Deep Anterior Lamellar Keratoplasty (DALK): Science and Surgery

Abstract

The last couple of decades have seen a paradigm shift in corneal transplant surgery. What started as a lamellar procedure with replacement of the anterior lamellae of the cornea, evolved into penetrating keratoplasty, which sustained for over 100 years, has come around one full circle with lamellar surgery once again taking the high ground. Modern lamellar surgery however, both anterior and posterior, is far more sophisticated and refined than anterior lamellar surgery of the past. Deep anterior lamellar keratoplasty is now established as the desired option for replacing diseased corneas where the endothelium is healthy and normal. It eliminates the risk of corneal graft failure secondary to endothelial rejection, which is a major problem with full-thickness corneal grafts. Suture-related complications and induced corneal astigmatism, representing the other major issues, remain, but the graft-host junction is stronger and less susceptible to dehiscence or rupture. The big bubble technique involving the intrastromal injection of air or viscoelastic to separate pre-Descemet's layer (Dua's layer, PDL) from the deep stroma in the type 1 big bubble, or Descemet's membrane (less commonly) from the PDL in the type 2 big bubble, is the most popular and most advocated technique. Various adaptations including use of the femtosecond laser and intraoperative optical coherence tomography have been introduced to increase predictability of big bubble formation. By this approach all the central cornea with the exception of Descemet's membrane alone (approximately 10-20 microns, type 2 big bubble) or with the PDL (total thickness of approximately 30-40 microns, type 1 big bubble) is replaced by healthy donor corneal tissue. The technique is challenging, with a steep learning curve. Intraoperative complications ranging from minor events like Descemet's membrane wrinkles/striae and micro- or macro-perforations to major events like rupture of the posterior layers or failure to obtain a big bubble, requiring conversion to penetrating keratoplasty, are well known. Epithelial and stroma rejection can occur, but the incidence is comparatively low; hence reduced duration of topical steroid medication and early removal of sutures, as wound healing is not affected by prolonged steroid usage, are recommended. Deep anterior lamellar keratoplasty led to the discovery of the pre-Descemet's layer (Dua's layer/Dua-Fine layer) knowledge of which has improved understanding of corneal surgery and corneal pathology. This approach, retaining the PDL, has a particular role in therapeutic keratoplasty as it is essentially an extraocular procedure reducing risk of intraocular spread of infection. The PDL, by virtue of its strength and elasticity, also allows the combined removal of cataract by phacoemulsification and lens implant or by pars plana vitrectomy and lens fragmentation, under its cover.