In Measuring Glaucoma Progression, Look at More Than Summary Metrics

In monitoring glaucomatous progression in the circumpapillary retinal nerve fiber layer (cRNFL), imaging analysis must be based on more than just summaries, according to research published in the Journal of Glaucoma.

“The most common optical coherence tomography (OCT) analysis [in glaucoma] involves measurement of the thickness of the cRNFL seen on circumpapillary b-scan images,” the New York-based research team explains. “These b-scan images are either obtained from OCT circle scans or derived from OCT cube scans.” In both cases, the cRNFL thickness is measured. 

At this point, “typically, the clinician looks at summary measures, such as the global average or the average of portions (e.g., quadrants) of the cRNFL thickness,” the investigators explain. These summary metrics, they say, can miss damage and ignore information identifiable on the actual circumpapillary b-scan image. The researchers argue that “these images should be examined the way a radiologist would examine a magnetic resonance imaging (MRI) scan, namely by closely inspecting the details of the OCT b-scan image.”

The investigators demonstrate this point by monitoring glaucomatous progression in the cRNFL, which is associated with the superior portion of the 24-2/30-2 visual field (VF). They aligned the scans obtained at different times in a region known to be prone to progression. With glaucoma, ophthalmologists will note a characteristic increase in both width and depth at the cRNFL, the report shows. 

While those changes are consistent, clinicians can “expect considerable variation in patterns of visual field (VF) loss from eye to eye. In addition, conventional metrics (global or quadrant cRNFL thickness) do not fully depict the progressive changes that can be appreciated by inspecting OCT images,” the researchers explain. 

To exemplify this, the researchers reviewed the cases of 24 eyes of 20 patients with glaucoma and evidence of progression on OCT. These patients were all examined with OCT disc cube scans on at least 3 separate visits with a mean follow-up of 7.4 years. 

Circumpapillary b-scans were derived after en face images were aligned to assure that the study region (ie, 0 to -135 degrees, where 0 degree is 9 o’clock, on a right eye) coincided. Within this region, a region of progression was defined based on the loss in cRNFL thickness between the initial and subsequent visits. Researchers determined the width of the region of progression, along with the locations of its leading (close to fixation) and trailing edges. They also measured the location and depth at the point of maximal loss, total loss, and average remaining RNFL at the region of progression. 

They found that the region of progression proceeded both toward and away from fixation. This region varied widely across all eyes in width (32 degrees to 131 degrees, mean 82.7 degrees), location, and loss at point of deepest loss (22 µm to 99 μm, mean 52.9 μm), as well as total cRNFL loss. This shows that progression is rarely a local phenomenon.

Additionally, the study demonstrates that glaucoma patients experience a wide variation in the pattern of loss. This was upheld even within the study’s region, which covers less than 40% of the disc. The researchers argue that these variations in location, width, depth, and heterogeneity of loss over time, mean that “simple metrics such [as] loss of global or quadrant or clock hour cRNFL thickness will not capture the nature of the loss in cRNFL.” 

The researchers predict that the pattern of cRNFL loss should be reflected in the pattern of VF loss in the associated region. In particular, wider and deeper VF defects are to be expected. However, a wide variation in the spatial patterns of local VF loss within the affected regions is likely, and these variations should, on average, be predictable from the changes in the OCT region of progression.

Clinicians must closely examine OCT images (b-scans), as well as the thickness and proba- bility maps, to understand the nature and location of progression for each individual eye, the study concludes. 

Disclosure: One of the study authors declared affiliations with relevant device manufacturers. Please see the original reference for a full list of authors’ disclosures.

Reference

Kim H, McKee W, Malendowicz K, et al. Local glaucomatous defects of the circumpapillary retinal nerve fiber layer show a variety of patterns of progression. J Glaucoma. 2020;29(10):857-863. doi: 10.1097/IJG.0000000000001620