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Purpose: To investigate the effects of stand-alone PRESERFLO MicroShunt (PFM) ab externo implantation on refractive and corneal astigmatism in the short (3 months) and in the long term (12 months).

Methods: This retrospective cohort study included eyes that underwent PFM implantation and had mandatory preoperative and 3-month postoperative subjective refraction data available (117 eyes from 108 patients), with 12-month postoperative data collected when available (56 eyes from 54 patients). Tomography measurements, when available, were obtained using Scheimpflug imaging. Refractive as well as anterior (ASA) and posterior surface astigmatism (PSA) were evaluated using vector analysis.

Results: Refractive cylinder showed a significant increase from -1.37 +/– 0.91 at baseline to -1.62 +/–1.06 at 3 months (p = 0.01) which was sustained at 12 months (p > 0.99). The magnitude of the Jackson cross cylinder projection to the 0 and 90° meridian, J0, rose from -0.09 +/– 0.63 at baseline to 0.11 +/– 0.73 D at 3 months (p < 0.001) and subsequently decreased slightly by 12 months (p = 0.1), whereas the 45 and 135° projection, J45, remained unchanged. In 25 eyes with tomography measurements, both the J0 and J45 components of ASA remained stable. However, on the posterior corneal surface, J0 changed significantly from -0.100 +/– 0.110 D at baseline to -0.154 +/– 0.166 D at 3 months (p = 0.03), while J45 was not affected (p = 0.5).

Conclusions: PFM implantation induces a small but significant with-the-rule shift in refractive astigmatism, sustained over 12 months. ASA remained stable, and although PSA changed significantly, its minimal magnitude and opposite directional shift do not explain the refractive findings. This supports a primarily non-corneal, IOP-reduction-related mechanism. Although modest, this shift may be clinically relevant in eyes with preexisting significant or near-relevant astigmatism and in toric or premium intraocular lens planning.