Bowman’s layer (BL) transplantation and keratoconus

[caption id="attachment_6197" align="alignnone" width="750"] Bowman’s layer graft in situ (arrows) without surrounding inflammation, oedema, or scarring advocated as a new treatment technique for preventing progression and to improve vision in keratoconus[/caption] Keratoconus has seen many advances in treatment and outcomes and the old policy of wait and watch until a penetrating keratoplasty (PK) becomes inevitable is not acceptable anymore. Newer varieties of contact lenses, intracorneal ring segments, corneal crosslinking (CXL), toric phakic intraocular lenses (IOLs) and deep anterior lamellar keratoplasty (DALK) have all changed the management of keratoconus dramatically. Mild cases are amenable to CXL with or without intracorneal ring segments, whereas moderately advanced cases of keratoconus can be stabilised with CXL followed by optical correction with glasses or phakic IOL in corneas that are not too irregular. However, patients with very thin corneas, contact lens intolerance or lack of improvement in vision with contact lenses, may need DALK or PK. It would be ideal to be able to abolish the need for DALK and PK completely. NEW TREATMENT TECHNIQUE Recently, the Bowman’s layer (BL) transplantation has been advocated as a new treatment technique for preventing progression and to improve vision in keratoconus. The aim of this new technique is to strengthen and flatten the cornea in patients with advanced keratoconus by mid-stromal transplantation of an isolated BL graft, thus enabling continued contact lens wear and avoiding complications that may be associated with a PK or DALK. This technique was developed by Dr Gerrit Melles and his group at the Netherlands Institute for Innovative Ocular Surgery (NIIOS), Amsterdam. “BL transplantation presents a new treatment option for patients with advanced keratoconus. By flattening and regularising the corneal surface, the operation aims to preserve/restore the recipient’s ability to wear rigid contact lenses, thereby enabling good functional vision and delaying or avoiding the need for either PK and DALK,” said Dr Melles. The principle behind BL transplantation is that the BL graft functions like a splint. Dr Jack Parker, also from the NIIOS, explained: “After manually dissecting a mid-stromal pocket within the recipient cornea, the donor tissue is slipped inside and unfolded. The subsequent healing response around the graft possibly flattens the cornea into a more normal and stable configuration. The perfect patients for the procedure are those with progressive advanced keratoconus, ineligible for UV-crosslinking and with good contact lens corrected vision, but with poor (or worsening) contact lens tolerance. Conversely, relatively poor candidates may include patients with large, dense central scarring.” BL transplantation consists of fashioning a mid-stromal pocket within the recipient cornea with the Melles DALK dissection spatulas, using the “air-endothelial” reflection to guide the depth of dissection. Once the pocket is created, a Sheets glide is placed into the mouth of the wound, and the BL graft is placed on top, where it is pushed into the pocket, unfolded and stretched out to the corneal periphery using a blunt cannula. “Our experience with endothelial keratoplasty (EK) taught us that corneal surface incisions (as with PK and even DALK) may entail some intrinsic risks, since those operations predispose to a variety of problems, including suture related and wound healing difficulties, ocular surface challenges, and the risk of allograft reaction and graft rejection. Therefore, we sought a similar solution as with EK: namely, to replace full thickness corneal transplantation with a less invasive alternative. Because BL transplantation involves no cornea surface incisions or sutures, the operation may minimise these risks, and because the graft is acellular, theoretically the threat of allograft reaction and graft rejection may be diminished,” said Dr Melles. NORMAL SURFACE Korine van Dijk, who is also part of this project, said in their first cohort of patients to receive BL transplantation, the average amount of corneal flattening experienced was eight dioptres. "All were able to comfortably wear scleral contact lenses postoperatively, and 90% had their previously progressive disease arrested,” she said. “After BL transplantation, average spectacle corrected vision increased by two Snellen lines, whereas contact lens corrected vision remained generally unchanged. Surprisingly, though, the majority of all patients – including those with stable or worsened objective vision after surgery – reported improved daily, subjective visual acuity, which may relate to a more ‘normal’ ocular surface,” she added. In the published study, it was encouraging to note that of the 20 patients included for analysis, the cornea stabilised in 18 and only two patients showed continued steepening of the corneal curvature despite the BL inlay. BL graft preparation remains a somewhat challenging prospect, and benefits greatly from a well-trained eye banking staff. A 9-11mm BL is harvested using a custom-made stripper. It is then submerged in ethanol 70% to remove remnant epithelial cells and stored in organ culture medium at 31°C until transplantation, at which time it is 
again washed sequentially in 70% 
alcohol followed by balanced salt solution and then stained with trypan blue before insertion. Performing the manual mid-stromal dissection without perforating may also be difficult, depending on the thinness of the recipient cornea, and in the original study two cases out of a total of 22 eyes had an intraoperative perforation of Descemet’s membrane during 
manual dissection. "The primary limitation to BL transplantation is that the operation may be unsuitable for patients with extremely poor contact-lens corrected visual acuity, since the operation does not generally result in a measurable improvement in that category. However, as with EK, the evolution of anterior lamellar surgeries will probably continue in the direction of selective, minimally invasive, targeted therapies for peculiar problems, rather than indiscriminate full (or nearly full) thickness corneal replacement,” said Dr Melles. To conclude, this new technique aims at restoring the potential functionality of the BL for corneal stabilisation in keratoconic corneas which generally show fragmentation of the BL. Once corneal stabilisation is achieved, the patient can continue contact lens wear for visual rehabilitation. The procedure does not aim at improving best corrected visual acuity (BCVA) and patients with a poor BCVA may benefit more with a DALK or a PK.  

Tags: bowman's layer, keratoconus