Symmetry of Corneal Hysteresis in Relation to IOP and Central Corneal Thickness

Abstract

Purpose:: Corneal hysteresis (CH) is a newly characterized viscoelastic property of the cornea. The Ocular Response Analyzer (ORA) measures this biomechanical property of the cornea in addition to measuring the intraocular pressure (IOP) using non-contact tonometry (NCT). If corneal hysteresis is an intrinsic property of the cornea, symmetry in hysteresis values is expected between eyes. This study examines the symmetry of hysteresis between eyes in patients with and without anterior segment penetrating surgery (ASPS), and characterizes hysteresis relative to IOP and central corneal thickness (CCT).

Methods:: The Ocular Response Analyzer (ORA; Reichert, Buffalo, NY) was used to measure CH and NCT in 276 subjects presenting to the Glaucoma Service at the Bascom Palmer Eye Institute. Central corneal thickness was measured with ultrasound pachymetry (DGH Technology, Exton, PA). IOP was re-measured in the clinic using Goldmann applanation tonometry (GAT) and/or Tonopen (Medtronic, Jacksonville, FL). All study subject ocular histories were reviewed by a glaucoma specialist to confirm diagnosis and the validity of ORA measurements.

Results:: Among all subjects, a strong correlation between right and left eyes was present with regard to CCT (r=0.85, p<0.001). A similarly statistically significant correlation also exists with CH between paired eyes (r =0.54, p<0.001). In those subjects with no ASPS, the result is similar (r =0.58, p<0.001). In those subjects with no ASPS, the absolute difference of CH between eyes is 1.2 mmHg (s.d. 1.5), while in those subjects with ASPS in only one eye, the absolute value of the difference between eyes is 1.7 mmHg (s.d. 1.6). This difference is statistically significant (p = 0.031, t-test). Among all subjects, the correlation between the magnitude of CH and CCT (r =0.3, p<0.001) and CH and IOP (r = -0.2, p<0.001) was small but statistically significant.

Conclusions:: Corneal hysteresis and intraocular pressure have small but significant correlations with corneal hysteresis between eyes in all subjects, and there was a significantly greater difference in the magnitude of CH in those eyes which had surgery compared to those eyes that did not. Higher magnitude CH was associated with thicker CCTs and lower IOPs. Further investigation will determine if the association between CH and surgery is due to surgery itself or the pathology of the eye that necessitated ocular surgery.

Keywords: clinical (human) or epidemiologic studies: risk factor assessment • visual fields • intraocular pressure