Effects of Refractive Power on Macular Thickness Measurement Using Spectral-Domain Optical Coherence Tomography

Fixation stability and deviation in optical coherence tomography angiography using soft contact lens correction in myopes

To compare fixation deviation and stability with soft contact lens correction and device built-in auto-focus system during optical coherence tomography angiography (OCTA). This observational study measured OCTA metrics first with contact lens correction, followed by removal of contact lenses, using the device auto-focus system at a University Optometry Clinic, Hong Kong. All participants were habitual soft contact lens wearers with either low or high myopia. OCTA measurements were obtained using a spectral domain OCTA. Fixation deviation was distance (in pixels) of the fovea to the center of the OCTA measurement grid. Fixation stability was test–retest repeatability (TRR) and coefficient of variation (CV) of fixation deviation from three consecutive OCTA measurements. OCTA metrics included vessel length density (VD), perfusion density (PD), and foveal avascular zone (FAZ) area. Averaged OCTA metrics were calculated from three measurements and compared between the two correction methods. The mean ± SD spherical equivalent of 74 eyes from 74 myopes measured was − 1.94D ± 0.75D in low myopes (n = 37) and − 7.97D ± 1.31D in high myopes (n = 37). When corrected with contact lenses, visual acuities of high myopes (median [IQR], − 0.06 [0.08] logMAR) and low myopes (− 0.02 ± 0.08 logMAR) were similar (P = 0.060), and with similar fixation deviation (5.0 ± 2.2 pixels vs 5.3 [3.6] pixels; P = 0.689). High myopes had poorer fixation stability than low myopes (TRR: 10.2 pixels vs 7.5 pixels; CV: 65% vs 54%, respectively). The worst fixation stability occurred when high myopes were corrected using the auto-focus system (TRR: 12.5 pixels, CV: 72%). The difference in VD and PD was within 1 mm⁻¹ and 1%, respectively. The FAZ area was similar. Difference in OCTA metrics was small in each refractive group (< 1 mm⁻¹ in VD, and < 2% in PD). High myopes had more stable fixation when corrected when wearing contact lenses. Subjects with good contact lens corrected visual acuity should wear their contact lenses during OCTA measurements.

The effect of contact lens wear on retinal spectral domain optical coherence tomography

BACKGROUND

This study assessed the impact of contact lens wear on retinal spectral domain optical coherence tomography (SD-OCT) image quality and macular thickness measurements, among subjects with myopia.

METHODS

This was a prospective study including 34 subjects (26.59 ± 3.19 years) with myopia or myopic astigmatism. Twelve were imaged wearing spherical soft contact lenses, eight non-contact lens wearers were imaged with a plano soft contact lens, and 14 with significant astigmatism were fitted with a rigid gas permeable (RGP) contact lens. For each group of contact lens types, the average image quality index (Q-index), and the average macular thickness measurements were compared between macular OCT scans obtained from the same eyes with and without a contact lens.

RESULTS

Among the subjects assessed with their habitual spherical soft lenses, the average Q-index was similar for scans acquired with and without a contact lens (30.10 ± 1.94 versus 31.03 ± 2.55; p = 0.18). Among non-contact lens wearers, the average Q-index was slightly higher for scans acquired without a contact lens, compared to scans with a plano contact lens (31.99 ± 2.06 versus 29.51 ± 1.56; p = 0.006). Among 14 subjects imaged wearing a fitted RGP contact lens, the Q-index was similar for scans acquired with and without a contact lens (29.04 ± 2.73 versus 28.75 ± 2.86; p = 0.78). In all groups, there were no correlations between the power of the sphere and change in the Q-index (that is, post- minus pre-contact lens Q-index), and no differences were found between OCT-derived macular thickness measurements from scans with and without a contact lens. The magnitude of cylinder was not correlated with the change in the Q-index in the habitual and RGP contact lens groups. However, an inverse correlation between cylinder power and change in the Q-index was found in the plano contact lens group.

CONCLUSION

In low to intermediate levels of myopia, with or without regular astigmatism, macular SD-OCT imaging does not merit placement of a soft or rigid contact lens, nor is there an added benefit from removing a habitual spherical soft lens prior to scanning.

Wide-field choroidal thickness in myopes and emmetropes

There is a paucity of knowledge regarding the normal in-vivo thickness of the choroid beyond the macula (~17°). In this study, the choroidal thickness of 27 healthy young adults was examined across the macular (the central 5 mm including the fovea, parafovea, and perifovea) and extra-macular (a 5-14 mm annulus including the near-periphery and periphery) regions using wide-field optical coherence tomography, and compared between emmetropes (n = 14) and myopes (n = 13). The choroid progressively thinned beyond the parafovea (350 ± 86 µm) towards the periphery (264 ± 44 µm), and was thickest superiorly (355 ± 76 µm) and thinnest nasally (290 ± 79 µm). Choroidal thickness also varied with refractive error; myopes exhibited a thinner choroid than emmetropes in the macular region (311 ± 88 vs. 383 ± 66 µm), however, this difference diminished towards the periphery (251 ± 48 vs. 277 ± 37 µm). Meridional variations in choroidal thickness were not different between myopes and emmetropes. In conclusion, the choroid was thickest within the perifovea; thinned substantially towards the periphery, and exhibited the minimum and maximum peripheral thinning superiorly and nasally across a 55° region respectively. Choroidal thinning associated with myopia was more pronounced in the macular than extra-macular regions.