Optic Nerve Imaging Technology: The Necessary Evil?

The growing use of optic nerve imaging devices has led to both excitement and frustration in being able to accurately diagnose and manage pre-perimetric glaucoma. After attending several instruction courses on the topic at the Academy’s Annual Meeting, I found that many practitioners including myself still came away less than confident in being able to appropriately interpret the information yielded from these devices. The last three Breakfast with the Experts roundtables I moderated on the topic (“Top Ten Tricks on Interpreting OCT,” “HRT” and “Gdx in Glaucoma”) have been consistently sold out and have given me invaluable feedback as to why these modalities have become such a “necessary evil.”  

In this article, I hope to give streamlined tips on obtaining and interpreting these often-confusing printouts, sharing some of the pearls and pitfalls I have learned along the way.

Top 10 Tricks on Obtaining and Interpreting Stratus OCT

1. Choose the right scan protocol
Because OCT is so versatile (its major strength), one must be careful that the appropriate scan protocol is selected. For glaucoma/optic nerve evaluations, the fast RNFL thickness (3.46) or fast optical disc scan are the preferred protocols.

The first is a widely used scan protocol for peripapillary RNFL measurements. It is comprised of three sequential circular scans (each with 256 axial scans), with a diameter of 3.4 mm centered at the optic nerve head.

Selecting the fast protocols is important so that motion artifact is minimized. Macular thickness protocols have obviously focused more on macular pathology, but recently, studies have reported their relevance in glaucoma evaluation as well.^1,2,3

2. Adjust scan position carefully
Though OCT does not rely on a manually drawn contour line like the HRT, the operator must still ensure that the scan ring is appropriately centered on the optic nerve. This starts with asking the patient to look at the green or yellow flashing light in his or her temporal field (yellow for patient with cataracts).

The operator may then also manually adjust the scan ring around the resulting grey-scale image of the disc to ensure it is as centered as possible. Proper patient fixation is necessary for this to occur. A quick look at the grey-scale image of the optic nerve to the extreme right on the printout can also be used to ensure the scan ring is centered appropriately (especially for clinicians who rely on technicians to perform the test). The scan position can have ramifications on how the signal profile analysis may be shifted and give erroneously diminished RNFL measurements.

3. Check signal strength
First, make sure you have a reliable test! Just as we have come to readily accept that when visual-field testing has poor reliability indices, we either discount it altogether or simply repeat it in the future, we should approach OCT imaging in the same way.

Here are a few measures you can take to maximize the signal strength and, hence, the validity of the test:

• Make sure you have adequate dilation.
• Focus the adjustment knob. (For example, in patients with moderate to high myopia, this knob should be dialed to match the patient’s refractive error.)
• Keep a bottle of artificial tears near your machine and have patients blink frequently, as ocular-surface irregularities can significantly reduce the signal strength of your test.
• An ideal signal strength should be >7. This numerical value is found just below the grey-scale image of the optic nerve. Do not buy into assurances that 5-6 is acceptable, especially in cases of an abnormal OCT. Such a test simply is not reliable. A recent study in Ophthalmology reported significant variations in RNFL measurements with varying signal strength, thus arguing for obtaining maximal possible signal strength.¹ Specifically, the superior and nasal quadrants were found to be most susceptible to changes in image quality when signal strength is low.⁴

If all fails, you may have to repeat the test on an alternative day or simply accept that OCT testing may not be suitable for the patient in question.

4. Do signal-profile analysis
The sinusoidal signal profile wave corresponds to the RNFL thickness profile 360 degrees around the optic nerve, starting from the temporal region and proceeding through the superior, inferior, nasal and back to the temporal region (TSINT).

The “double humps” in this profile thus correspond to the superior and inferior quadrants, where the RNFL is thickest.

• Look at the inferotemporal region of the profile first. Assess not only where the black line (your patient) lies in comparison to an age-matched control (green, yellow and red), but also the quality of this wave. Is it shifted erroneously (see #2)? How does it compare between the two eyes? Does it dip down only in the nasal area, where you find the optic nerve is perfectly healthy on clinical exam?
• Check for asymmetry. Invaluable information can be obtained by comparing the two eyes.
• The average RNFL thickness (last number on the bottom of the RNFL) measurements of above 80 have been cited as a relatively normal RNFL. The inferior and superior peaks (Imax and Smax) of over 125 also are also considered relatively normal. I have certainly encountered patients with “normal” average RNFL measurements of around 85 who show focal depressions in the RNFL analysis and have perimetric glaucoma; so, again, I focus on the quality of the wave more than just the numbers.
• The “OSI combination.” Recent studies have also been able to identify the most valuable combinations of ONH and RNFL parameters to identify eyes with glaucoma. “The best stand-alone diagnostic strategy is to classify an eye as glaucomatous if the overall, inferior or superior quadrants are below normal.”⁵

In terms of optic nerve parameters, the printout can also give you myriads of data in volumetric information, dimensional information and C:D ratios. Most every glaucoma expert I’ve heard speak on the topic rarely uses these specific numerical parameters in their clinical decision-making, so use caution before relying on them to assess progression.

5. Conduct sector analysis
These color-coded “pie charts” are broken down into quadrants as well as clock hours. They often make correlation of RNFL thickness profile easier to correlate with the clinical exam. The color-coding is simply a classification method to denote limited, age-matched controls. The numerical values listed along the perimeter of these pie charts correspond to the RNFL measurement for that corresponding quadrant/clock hour.

6. Check correlation to VF
Self-explanatory, but still important! To accurately confirm structural loss before functional loss requires that you periodically go back and correlate ON/RNFL analysis if/when VF defects develop.

7. Compare to the normative database with discretion
Be aware of what you are comparing to. The population-based profiles are derived from a very narrow population of “normals,” without much representation of various ethnicities.

There are 328 normal control subjects for the fast protocol: Asian =11, Indians = 3, Black = 27, Hispanic = 79, White = 205, Other = 3.

There are also no control patients with myopic discs, peripapillary atrophy or optic nerve drusen. Hence, any/all of these conditions may yield a significantly abnormal OCT printout when the patient does not have glaucoma.

8. Remember your pros and cons
This will allow you to always be aware of the limitations of this imaging technology and adjust your expectations accordingly.

9. Involve the patient
I routinely show the OCT printout to glaucoma/glaucoma-suspect patients with a brief, layman’s explanation of what the optic-nerve and retinal-nerve fiber layer looks like and what this means for their visual outcome.

More often than not, this gets them on board with what we are looking for and whether they would opt to observe things or get treatment proactively. Often patients have surprisingly strong personality types in determining the style of their care (conservative vs. aggressive); OCT allows me to tune in to that, thus helping to share the burden of care.

10. Consider cost/compliance
Many practitioners agree that this imaging technology is well worth the price and that the machine “pays for itself” within about a year of purchase, especially if you are using it to test for macular pathologies. However, you must be alert to the medical necessity of performing the test and the fact that you will actually have to do something with it after you take the picture.

Make sure you put your interpretation/assessment of the test on the printout and sign it — even if it is an unreliable test.

Most experts agree that one test per year is sufficient for pre-perimetric glaucoma and that there is rarely an indication for imaging in case of advanced glaucoma. (VF is usually more appropriate.)

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About the author: About the author: Nalini K. Aggarwal, MD, is an assistant professor and assistant chief of service at the University of Texas Southwestern Medical Center in Dallas.

Disclaimer: The views expressed in this article are those of the author(s) and do not necessarily reflect the official policy or position of University of Texas Southwestern Medical Center or the American Academy of Ophthalmology.