Corneal shape changes during accommodation may be a helpful sign for clinicians diagnosing corneal ectasia, a study in Translational Vision Science & Technology reports.

Since variation in changes to corneal conformation (i.e., curvature, height and thickness) may predict postoperative ectasia following refractive surgery, the researchers aimed to establish a baseline for these changes during ocular accommodation. They also wanted to clarify the role of biomechanical factors in predicting these changes in a population without corneal pathology.        

The study, conducted in New Zealand, used corneal tomography to assess 63 participants with normal corneas for both accommodated and un-accommodated states. The investigation included four diopters of physiological accommodation using near-acuity calibrated words.

The researchers noted anterior chamber depth was reduced by 0.10 ± 0.07mm with accommodation, and areas of statistically significant change in corneal curvatures were seen in all participants with accommodation. The study also found the mean anterior instantaneous corneal power increased in the superior-nasal periphery (0.1D) and decreased in the inferior-temporal periphery (0.1D).  The investigators found that corneal stiffness and the corneal deformation amplitude ratio predicted peripheral corneal curvature changes with accommodation.

The ability to screen for ectasia risk using conformational changes with accommodation remains unknown, as the subjects in the current study were all emmetropic and presumably at no risk of ectasia, the researchers noted. “Last, and perhaps more clinically relevant, would be the screening of patients with forme fruste, unilateral (in the uninvolved eye), or early keratoconus to detect those likely to develop keratoconus or progressive ectasia,” the researchers wrote in their paper. “If accommodative conformational changes could predict patients at high risk of progression, it may assist decision-making for the necessity and timing of corneal collagen crosslinking.” Serial tomography assessments, currently used to assess the criteria for crosslinking, can be expensive and time-consuming, leading to delays in treatment and deterioration in uncorrected visual acuity in some cases, they added.

Further studies are required to assess the magnitude of corneal changes during accommodation in patients with corneal ectasia, the study noted. 

Wallace HB, McKelvie J, Green C, et al. Corneal curvature: the influence of corneal accommodation and biomechanics on corneal shape. Transl Vis Sci Technol. 2019 Jul;8(4):5.