When using en-face OCT angiography (OCT-A), a new study has found that with-the-rule (WTR) astigmatism may affect the data interpretation, given that increasing levels of astigmatism led to diminishing parafoveal vessel length and perfusion density that is more symmetrical in vertical quadrants and most pronounced in the nasal region.

The research team attributes their findings to a high prevalence of horizontal vessels nasally, and the horizontal optical defocus induced by WTR astigmatism.

OCT-A allows for noninvasive evaluation of the retinal vascular architecture, rapid image acquisition, depth-resolved image segmentation, as well as the avoidance of discomfort and associated risk of intravenous fluorescein dye injection; however, OCTA has its own shortcomings including image distortion, de-centration, defocus, and uncorrected axial length, which may negatively affect its reproducibility and reliability, says researcher Jesse Jung, MD, who is a partner at East Bay Retina Consultants in Oakland, CA.

 Even with these limitations, most issues can be corrected by proper image acquisition, he says. Still, although any issues can be mitigated with proper technician training, inherent anatomical aspects of eyes, such as spherical refraction, need to be corrected by the devices themselves, he adds.

“In current OCT-A systems, the telecentric lens can adjust for spherical refractive error, but the exact affect of astigmatism has not been elucidated,” Dr. Jung says.

To understand the influence of astigmatism, Dr. Jung and his team tested the effect of WTR cylindrical astigmatism on quantitative vessel density metrics.

Using a 3x3mm scan pattern, the investigators imaged 15 eyes of 15 patients including normal eyes without astigmatism and those with 0.75D, 1.75D and 2.75D of WTR astigmatism. Quantitative parameters, including FAZ metrics, parafoveal vessel length density and perfusion density, were corrected for magnification secondary to axial length and analyzed.

For every 1D increase in induced WTR astigmatism, the researchers observed an associated, statistically significant decrease in vessel length and perfusion density within all of the ETDRS (Early Treatment Diabetic Retinopathy Study) inner-ring quadrants, and especially nasally.

Additionally, for every 1D increase in induced astigmatism, the resulting decrease in the inner-ring superior quadrant was 12% greater for vessel density and 16% greater for perfusion density compared with the inferior quadrant.

The study also noted the resulting decrease in the inner-ring nasal quadrant was 40% greater for vessel length density and 48% greater for perfusion density vs. the temporal quadrant.

Since all four quadrants in the ETDRS fields have horizontal and vertical vessels within the fields, the influence of WTR would induce astigmatic power along the horizontally oriented vessels, therefore blurring these vessels and sparing the vertical vessels in all fields, Dr. Jung notes.

As seen histology and on OCT-A, the superficial capillary plexus vessels follow parallel to the nerve fiber layer; therefore, the effect of WTR astigmatism has a more profound affect on horizontally oriented vessels (i.e. the nasal macula). This is why there is a more profound difference in the horizontal meridian (nasal vs. temporal) compared with the vertical meridian, Dr. Jung explains.

“All quadrants will be affected by induced astigmatism because there are horizontal capillaries in each quadrant, just more so in the nasal quadrant because it has more horizontal capillaries,” he says. 

Jung JJ, Soh YQm Sha P, et al. Effects of induced astigmatism on spectral domain-OCT angiography quantitative metrics. American Journal Ophthalmology. July 16, 2020. [Epub ahead of print].