Astigmatic axis affects visual acuity after refractive surgery

The axis of astigmatism has a small but definite effect on uncorrected distance vision in eyes that achieved emmetropic spherical equivalent after laser refractive surgery, according to a study.

A retrospective cohort of 17,436 eyes of 8,718 patients who underwent LASIK or PRK at the Care-Vision Laser Centers in Tel Aviv, Israel, was divided into three groups according to the steep astigmatic axis: with-the rule (WTR), oblique and against-the-rule (ATR). The oblique group was further divided into oblique WTR and oblique ATR. Uncorrected distance visual acuity (UDVA) was compared among the three main groups and between the two oblique groups.

Significantly better UDVA (logMAR 0.01 ± 0.08) was found in the WTR group as compared with both the oblique (logMAR 0.02 ± 0.09) and ATR (logMAR 0.02 ± 0.10) groups (P < .001). Of the two subgroups with oblique astigmatism, the WTR subgroup had slightly better outcomes than the ATR subgroup (P < .001). Multiple regression analysis demonstrated that, in addition to the amount of astigmatism, the axis of astigmatism had a significant role in these results.

According to the authors, “the effect of the axis on UDVA stems from blur tolerance.” Due to the development of the primary visual cortex, vertical blur might be better tolerated than oblique and horizontal blur. Another possible mechanism is the pinhole effect. Although no evidence exists in the literature to support this theory, the authors suggested that “narrowing the palpebral fissure causes a pinhole effect that may improve UDVA in WTR astigmatism more than in ATR astigmatism.” – by Michela Cimberle

Disclosures : The authors report no relevant financial disclosures.

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Source:

Mimouni M, et al. Eur J Ophthalmol. 2017;doi:10.5301/ejo.5000890.