Bidirectional trans-synaptic axonal degeneration in a hardwired pathway. (a) Normal situation. (b) Axonotmesis of the first axon results in anterograde axonal degeneration (red arrow to the right). This process can pass on to the second axons trans-synaptically. (c) Likewise, axonotmesis of the second axon results in retrograde degeneration. Again this process can pass on the first axon trans-synaptically. Logically, the process will always be bidirectional
cSLO images (left) and corresponding (green reference lines) OCT B-scans (right). (a) Optic nerve head. Segmented retinal layers are shown for a peripapillary ring scan (green circle in Confocal scanning laser ophthalmoscopy (cSLO) image to the left). Note the very thin GCL close the optic nerve head explaining why here the combined GCL-IPL (also called ganglion cell complex, GCC) were shown for segmentation. The blue line in the cSLO images indicates the physiological tilt of 7° of the foveola with respect to the optic disk. (b) Macular region. Segmented layers are shown for a B-scan crossing the foveola (green vertical line in cSLO image) constituting the macular volume scan. Note the absence of inner retinal layers from the foveola. RNFL retinal nerve fiber layer, GCL ganglion cell layer, IPL inner plexiform layer, INL inner nuclear layer, OPL outer plexiform layer, ONL outer nuclear layer. Figure reprinted with permission from (28)
In addition to quantitative OCT data, qualitative assessment has become important (12). Microcystic changes present in the INL have been called microcystic macular oedema (MMO) according to British and microcystic macular edema (MME) according to US spelling (Fig. 3). While these changes are not specific for MS (13, 14), they are of potential prognostic value in MS (15, 16). Therefore, retrograde maculopathy should be the preferred terminology (17). This chapter describes a protocol for retinal OCT of the peripapillary and macular regions useful for the assessment of neurodegeneration in MS.
Microcystic macular edema (MME) of the INL in a patient with RRMS who had experienced optic neuritis in this eye (13). The INL is thickened (72 μm) at site of MME compared to the opposite site (52 μm). The vertical green line in the cSLO image (left) indicates placement of the OCT B-scan shown to the right
A retinal OCT device. At the time of writing, this should be a spectral domain OCT device, ideally with an eye tracking function for optimal reproducibility (18, 19). An updated list of the United States Food and Drug Administration (FDA)-approved devices can be found on: www.fda.gov/MedicalDevices. Please consult the manual of your device for specific instructions and safety regulations.
The software version at the time of writing is 22.214.171.124 for the acquisition module and 126.96.36.199 for the viewing module. All data are stored in the “Heidelberg Eye Explorer” database version 188.8.131.52. Please take a second to check which software versions your system has. To do so, click on “Help” at the menu bar and then click on “About….” The new window will list all software versions. Please note that automated layer segmentation may not be available for older software versions.
3.1 Activating the OCT Device
Prepare the device: (a) pull the camera all the way back, (b) clean forehead rest and chin rest, (c) check that the lens is clear, and (d) adjust table height and chin rest so that the canthus marker (see Fig. 4) on the right is at the patient’s eye height and that the patient feels comfortable.
Prepare the patient and inform him/her that there will be a high-pitched noise during the operation. Offer ear plugs in case the patient is phonophobic and cannot tolerate the noise.
Start the software, which will open the database on the screen. Create a new patient file. Alternatively open an old patient file if you have already examined this patient before. Start a new examination. You will be guided to a number of windows allowing you to enter the patient’s demographic information. Additional information may be added if required.
Select the acquisition mode. You will note that the square on the bottom right of the screen turns yellow. It will take a few seconds before the computer has calibrated and started up the OCT camera. The OCT camera is working when the high-pitched sound starts. The sound is caused by the fast-turning motor that is needed to rotate the mirrors that reflect the light needed for OCT image acquisition. You will see immediately the cSLO image in the large window on the left of the screen. It takes another few seconds before the turning mirror wheel provides enough light information to create an OCT image signal, which will appear in the window on the right side of the screen.
Explain to the patient what will happen (see Table 1). Next, select a fixation target. There are two options: (a) an internal fixation target is selected by clicking on the point raster (nine points) on the bottom of the screen. The easiest option is to select the point in the middle of the raster by clicking on it. The point will change color from black to blue (on the PC screen), indicating that it is working. The patient sees a blue dot. Some patients may have difficulties seeing the blue internal fixation target either because the vision in the eye to be examined is poor (e.g., optic neuritis) or for other reasons. In this case (b) an external fixation target is needed (see Fig. 4). Needless to say that the patient will look at the external fixation target with the eye that is not being examined by OCT. In some cases a moving fingertip may be easier for a patient than the external fixation lamp. In very few cases with extremely poor vision, an imaginary target will be needed and you will need to instruct the patient to look a tiny bit more to the left/right/up/down.
Always explain to the patient what he/she will see during the OCT examination during which his/her eye is investigated
The patient will see either a red circle (in the case of pRNFL) or a set of red parallel lines (in the case of the macular scan). As the OCT images are taken, the red light will move. This is particularly noticeable for the red lines. Tell the patient that the eye always reacts sensitively to movement and that this is normal that he/she will eventually look at the moving target. When he/she does so, the OCT measurement will stop (fixation loss) and start again when he/she looks back at the fixation target. Tell the patient that the same will happen if he/she blinks or sits back for a moment. This is not a problem and will only slightly prolong the time of the OCT image acquisition
There are a few patients for whom maintaining fixation will be difficult because of the underlying neurological disease (e.g., nystagmus, oscillopsia). In these patients it is reasonable to turn the automated retina tracking (ART) function off in order to shorten the image acquisition time. Please make a note if you turn ART off. If ART is turned off, this information will automatically be added by the software to the data file
Now ask the patient to sit forward and rest his/her chin on the chin rest and lean his/her forehead against the headrest. Adjust the height of the patient’s head such that the canthus of the eye (lateral line between upper and lower lid) is approximately in line with the canthus mark on the metal rod to the right of the camera. Ask the patient if he/she can see the blue light in the camera (internal fixation target). If not, try to provide an alternative internal fixation target or switch to an external fixation target. Good visual fixation is not mandatory but will facilitate and speed-up retinal OCT image acquisition. Encourage patients to blink in between for their own comfort.
After selecting the category “Axonal” in the “Application” section, a specific set of “Preset” buttons becomes available. The axonal setting was developed specifically for neurological conditions. The orientation of the OCT B-scans is vertical. This is because all axons projecting from the macular to the optic nerve head will be captured. In contrast the slightly faster horizontally orientated B-scans need to interpolate in between axonal projections with less accurate volumetric information.
3.2 Acquisition of the Peripapillary Ring Scan
Choose the peripapillary ring scan from the preset buttons. Set resolution and averaging (ART) as wanted. For imaging of MMO an ART of 3–9 may be advantageous. For good contrast images best suited for segmentation, a higher ART is better. There is no golden rule to this. In a patient who can fixate well you will get a suitable image with a low ART. In a patient with poor fixation or opacifications in the media you may need more. Remember the higher the ART, the larger the scanning time. Not all patients may easily tolerate a long scanning time.
Activate the “Eye-Tracker” by pressing the button on the joystick for about 2 s.
Define the exact position of the fovea and mark this position (blue line in the SLO image in Fig. 2a).
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