Getting the most out

Glaucoma diagnosis using OCT

Step-by-step approach holds the key to accurate clinical application of OCT information. Cheryl Guttman Krader reports

Structural imaging with optical coherence tomography (OCT) is a core component of the evaluation for glaucoma diagnosis and monitoring, but the report needs to be interpreted carefully and with all of the patient’s clinical data taken into consideration, said David Garway-Heath MD, at the 2020 ESCRS Virtual meeting.

Dr Garway-Heath, Glaucoma UK Professor of Ophthalmology, University College London, London, England, underscored these points by adapting a quote from Mark Twain. The American author/humourist observed the hazard from not reading the newspaper but also from newspaper reading. Dr GarwayHeath said: “If you are not imaging with OCT, you will be relatively uninformed on glaucoma diagnosis, but if you just read the OCT report without interpreting it, you could be misinformed.”

Emphasising that OCT imaging is just part of the diagnostic workup, Dr Garway-Heath said: “Structure and function provide complementary information. Look for structure-structure and structure-function concordance, and remember you still need your clinical acumen because there are things that OCT cannot do. Look at the optic nerve for features such as optic nerve haemorrhage, which is an important prognostic factor for glaucoma and glaucoma progression, and for lesions in the retina and/or defects in the retinal nerve fibre layer that might explain visual field loss.”

He continued: “Even when there is structure-function concordance, you cannot necessarily make the diagnosis of glaucoma without excluding other potential causes. Finally, if you are uncertain of an abnormality, monitor for change over time that will tend to occur if the abnormality is due to glaucoma.”

REVIEWING THE OCT REPORT Providing advice for interpreting the OCT report, he first acknowledged that the software generates “an overwhelming number of parameters” to look at. To address this challenge, the first tip was to concentrate on the peripapillary nerve fibre layer (PP NFL) and the ganglion cell complex (GCC) thickness because those two anatomical structures provide the most information.

Next, he outlined a systematic approach to reviewing the OCT report, adding as a caveat that not all OCT devices provide all of the features he would be describing. The systematic approach is comprised of five steps and begins with a check of the retinal NFL segmentation, looking for segmentation errors and artifacts as a means of assessing the accuracy of the software’s segmentation.

“Some OCT devices allow the user to go back into the software and correct the segmentation. If that is not possible with your system, exercise caution when interpreting the result,” Dr GarwayHeath suggested.

The second step is to look for structureto-structure concordance between regions of retinal NFL thinning in the en face and PP NFL thickness maps. Here, Dr GarwayHeath advised caution, noting devicerelated differences in the displayed location of the temporal part of the PP NFL.

While there is a general movement towards showing the temporal part in the centre of the scan, with some machines it may still appear at the edge of the scan, he cautioned.

If you just read the OCT report without interpreting it, you could be misinformed on glaucoma diagnosis David Garway-Heath MD

In addition, availability of the en face maps also varies. These maps, which are very useful for identifying the pattern of NFL thinning that is associated with glaucoma, are provided by all sweptsource OCT platforms, but not by all time-domain machines.

The third step in the systematic review of the OCT report is to look for topographical correspondence between the structural parameters (NFL thinning and ganglion cell loss) and visual field sensitivity loss. With some OCT devices, a map will be produced that aids in the check for concordance, Dr Garway-Heath said.

MACULA VULNERABILITY ZONE Next, clinicians should look for damage in the macula vulnerability zone in both the PP NFL scan and the macula ganglion cell thickness scan. This information is particularly useful when assessing the OCT report for early glaucoma because it can show damage in the absence of any functional loss in the 24-2 visual field test.

“If there is thinning in the OCT scan, do a 10-2 visual field to pick up visual field loss in this region,” Dr GarwayHeath advised.

The last step is to look for correspondence between the macula and NFL maps and the visual field sensitivity loss to check for structure-to-structure and structure-to-function concordance.

He warned against automatically interpreting red in the report is a hallmark of glaucoma. The finding needs to be considered with particular caution in myopic eyes that have unique anatomic variations, he emphasised.

“In myopic eyes the thicker bundles in the superotemporal and inferotemporal parts of the nerve are closer to the temporal region. Consequently, parts of the NFL thickness sometimes fall outside normal ranges in an otherwise healthy eye without glaucoma. In addition, because the measurements are taken further away from the edge of the nerve in a myopic eye, the NFL measurements in the myopic eye are thinner than the average,” Dr Garway-Heath explained.