There is no single cut-off that suits everyone, but a prescription is too high for laser eye surgery when treating it would leave the cornea structurally unsafe or vision unpredictably poor.
Safety limits and corneal structure
High corrections remove more tissue, so planning tools focus on how much cornea would remain and how strong it will be.
- Predicted residual stromal thickness below accepted safety thresholds increases ectasia risk
- High percentage tissue altered (PTA) values are linked to post-LASIK ectasia
- Very steep or thin corneas may already be biomechanically vulnerable
- High myopia combined with significant astigmatism often reaches the limit sooner
- Abnormal topography or tomography can make even modest prescriptions unsafe
Alternatives when limits are exceeded
When the planned ablation would be unsafe, surgeons usually recommend different approaches rather than “pushing” the cornea.
- Surface laser (PRK) or SMILE with more conservative targets
- Phakic intraocular lenses such as ICLs for very high prescriptions
- Refractive lens exchange in older patients with early lens changes
- Continuing with contact lenses or glasses if surgery offers no safe benefit
- Shared decision-making to balance visual goals with long-term corneal health
