and can lead to complete blindness. Because the progression of visual loss is relatively slow in patients with RP, detecting change during the course of a typical clinical trial presents a challenge. Frequency domain optical coherence tomography (fdOCT) offers a possible method for following disease progression in clinical studies of RP. With fdOCT, anatomically distinct layers of the outer retina can be visualized.
1,2 Various aspects of the outer retina, as seen on the fdOCT, have been quantitatively measured in eyes with RP. A number of studies have reported a decrease in thickness of the total receptor, outer nuclear layer (ONL), and/or outer segment (OS) due to RP.
3–13 Further, the thickness of these layers in the transition zone between healthy and severely affected retina provides a possible model of disease progression.
9–11 The earliest change seen in the transition zone with fdOCT is a decrease in the thickness of the OS layer, followed by decreases in the ONL.
10,11 With further loss in vision, the OS region disappears completely. This disappearance of the OS region does not occur until 9 to 10 dB of sensitivity has been lost as measured with static automated perimetry.
8,12,13 However, the point at which the OS region disappears is a marker of the edge of the usable visual field,
12 meaning the location at which visual sensitivity shows a precipitous drop. Most important, change in the location at which the OS layer is no longer present appears to be an excellent measure of progression of visual loss in patients with RP.
12,13
However, the OS is less than 40 μm
4,8 thick and thus measures of its thickness are variable. Fortunately, it is not necessary to measure OS thickness in order to estimate the point on the retina where it disappears. One can simply measure the point at which the inner segment ellipsoid zone (EZ) is no longer present. The EZ band (also known as inner segment [IS]/OS band) is thought by some
2 to be due to light scattered by the mitochondria of the ellipsoid region of the IS, and by others
14 to be due to the change in refractive indices of the IS and OS. In either case, by convention, the OS thickness is measured between the EZ band and the proximal border of the retinal pigment epithelium (RPE). Thus, when the EZ band disappears (i.e., is no longer discernible from the RPE border), the OS thickness is by definition zero.
While previous studies have shown that the EZ band can be used to track annual progression,
12,13 it is not clear if this is the best OCT measure. The fdOCT volume scans offer a number of possible measures of the thickness and volume of the OS layer and ONL. Here we compare various outer retinal measures derived from fdOCT line and volume scans in a group of x-linked (xl) RP patients who were followed over time.