A key tool in glaucoma diagnosis is evaluation of abnormalities in the optic nerve head and retinal nerve fiber layer (RNFL). However, what constitutes “abnormal” findings? Given the well-known ocular anatomical differences between ethnicities, researchers at the Scheie Eye Institute of the University of Pennsylvania believe it is important to examine whether an OCT’s normative database is accurate for classifying high-risk minority populations. They suggest an RNFL normative database specific to African-American patients may be more useful in detecting and monitoring glaucoma in this overaffected population. 

The team recently evaluated differences in peripapillary RNFL thickness between healthy African Americans and the Cirrus HD-OCT normative database. Their study included 466 eyes from the control cohort of the Primary Open-angle African American Glaucoma Genetics study. Using both an age-stratified and a linear regression method, the researchers reclassified white-green-yellow-red color probability codes for RNFL thicknesses by quadrant.

 Average RNFL thickness in the African-American POAG cohort was thinner than the Cirrus normative database in all except the nasal quadrant. The original color codes of the POAG cohort did not fall into the expected distributions, with more RNFL measurements assigned as white and red codes than expected (9.5% and 1.7%, respectively) and fewer measurements assigned as green and yellow codes than expected (85.3% and 3.5%, respectively).

 Compared with the original Cirrus results, the linear regression reclassification produced color codes closest to the expected distributions. The proportion of abnormal results actually shifted closer to the expected 5% in the nasal (1.3% vs. 3.0%, respectively) and temporal (8.2% vs. 3.6%, respectively) quadrants.

 “In the temporal quadrant, existing RNFL thicknesses coded by the Cirrus machine are likely to overdiagnose abnormal optic nerves in African Americans; whereas in the nasal quadrant, abnormal thinning may be more likely to be coded as normal,” the researchers wrote in their paper.

 “The current Cirrus algorithm does not account for this variation in RNFL thickness across ethnicities, limiting the ability to properly discriminate between normal and abnormal optic nerve,” the researchers concluded. “Reclassification of the color codes in an African American population produces color code distributions closest to the expected percentiles, particularly in the temporal and nasal quadrants.”

Addis V, Chan L, Chen J, et al. Evaluation of the Cirrus HD-OCT normative database probability codes in an African American population. Ophthalmology. May 22, 2021. [Epub ahead of print].