Good-quality images are key for interpreting OCT angiography (OCT-A) images correctly, and as the sensitivity of instruments increases, they run the risk of error from even marginal abnormalities of the ocular media. Image quality may be affected by a number of factors, including scan pattern, retinal disease, signal strength, axial length and the patient’s ability to hold their eye open for a period of time. The latter, researchers note, may “induce instability of the tear film,” affecting the quality of the scan.
A recent study found that OCT-A’s repeatability and signal strength may indeed be influenced by a patient’s dry eye status. Researchers hypothesized that an unstable tear film could affect the quality of OCT-A and compared the test’s repeatability based on tear break-up time (TBUT).
They performed 3x3mm OCT-A twice in 112 eyes, which were divided into three groups according to their TBUT (Group 1: five seconds or less; Group 2: six to 10 seconds; and Group 3: 11 seconds or more).
The researchers found a significant difference among OCT-A signal strengths in the three groups (9.1, 9.5 and 9.5in groups 1, 2 and 3, respectively). They also found that the intraclass correlation coefficient values of vessel density increased in order from Group 1 to Group 3. Coefficient of variation and test-retest standard deviation values decreased significantly in order from Group 1 to Group 3.
"The tear film is the first entry point of light,” the researchers wrote in their paper. “When being examined by OCT-A, patients keep their eyes open for a certain period, which may induce instability of the tear film. Several studies had reported that tear break-up reduced the optical quality.”
They believe that tear film instability leads to higher corneal aberrations that attenuate and defocus the OCT-A beam. “We found TBUT was the significant factor influencing OCT-A repeatability in multivariate analyses,” they wrote.
The researchers concluded that OCT-A’s repeatability tended to decrease with a shorter TBUT. “When the TBUT is less than five seconds, care must be taken to interpret the OCT-A results correctly,” they noted.
Lee WH, Lim HB, Kim J, et al. Repeatability of macular microvasculature measurements using OCT angiography according to tear break up time in dry eye disease. Retina. April 1, 2021. [Epub ahead of print].