The correct answer is yes, it is significant. The other correct answer is no, it is not significant. Both are correct, let me explain.
I am sure we have all asked ourselves similar questions when interpreting OCT results. Is the change that the OCT is reporting significant or not? If it is significant, then invariably, therapy needs to change. If it’s not, then maintaining the status quo is generally OK. Guess wrong, and the patient loses vision. So how do you know exactly what is significant and what is not significant?
The answer depends on two things: your instrument, and what you are looking at in the eye. Let’s take a look first at the instrument.
Many eye care practitioners have incorporated OCT technology into their offices. Some have played a role in selecting which instrument was incorporated, whereas others were not involved in the decision-making process. Either way, you need to know what your instrument does and what it does not do.
Resolution and Image Registration
What is the resolution of your instrument? You should have that answer at your fingertips. Is it two microns or ten? This of course presupposes a pristine image capture, but if the resolution of your instrument is two microns, then a follow-up scan of eight microns of difference is most likely significant. If on the other hand, the image resolution is ten microns, then a difference between the baseline scan and the subsequent scan of eight microns may in fact not be significant. Part and parcel of this discussion is image registration, in other words, do subsequent OCT scans obtain data in exactly the same place as the baseline scan. If they are not taken in the same place, then you’re really comparing apples to oranges.
Take for example a case of CSCR that is perfectly dome-shaped and not asymmetric in any way – a perfect dome – and let’s say your first horizontal OCT scan is obtained exactly at the apex of the domed retina. This will be the point of maximum elevation in this hypothetical case. If your subsequent scan is taken close to, but not exactly in the same location, say a few microns superior or inferior to the baseline scan, then the follow-up scan would inherently, by not being at the point of maximum elevation, read lower, leading you to believe the condition has improved. In this example, the follow-up has not been registered to the baseline, and while taken close together, they were not taken in the same exact location. If the image resolution is two microns, but the scans are not taken in the same exact place, then that info becomes hard to interpret. It becomes even harder if the resolution is ten microns and there is not adequate image resolution.
Of course, image quality plays a big role in being able to maximize the interpretation of the data. You need high quality images, and media opacities among other things, can degrade image quality. Poor quality images further make interpretation difficult.
What You’re Looking At
What you are looking at, i.e., what you are imaging, also plays a role in determining stability. For example, retinal thickness, in a normal healthy retina, thickness varies depending on where you are concentrating in the retina. The foveal avascular zone is thicker than the foveola, and is thicker than the immediately surrounding retina, especially when looking temporal to the foveola. In the OCT scan pictured below, retinal thickness, as mentioned above, varies, but across the board, it will be about 250 microns thick. Would an eight-micron difference from one scan to another be significant if we’re looking at the same point (image registration)? The answer: If both scans were pristine, image resolution was two microns, and there was image registration, the answer is probably yes, but, in reality, image quality can cloud this issue.
However, if we were looking at just the ganglion cell layer of the macula, as opposed to the entire retinal thickness, and we see an eight-micron difference between baseline and follow-up scan, that would be hugely significant! Why? Because, as we can see in the image below the ganglion cell layer (in a healthy patient) is only about 40 microns thick, so an eight-micron difference is almost a 25% variance. Your OCT certainly has better than a 25% resolution quotient. That practically means that an eight-micron decrease in ganglion cell thickness would equate to a 25% decrease in thickness…something that doesn’t happen because of image resolution. That is progression. Independent of the image resolution of your instrument.
Finally in the case of RNFL thickness, as seen in the image below, RNFL circle scan thickness will vary depending on what sector of the curcumpapillary RNFL you are looking at, with the superior and inferior temporal sectors being the thickest. These scans are plotted as TSNIT or NSTIN graphs and can vary normally from 40 to 100 microns. In the scan below, where the marker is placed, we see a four-micron difference from baseline scan. Is that significant? Probably not, as the areas on each side of that marker show no change. While there is a ‘dip’ in the RNFL scan where the marker is, that’s not enough of a difference to warrant changing therapy or the management plan, but it’s an area to watch in moving forward.
Where you look…what you look with….and the nuances of the instrument, all play a role in determining change over time. Gross change on the OCT is easy to see. However, it’s also easy to see without the OCT when you get right down to it. Use the OCT to determine subtle changes, but know how to use the information provided. Otherwise, you’ll end up over or under treating individuals.