Tomographic and Biomechanical Index Spots Keratoconus With Normal Tomography

Tomographic and biomechanical index can be used to differentiate between patients with very asymmetric corneal ectasia (VAE) in 1 eye and no apparent ectasia in the contralateral eye, based on tomographic assessment, and patients without ectatic disorders, according to research published in Cornea.

Researchers enrolled 105 individuals in the prospective study and matched patients with VAE (n=21; mean age 26.2 years; 33% women) in a 1:4 ratio with control group participants (n=84; mean age, 30.68 years; 55% women). The team compared the ability of tomographic and biomechanical index, strict biomechanical parameters, and strict tomographic parameters to discriminate eyes in the control group from eyes with early keratoconus.

A skewed asymmetric bow tie or inferior steepening plus 1 or more clinical findings, including Munson sign, Vogt striae, Fleischer risk, apical thinning, scissoring on retinoscopy, and Rizzuti sign constituted a clinical ectasia diagnosis (VAE-E). Findings from these eyes were compared with eyes without apparent ectasia based on normal tomography (VAE-NT).

Among the 42 biomechanical parameters examined, 36 differed significantly between eyes with VAE-E and the eyes of control group participants (all P <.01). A total of 24 biomechanical parameters differed significantly between control and VAE-NT eyes (all P ≤.04).

Overall, tomographic and biomechanical index demonstrated the most usefulness for determining keratoconus in eyes with normal tomography.

“At a threshold of 0.72, the [tomographic and biomechanical index] had 99% sensitivity, 67% specificity, and 92% accuracy in distinguishing normal and VAE-NT corneas,” according to the report.

The receiver operating characteristic curve (ROC) assessment confirmed the ability of tomographic and biomechanical index to distinguish between corneas with VAE-E (area under the ROC [AUROC], 1.00; 95% CI, 1.00–1.00). Despite this AUROC value, the method was not statistically superior to another index that used strict biomechanical parameters (P =.27) and another combining elevation tomography with tomographic thickness evaluation (P =.34).

“This analysis was performed specifically to determine whether the corneal biomechanical compromise that precedes corneal tomographic changes can be detected,” according to the study authors. “The results of the current study show that these parameters are not statistically different in their ability to discriminate between normal and ectatic corneas. The [tomographic and biomechanical index], however, was statistically superior . . .”

Study limitations include a small sample size and the possibility of mild ectasia among some control group participants.