Thinnest pachymetry and nasal-temporal asymmetry in myopia and emmetropia by Pentacam

Purpose: To assess the association between thinnest point pachymetry and nasal-temporal asymmetry in healthy Caucasian individuals with myopia and emmetropia. Methods: One hundred and four healthy prepresbyopic subjects (49 myopes, 55 emmetropes) were assessed using the Pentacam Scheimpflug system. One eye per subject and the mean of two measurements were used for analysis. Corneal thickness was extracted from topographic maps including the apex (CCT) and peripheral nasal and temporal points up to 5 mm away from the centre. Thinnest pachymetry (TP) was recorded and the apex-to-thinnest-pachymetry difference (ATD) and apex-to-thinnest ratio (ATR) generated. Absolute nasal-temporal pachymetry differences (NTD) were calculated by subtracting temporal from corresponding nasal values. The relative peripheral asymmetry was expressed as the nasal-temporal ratio (NTR) between corresponding locations. Simple linear regression was used to assess the relationship between ATR and nasal-temporal asymmetry for all as well as myopic and emmetropic eyes.Results: The mean age of all subjects was 24±6 years. The mean spherical equivalent was -1.43±1.95 D for all subjects, and -3.05±1.75 D and 0.01±0.19 D for the myopic and emmetropic sub-groups respectively. CCT averaged 554±32 µm and TP 550±32 µm, with a mean ATD of 4.51±2.24 µm and an ATR of 1.008±0.004. Corneal thickness increased asymmetrically with increasing distance from the apex, with a mean NTD of 58±22 µm and a mean NTR of 1.087±0.034 (at 4 mm from apex). Thinnest pachymetry was related to apex thickness (r=0.998, p<0.001). ATR was associated with NTR (r=0.485, p<0.001) and ATD with NTD (r=0.466, p<0.001). These relationships were noted for both groups of myopic and emmetropic eyes (p<0.001).Conclusions: In normal corneas without focal thinning the difference and the ratio between apex and thinnest pachymetry are related to the nasal-temporal asymmetry. Further studies on eyes with progressive central or peripheral thinning would be useful to investigate the theory whether peripheral asymmetry is a potential driver for corneal thinning disorders and whether relationships exist between anterior and posterior segment asymmetry.