[Cross-linking treatment of ultrathin corneas: the ELZA-sub400 protocol : Individualized adaptation of fluence as paradigm shift in CXL treatment of thin corneal conditions].

BACKGROUND

The Dresden corneal cross-linking (CXL) protocol requires a minimum corneal thickness of 400 µm to protect the endothelium from UV (ultraviolet) damage; however, many patients with advanced corneal ectatic disease have thinner corneas and are excluded from treatment. Earlier approaches to circumvent this limitation, hypo-osmolar swelling, contact lens-assisted CXL, epithelial island techniques, showed relevant shortcomings. The ELZA-sub400 protocol takes a fundamentally different approach: the UV fluence is individually adapted to pachymetry.

METHODS

This narrative review is based on current literature and clinical experience data from the ELZA Institute. The photochemical principles, the algorithm of the sub400 protocol, clinical outcomes and practical implementation are presented.

RESULTS

The sub400 protocol is based on an algorithm integrating Fick's law of diffusion for riboflavin and oxygen and the Lambert-Beer law for UV absorption. By individually adjusting the duration of UV irradiation to the pachymetrically measured stromal thickness, effective cross-linking is achieved while maintaining a 70 µm safety margin above the endothelium. A prospective study of 39 eyes (corneal thickness 214-398 µm) demonstrated a 90% success rate with no signs of endothelial decompensation. The protocol has also been applied in keratoglobus (1 case; tomographic stabilization at 32 months, no endothelial decompensation) [4].

CONCLUSION

The ELZA-sub400 protocol enables safe CXL even in corneas well below 400 µm thick for the first time. It adapts the technique to the individual anatomy rather than adapting the anatomy to the technique. Implementation requires only ultrasound pachymetry and a 3 mW/cm2 UV light source and is therefore feasible on most available CXL devices.