Background: To investigate the relationship between the change in the manifest refractive error (Delta M), the change in apical corneal power (Delta ACP) and initial corneal asphericity (Q) in overnight orthokeratology (ortho-K). Methods: One hundred and twenty-eight clinical records of children undergoing ortho-K from a university optometry clinic were reviewed. The refractive and topographical data at baseline and at two-week visit of 58 patients who fulfilled the inclusion criteria were retrieved and analysed. Results: Significant differences (p < 0.001) between the change in manifest refractive error and changes in the apical corneal power or the maximum change in corneal power (Delta MCP) within the treatment zone were found. Linear regression analysis was used to describe the change in manifest refractive error and the change in apical corneal power, and the change in manifest refractive error and the maximum change in corneal power, with the equations: Delta M = 0.91 Delta ACP + 0.57 (r = 0.78, p < 0.001) and Delta M = 0.93 Delta MCP + 0.01 (r = 0.79, p < 0.001) respectively. On average, the change in apical corneal power underestimated the change in manifest refractive error by 0.34 +/- 0.57 D; whereas on average, the maximum change in corneal power overestimated the change in manifest refractive error by 0.23 +/- 0.57 D (paired-t-tests, p < 0.001). A low but significant correlation between initial corneal asphericity and the change in manifest refractive error (Spearman r = -0.33, p = 0.01) was observed. Conclusions: The change in apical corneal power underestimates the change in manifest refractive error in ortho-K, whereas the maximum change in corneal power overestimates this parameter. Compared with retinoscopy and autorefraction, the change in apical corneal power is still useful for estimation of the change in manifest refractive error. Although the maximum change in corneal power appears to give a closer estimation of the change in manifest refractive error than the change in apical corneal power, there is no advantage in the use of maximum corneal power (manually located) instead of apical corneal power (a default given by the topographer) to estimate the change in manifest refractive error, as there is no significant difference in the estimations by either parameter. Initial corneal asphericity measured by the Medmont E300 corneal topographer has limited usage in predicting the change in manifest refractive error in overnight ortho-K.
