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OCT thickness map most useful in detection of early glaucoma RNFL defects
Classification based on a normative database comparison leads to false readings because topographic distribution of RNFL varies between patients.
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Thickness maps generated with spectral-domain optical coherence tomography identified retinal nerve fiber layer defects in glaucomatous eyes more effectively than clock-hour and deviation maps, according to a study.
Investigators evaluated the accuracy of clock-hour, deviation and thickness maps created with the Cirrus high-definition spectral-domain platform (HD-OCT, Carl Zeiss Meditec) to detect retinal nerve fiber layer (RNFL) defects previously identified in red-free fundus photographs.
“The most important finding was that photographic RNFL defect was misidentified in clock-hour map or deviation map of Cirrus HD-OCT, whereas the RNFL thickness map of Cirrus HD-OCT detected all of the photographic RNFL defects. Therefore, RNFL thickness map of Cirrus HD-OCT may be a useful tool for the detection of RNFL defect,” Young Hoon Hwang, MD, the corresponding author, said in an email interview.
“When interpreting OCT results, many doctors detect RNFL abnormality based on a clock-hour map of OCT. However, it can lead to a misidentification of RNFL abnormality. When assessing RNFL by using OCT, a RNFL thickness map should be inspected,” Hwang said.
Young Hoon Hwang
Study design and methods
The study, published in Ophthalmology, included 295 eyes with early glaucomatous RNFL defects; a control group comprised 200 healthy, age-matched eyes.
Patient selection was critical in terms of disease progression and image quality, Hwang said.
“In the early stage of glaucoma, structural change in RNFL is more prominent than functional changes,” he said. “Thus, to investigate the glaucoma diagnostic ability of RNFL assessment by using Cirrus HD-OCT, only eyes with early glaucoma were included. In addition, to compare the RNFL defect detection ability among red-free fundus photograph, clock-hour map, deviation map and thickness map of Cirrus HD-OCT, localized RNFL defects with clear margins were evaluated.”
The investigators assessed visual acuity, refractive error, IOP, slit lamp biomicroscopy, fundus photos, visual fields and RNFL measurements taken with HD-OCT.
Two observers compared HD-OCT clock-hour, deviation and thickness maps with red-free fundus photographs of RNFL defects.
Results and conclusions
Among the 295 defects identified in red-free fundus photographs, 83 defects (28.1%) were misidentified in the clock-hour map, 27 (9.2%) were misidentified in the deviation map and none were misidentified in the thickness map.
In the control group, 25 eyes (12.5%) showed false-positive findings in the clock-hour map, 30 (15%) showed false-positive findings in the deviation map and 12 (6%) showed false-positive findings in the thickness map.
HD-OCT RNFL thickness maps had the lowest rate of false-positive RNFL color codes.
Clock-hour and deviation maps based on normative database comparisons typically yield spurious findings, Hwang said.
“Given that topographic distribution of RNFL may vary person to person, a classification based on the normative database comparison can lead to erroneous judgment. For instance, a myopic eye can have a thicker RNFL in the temporal area and a thinner RNFL in the superior/inferior area compared with normative database, and such difference may lead to misdiagnosis based on normative database comparison,” he said. “The thickness map can demonstrate the abnormalities of RNFL regardless of various factors which can affect normative database comparison.” – by Matt Hasson