Pre-Descemetic DALK

Rishi Swarup, FRCS and T. Suchi Smitha, MS, DNB, FCRS

Deep anterior lamellar keratoplasty (DALK) aims at retaining the host Descemet’s membrane (DM) and endothelium while removing the stromal layers completely or near completely wherever possible. Among the various techniques described for DALK, the big bubble technique achieves complete removal of stromal fibers and gives a Descemetic DALK (D-DALK), which is considered ideal, as visual results of D-DALK are comparable to penetrating keratoplasty (PK).¹ Nearly all other methods described for DALK may retain deep stromal fibers to a varying extent, leading to a pre Dua’s or pre-Descemetic DALK (PD-DALK), or a near-Descemetic DALK (Figure 6-1).

The level of dissection by the surgeon often depends on the indication for surgery, expertise of the surgeon, and preexisting scar or breach in DM (where the surgeon chooses to retain deeper layers of stroma to prevent perforation of DM, thus avoiding conversion of the procedure to full-thickness keratoplasty). The main concern regarding D-DALK is the unpredictability of the surgical procedure, especially in the hands of a novice surgeon, perforation of DM being the main concern.² In case perforation occurs, even if a lamellar procedure can be completed, it may sometimes lead to a low-quality corneal bed. Also, at times, a donor cornea with good quality endothelium may not be available, in the event the procedure needs to be converted to a PK. PD-DALK may have a higher chance of safety against perforation of DM compared to D-DALK, especially in situations where there is a possible weakness in DM or in the hands of a less-experienced surgeon.

Though earlier studies have shown anterior lamellar keratoplasty (ALK) to be inferior to PK as far as visual results are concerned, with regards to PD-DALK, there is some evidence at present to show that if the rear stroma is healthy, and if very little stroma is left behind during dissection, it may not hamper the final visual outcome.3 Thus, intentional sparing of the deepest stromal layers could be considered in order to prevent intraoperative DM rupture in selected cases where DM baring is difficult (Figures 6-2 and 6-3).

Specific indications for PD-DALK include keratoconus eyes that have had previous hydrops, eyes with traumatic penetrating injuries to the central cornea, or severe microbial infections with residual scarring close to DM,4 iatrogenic ectasia post-keratotomy with deep cuts,⁵ and cases where the big bubble technique of DALK should be avoided to prevent perforation of DM (Figures 6-4 and 6-5). It may also be done for acute hydrops using the Jacob modified technique^6,7 as described in detail elsewhere in this book.

Where possible, the surgeon should aim for a D-DALK (ie, to bare the Dua’s layer ideally at least in the visual axis). The most accepted technique of D-DALK is currently the big bubble technique, invented by Anwar and Teichmann8,9; however, in this technique, if the surgeon fails to obtain a big bubble he or she may choose to dissect layer-by-layer to reach as close as possible to DM. Due to the presence of thin stromal support for the DM, the chances of salvaging a PD-DALK may be better than D-DALK in the event of perforation of DM during dissection, and the thin stromal support can often prevent a microperforation from becoming a macroperforation¹⁰ (Figure 6-6).

If sharp dissectors are used as in Conventional Manual (Cut and Peel) DALK, which involves layer-by-layer excision of the stroma until the desired depth is reached, there may be a higher risk of perforation from the dissectors.

The Melles technique of PD-DALK (described elsewhere in detail in this book), involving utilization of the air-endothelial reflex and introduction of curved dissectors in the deep stroma, may be safer, but can give suboptimal results due to higher possibility of interface scarring between host stromal bed and donor stromal button.⁸

In case a PD-DALK is primarily planned, during initial dissection, use of a diamond knife with calibration set at 50 μm less than thinnest corneal pachymetry has been described, in order to spare a thin deep stromal layer along with DM and endothelium. Subsequently, the pocket created at the desired depth can be enlarged with dissectors to get the right plane. Viscoelastic may be used in aiding dissection.¹¹

The Groove and Peel technique¹² is yet another safe technique of achieving a near D-DALK and overcomes many of these difficulties without the need for expensive instrumentation or tremendous surgical skill, making it the ideal technique for the novice lamellar surgeon (Figures 6-7 and 6-8). It involves partial-thickness trephination of the cornea using a simple or guarded trephine until about 50% to 75% depth. At one point in the trephination groove, the groove is slightly deepened using a Bard Parker blade (Aspen Surgical) to create a stromal pocket. Blunt-tipped curved corneal scissors are introduced into this pocket with the blades of the scissors oriented horizontally and the tips closed. Once the tips are intrastromal, the blades are slowly opened to separate the collagen lamellae and, hence, widen the intrastromal pocket (Figures 6-9 through 6-11). The anterior layer of this pocket is cut with the scissors and the dissection is advanced in a similar manner along the trephination groove. This blunt dissection is slowly progressed along the entire length of the trephination mark until a circular stromal gutter is created. As the dissection proceeds from one end to the other, the plane of separation gradually advances deeper in the stroma in a spiraling fashion with each pass such that one may complete 2 or 3 turns of the spiral before the desired depth is achieved. This spiraling dissection is to be halted once the Dua’s layer is visualized, seen as a glistening clear membrane devoid of any significant overlying stromal attachments. At this point, making a paracentesis, helps to decompress the anterior chamber and reduces the chances of DM perforation. Once the correct plane is reached, the dissection is free of any resistance and the exposure of Dua’s layer is slowly increased until 3 to 4 clock hours. The stromal edge is then grasped and tented up (see Figures 6-9, 6-11, and 6-12). Stretch forces, thus created, help cleave the microscopic adhesions between the deep stroma and DM. In one continuous motion, the entire stroma within the trephination groove is easily peeled away using traction forces distributed over a large surface area of the DM (see Figure 6-9F). Counter-traction may be provided by means of a fixation forceps at the limbus opposite to the direction of the applied force. A relatively easy peeling off of the stroma from the DM has been noted in eyes with keratoglobus (unpublished observation), thus making this technique possibly very predictable and simpler in such cases and where the cornea has become extremely thinned out.

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