This is a complication of corneal refractive surgery. Ectasia may be likened to corneal warpage and may occur many years after apparently succesful surgery. Patients with ectasia probably represent:-
i) patients with weak cornea prior to surgery (eg keratoconus, pellucid marginal degeneration)
ii) residual stromal bed too thin to maintain structural integrity
iii) patients who were later going to develop keratoconus
Risk factors include:-
i) Preoperative corneal topographic abnormalities
ii) low residual stromal bed thickness
iii) Young age
iv) High myopia.
Abnormal preoperative corneal topography is the most important predictor of possible postoperative ectasia. Useful topographic indicators are:-
i) Inferior-superior differences in corneal power around the visual axis (greater than 1.4 dioptres). This is a central keratometric index and an indicator of forme fruste keratoconus
ii) Asymmetric inferior corneal steepening – ie steepening in the inferior peripheral cornea – of the sort seen in pellucid marginal degeneration
iii) Skewed radial axes above and below the horizontal meridian (asymmetric bow-tie)
iv) Asymmetry of the topographic pattern between eyes.
Ectasia can develop in patients with entirely normal preoperative topography.