Transitions to Abnormal Thickness in the Outer Segment and Outer Nuclear Layers
Our primary focus here is the receptor region. A is the horizontal scan of one of the patients with the central portion expanded in B and C. The blue curves in D and E show the thickness of the OS layer and ONL+ as a function of distance from the center of the fovea. The black curves in these panels are the mean (bold) and 95% confidence intervals (CI) (thin) for the group of 30 controls. As shown by the purple circles in D, we marked the nasal and temporal points at which the patient's OS thickness fell below the CI, that is, below the mean minus 2 standard deviations of the controls. E shows a similar analysis for the ONL+, where the red circles mark the point at which the thickness falls below the CI.
Eight of the 13 patients had a foveal region in which the OS thickness was within the CI, although this region was ≤100 μm in two cases. We call this central portion of the retina region A. Region A has an OS thickness within the normal CI.
In all 13 patients, the ONL+ was within normal limits in the center of the field, including in region A. However, there was typically a region, beyond region A, with an abnormal OS thickness, but with an ONL+ thickness within the normal CI. We call this region B. Operationally, region B was defined as the portion of the scan between the purple (OS below CI) and red (ONL+ below CI) symbols in D and E. The red bars at the bottom of E show the extent of this region for this patient. For the nasal retina, there was a measurable region B in 11 patients; in one patient it was not present (i.e., the purple and red symbols coincided); and in one patient the OS region, as well as the other layers, was normal to the end of the scan. For the temporal retina, there was a measurable region B in 10 patients, in two patients it was not present (purple and red symbols coincided), and in one patient the OS region was normal to the end of the scan. The median extent of region B was 460 μm (nasal) and 530 μm (temporal), with ranges of 0 to 1410 μm (nasal) and 0 to 2240 μm (temporal).
Transitions to Asymptotic Thickness in the Outer Segment and Outer Nuclear Layers
Two other transition points were identified in the OS and ONL+ thickness profiles. First, for the OS layer (D), we marked the point, green square, at which the OS thickness fell to 0, that is, the IS/OS line (dark blue in C) approached the OS/RPE border (purple) and disappeared. This point nearly always occurred at a more eccentric location than did the end of region B, the end of the normal ONL thickness: that is, there was typically a region where both the OS and ONL+ were present, but abnormal in thickness. We call this region C; its extent in E is shown as the green bars at the bottom of the figure. A measurable region C was present in all but 3 of the 24 hemifields with abnormal OS regions. The median extent of region C in these 24 hemifields was 700 μm (nasal) and 640 μm (temporal), with ranges of 120 to 2120 μm (nasal) and 270 to 2650 μm (temporal).
Beyond the point at which the OS layer disappeared, there was also typically a region over which the ONL+ continued to decrease until it asymptoted at a thickness of approximately 25 to 30 μm (dotted light blue line in E). The point at which this asympototic thickness was reached was approximated as shown in E by the blue squares. To aid in estimating these points, a horizontal line was drawn through the more peripheral points as shown by the dotted blue line in E. In all but two of the 24 hemifields (same patient), this point occurred at a more eccentric location than did the end of the OS layer. Thus, there was typically a region D where the OS layer was missing on the OCT scan, but the ONL+ layer had not yet reached an asymptotic thickness. The blue bar at the bottom of E indicates the extent of this region. The median extent of region D in these 22 hemifields was 965 μm (nasal) and 690 μm (temporal), with ranges of 0 to 1800 μm (nasal) and 170 to 2980 μm (temporal).
Finally, we defined region E, as the region over which the OS layer was not present and the ONL+ had reached an asymptote. Of the 24 hemifields with abnormal OS regions, 23 showed a region E, and this region extended to the end of the scan.
The Outer Limiting Membrane and the Inner Segment
A measure of receptor IS thickness could have been obtained as well by taking the difference between the OLM and the IS/OS borders. Instead, we marked the position at which the OLM disappeared (i.e., there was no longer a measurable IS thickness). The termination of the OLM could be identified in 22 hemifields. In three hemifields the OLM extended to the end of scan, and in one scan it was difficult to segment. In all 22 cases, the OLM terminated after the OS layer terminated (IS/OS border disappeared). The termination of the OLM in B is shown by the red arrows.
Regions of the Transition Zone
summarizes our working framework for categorizing the pattern of outer retinal loss across the transition zone from healthy to affected retina. is a summary of the thickness of the OS layer (panels A and B) and ONL+ (panels C and D) for each patient (different color symbols) analyzed separately for the nasal (left column) and temporal (right panel) halves of the scans. Each point in is the patient's average thickness expressed relative to the thickness of the controls averaged over the same portion of the retina. Consider the OS region first (panels A and B). As expected, the OS thickness is near, but typically below, the mean normal value (1.0) in region A and then falls to 0 in region D. The ONL+ (panels C and D) starts near the control values and falls to a smaller (asymptotic) value in region E. Recall by definition, the OS thickness is within the normal CI in region A, while the ONL+ is within the normal CI in regions A and B. The points, however, fall below the mean normal value of 1.0 because we define the end of these regions based on when the thickness falls outside the CI, not when it falls below the mean of the normal values.
Changes Seen in Outer Retina across Transition Zone
Figure 3. (A) Symbols are the patients' OS layer thickness in the nasal retina normalized by dividing by the mean thickness for the 30 controls over the same portion of the retina. (B) Same as in (A), but for the temporal retina. (C) Same as in (A), but for the (more ...)
Because the width of any given region differs across hemifields, it is not possible to relate the data in to confidence intervals based on our controls. The same data are shown in A–D as z-scores (i.e., the number of standard deviations above or below the mean of the controls). To obtain these z-scores, the thickness of each region of each hemifield was related to the distribution of the 30 control thicknesses for the same region width. The black diamonds show the average ±1 SE for the patients. As expected, in region A nearly all the points fall above −2 SD (lower dotted line) for both the OS and ONL+ layers. Likewise, for region B, nearly all the points for the ONL+ layer (C, D) fall above this limit, while all the points for the OS layer (A, B) fall below. Thus, and supply quantitative validation of our framework in . Although our framework is not meant to be a quantitative model, from it appears that OS length decreases by 40% before ONL+ thickness is affected (region B) and that an 80% reduction in OS is associated with only about a 25% decrease in ONL+ (region C).
Figure 4. (A) Symbols are the patients' OS layer thickness in the nasal retina expressed as z-scores. The difference between a patient's OS thickness and the mean thickness for the 30 controls, over the same portion of the retina, was divided by the SD of the 30 (more ...)
Although our primary purpose was to categorize the outer layers of the retina, we also measured the thickness of the inner retinal layers. shows the z
-score presentation, as in , for the INL, RGC+IPL layer, and RNFL. In A and B, the INL thickness is, on average, close to the mean of the controls, although on the nasal side, over 80% of the points in A are thinner than the controls, i.e., fall below the dashed line. On the temporal side, the INL layer is thicker than the controls for three hemifields in two to four of the regions. However, there does not appear to be any evidence for a region-specific change in the INL. On the other hand, for some hemifields the RGC+IPL thickness in the temporal retina and the RNFL thickness in the nasal retina are thicker than normal in regions D and E, consistent with our previous findings.4
Note that the RNFL of the temporal retina is not shown because it is the horizontal raphe region and thus very thin in controls.
Same as in but for INL (A, B), RGC+IPL layer (C, D), and RNFL (E).