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Andrew KC Lam, Kenny KH Lau, Ho-yin Wong, Jasmine PK Lam, Man-For Yeung; Fixation stability using soft contact lenses versus built-in auto-focus in OCTA of myopes. Invest. Ophthalmol. Vis. Sci. 2021;62(8):1883.
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© ARVO (1962-2015); The Authors (2016-present)
High myopia has been found to be associated with reduced retinal capillary vasculature. Optical defocus could lead to erroneous OCTA measurement. Some OCTA systems have built-in ocular lenses to compensate refractive defocus. We hypothesized that inadequate optical correction could lead to fixation instability, hence affecting OCTA measurements.
Seventy eyes of 70 myopes (35: spherical equivalent >-3D, 35: spherical equivalent <-6D) had OCTA measured using a Cirrus SD-OCT. Three measurements were obtained using a 6x6 mm scan (350x350 pixels) centered at the fovea, first with soft contact lens correction, followed by using the built-in Auto Focus correction after removal of contact lenses. Valid OCTA scans had signal strength of ≧7 and no obvious artefacts. Fixation stability of the 3 OCTA scans was analyzed in terms of deviation (in pixel) of the fovea from the centre of the 350x350 grid (Figure 1). OCTA metrics, namely vessel length density (VD) and perfusion area density (PD) using the ETDRS format, and foveal avascular zone (FAZ) were averaged from the three OCTA scans and compared between different correction methods.
Low myopes (-1.89D±0.75D) and high myopes (-8.10D±1.34D) had similar age. Fixation stability of low myopes from three OCTA scans (test-retest repeatability) was similar between the two correction modes (contact lens: 10 pixels vs Auto Focus: 7 pixels). High myopes had poor fixation stability when corrected with Auto Focus (test-retest repeatability: 17 pixels) compared with using contact lenses (12 pixels). Signal strength was higher in low myopes than high myopes when corrected with contact lenses (8.96 vs 8.40) and Auto Focus (8.82 vs 8.17). When combining the two groups, only VD at the outer ring and PD of the entire 6mm circle showed significant difference between the two correction methods. VD and PD results were smaller when using Auto Focus. No significant difference in FAZ was found.
Fixation was more stable in OCTA measurement when myopes were corrected with soft contact lenses, especially high myopes. In general, VD, PD, and FAZ were similar using different correction methods that could be due to using averaged results from three OCTA scans. Multiple OCTA scan averaging approach is recommended.
This is a 2021 ARVO Annual Meeting abstract.
Fixation deviation (FD) in pixel was calculated as distance between grid center and foveal center using the formula below. FD = √(175-x)^2+(175-y)^2
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