Glaucoma type influences visual field damage
Patterns of visual field damage vary depending on whether a patient with advanced glaucoma has primary open-angle glaucoma or normal tension glaucoma, according to a study in the Journal of Glaucoma.
“This study is unique in that it included only patients with advanced glaucoma and focused on the identification of differences in patterns of the functional defects and their association with structural parameters in these two types of glaucoma,” Mi Sun Sung, MD, of Chonnam National University Medical School and Hospital in South Korea, and colleagues wrote.
In a cross-sectional study, Sung and colleagues reviewed medical records of 97 patients who were diagnosed with advanced glaucoma between October 2019 and March 2021, of whom 59 had primary open-angle glaucoma (POAG) and 38 had normal tension glaucoma (NTG).
Researchers measured peripapillary retinal nerve fiber layer (RNFL) and macular thickness to examine differences in structure-function relationships between POAG and NTG groups.
On the superior nasal quadrant, both POAG and NTG groups had a severely depressed visual field (VF); however, more severe VF defects were observed in those with NTG, indicating increased asymmetry.
Both groups had relatively preserved VF on the inferior temporal quadrant and central area, but the POAG group had poorer VF on the inferior temporal quadrant and more diffusely distributed VF defects.
Further, structure-function relationships in total macula and macular RNFL thickness demonstrated different characteristics based on POAG or NTG classification, with NTG eyes showing better relationships in the inferior and nasal-inferior, temporal-inferior and superior and nasal-superior regions compared with POAG eyes.
“Considering that damage in different subfields of the VF significantly contributed to the success in performing different daily tasks, our findings may help clinicians understand the difficulties encountered by patients with different subtypes of glaucoma and may provide clinically meaningful information to minimize glaucoma-related visual disability,” Sung and colleagues wrote.
Perspective
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The strength of the structure-function relationship is known to be weaker in the early stage of glaucoma, reaching its peak in the moderate stage. In advanced glaucoma, structural measures such as OCT have limited value because of a floor effect, while the results of visual fields are unreliable and inconsistent, making the true extent of damage difficult to determine. The structure-function relationship difference between NTG and POAG remains poorly understood, with some studies finding no significant variation. However, most studies focus primarily on early-to-moderate glaucoma.
In this study, Sung and colleagues found that in advanced glaucoma POAG patients had more diffusely distributed VF defects, while NTG sufferers had more severe VF defects at the superior nasal quadrant. However, macular RNFL thickness in NTG eyes showed a better-structure function relationship.
The true connection between NTG and POAG remains controversial, but they are clearly related conditions. Glaucoma of all types can follow a variable course, and we must diligently monitor each patient carefully without pretending to know individual outcomes. Future research will provide greater insight into glaucoma staging, but listening to a patient’s lived experience is particularly critical, as clinical tests are more likely to fail with advanced disease.
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