Effect of changes in the “corneal rigidity / intraocular pressure” balance on corneal topography in keratoconus

Background. Corneal refractogenesis is influenced by intraocular pressure (IOP), which helps shape the anterior segment profile. Reduced corneal structural strength in corneal ectatic disorders and after laser refractive procedures diminishes resistance to IOP, disrupting the corneal rigidity/IOP balance and promoting the development or progression of corneal ectasia.

Purpose: To assess patterns of local corneal topography associated with changes in the corneal rigidity/IOP balance in keratoconus.

Materials and methods. A total of 96 patients were allocated to four groups. Group 1 (31 patients; 62 eyes without ophthalmic pathology) served to define the reference range of corneal topographic parameters. Group 2 (23 patients; 46 eyes without corneal pathology) constituted the comparison cohort. Group 3 (21 patients; 42 eyes) had confirmed keratoconus (stages I–III). Group 4 (21 patients; 40 eyes) was examined 1 month after laser refractive surgery (LASIK or SMILE). All participants underwent a vacuum-compression test (VCT); corneal topographic indices were recorded before and during transient, vacuum-induced IOP elevation.

Results. With IOP elevation, Group 2 exhibited localized peripheral corneal protrusion. During the vacuum-compression test (VCT) in Group 3 (keratoconus), the corneal apex flattened, while the periphery consistently bulged. IOP increased significantly in Groups 2 and 3 (both p < 0.05). In Group 3, an IOP rise of 13.2 ± 0.9 mmHg (vacuum level 80 mmHg) was accompanied by a decrease in corneal hysteresis (CH) of 4.1 ± 0.8 mmHg; in the control group, CH decreased by 2.4 ± 1.5 mmHg. In Group 4, the corneal rigidity–IOP balance was disrupted due to a localized, clinically insignificant ectasia along the flap margin.

Conclusion. Transient IOP elevation caused corneal deformation characterized by apical flattening and mid-peripheral steepening, indicating areas of reduced corneal rigidity. Structural alterations after laser refractive surgery weaken the cornea between the optical zone and the limbus, leading to predominantly peripheral deformation that may contribute to the development or progression of corneal ectasia.

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