DALK is a partial-thickness corneal transplant which involves only the donor stroma, leaving the recipient's own Descemet membrane and endothelium. The Penetrating Keratoplasty (PK) technique largely overshadowed Lamellar Keratoplasty (LK) because LK had poor visual outcomes related to irregularity of the dissected surfaces and scarring in the tissue interfaces. Recent improvements in surgical instruments and introduction of new techniques of maximum depth of corneal dissection as well as the reduced risk of immunological rejection compared to PK have renewed the interest of corneal surgeons. The procedure offers advantages over penetrating keratoplasty (PKP), including a “closed-system” operation and a decreased risk of postoperative immune rejection.

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History

Selective transplantation of a patient’s stroma without the endothelium originated over 150 years ago with the pioneer work of von Hippel and Filatov. Paufique later refined the technique by introducing dedicated instruments for stromal dissection. In 1959, Hallerman became the first surgeon to perform a deep stromal dissection close to DM and transplant a full-thickness donor button that included DM and endothelium. Anwar suggested in 1974 that visual outcomes are improved when the donor DM and endothelium are removed, thus creating a smoother interface. In 1984, Archila was the first to use air injection to facilitate the creation of a deep stromal plane. Other techniques such as hydrodelamination and manual dissection were subsequently proposed. Most recently, the forceful intrastromal injection air or viscoelastic to create a supra-Descemet cleavage plane and reduce the risk of intraoperative perforation was proposed and remains the preferred technique for DALK operations today

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Indications

DALK is most useful for treating corneal pathologies in the circumstances of a normally functioning endothelium. Some common indications for performing DALK include:

Tectonic:

1. Descemetocele: DALK may be performed both as a tectonic and optical means to restore ocular integrity. Therapeutic DALK can be performed in cases with healed                       infectious keratitis and descemetocele
2. Pellucid marginal degeneration
3. Sterile Mooren’s ulcer

Optical:

• Corneal ectasias

1. Keratoconus: The most common indication for DALK. Keratoconic patients are typically young and have healthy endothelium. However, there is controversy regarding visual     outcomes of DALK compared to PK with some sources stating that the outcomes are comparable while others reporting that DALK yields inferior visual outcomes.
2. Pellucid marginal degeneration (PMD): DALK is a useful surgical alternative to PK in the management of PMD. The technique provides useful visual rehabilitation in patients       with PMD even in the presence of previous corneal perforation.
3. Postrefractive surgery keratectasia: PK is considered the preferred approach for post- LASIK keratectasia (PPLK) with successful results. Due to success in treating                        keratoconus, DALK has also been gaining favor in managing iatrogenic keratectasia.
4. Refractive keratectomy haze, radial keratotomy, myopic keratomileusis, myopic keratomileusis cryo, persistent folds in the LASIK flap, and intracorneal ring segment                   complications.
• Corneal scars: DALK can be successfully performed for cases with corneal scars not involving the Decemet’s membrane. Dalk has been successfully used in scars secondary to     trauma, surgical, chemical injury, herpetic, post-bacterial/fungal keratitis, and other superficial scars.
• Corneal stromal dystrophies: Patients with Avellino, lattice and granular corneal dystrophies are good candidates for DALK because this technique maximizes depth of                 dissection and provides patients with acceptable visual acuity. DALK is not suitable for macular corneal dystrophy, as it might show progressive decrease in the corneal                endothelium postoperatively. DALK also has utility for correcting Bowman’s membrane ReisBücklers’ dystrophies and map-dot-fingerprint dystrophy with recurrent                     erosions.
• Ocular surface disease: Severe surface disease with limbal stem cell deficiency is a common presentation of trachomatous keratopahty, Stevens-Johnson syndrome, ocular        cicatrical pemphigoid and chemical/thermal burns. DALK is also used for corneal degeneration such as in patients with Salzmann’s nodular degeneration, climatic           degeneration and band keratopathy.

• Corneal clouding due to mucopolysaccharidosis: DALK is ideal since corneal clouding in these genetic diseases is due to glycosaminoglycans accumulating in the corneal          stroma, sparing the endothelium.

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Therapeutic:

• some resistant corneal infections.
• dermoids, some tumors
• inflammatory mass
• perforations

Contraindications

Anterior lamellar keratoplasty is contraindicated in cases of unhealthy endothelium or DM. The big-bubble technique (see below) cannot be performed in the presence of a pre-existing break in DM (e.g., following hydrops in keratoconic eyes) or deep scars involving DM

Preoperative Considerations

Advantages of DALK over PK include better long-term graft survival, minimal steroid-related complications, and easier follow up, and the ability to use a lower quality donor. In addition, larger grafts can be transplanted without an increased rejection risk21.

Techniques

DALK techniques can be divided into two main categories. In "pre-Descemet membrane" procedures, some stromal layers remain adherent to DM. "Descemet membrane" procedures involve complete stromal dissection and baring of DM. In all the techniques a partial-thickness trephination of the cornea is performed depending on pachymetry.

Manual intrastromal dissection

A partial thickness trephination (approximately two-thirds) is performed, followed by excision of the superficial stroma by manual dissection using a bevel up crescent knife. Maintaining a consistent depth is important to create a recipient bed of even thickness. Melles introduced a technique of using an air bubble in the anterior chamber to create a mirror image of the dissector blade, which serves as an indicator of stromal depth (air-guided deep stromal dissection). Exposed DM can be recognized by its smooth, glossy appearance. If DM is exposed, creation of a peripheral paracentesis to reduce intraocular pressure is recommended. A full or partial-thickness stromal button can be sutured in place once the anterior layers are removed. Disadvantages of manual dissection include the possible creation of an irregular stromal bed that can cause astigmatism and scarring at the graft-host interface. There is also a high rate of perforation.

Automated therapeutic lamellar keratoplasty

A microkeratome can be used to excise the diseased corneal layers with precision and high optical quality. The surgeon can choose the depth of the cut (typically ranging from 130 to 450 um). The donor cornea can also be cut using a microkeratome to the desired thickness and then sutured into the recipient bed.

Cleavage separation

The “big-bubble technique” was developed by Anwar and Teichman by utilizing the naturally occurring virtual space between the posterior stroma and DM. A 300 to 400 um deep trephination is performed first. Next, a 27 or 30 gauge needle, bent to 30 to 40° 5 mm from its tip is advanced with the bevel facing down through the trephination groove and into the paracentral corneal stroma parallel to DM. Air is then injected forcibly either through the needle or through a Sarnicola or Fogla cannula. This causes whitening of the peripheral stroma as a large air bubble enters the pre-DM plane. Viscoelastic or saline solution can also be used for this step, though it diffuses through the stroma less easily and carries a higher risk of perforation. The epithelium and anterior stroma can then be removed. A sharp 30° blade is then used to puncture the anterior (posterior stromal) wall of the bubble, and blunt scissors can be used to cut the stroma into quadrants and excise it within the trephination. Once DM is bared, the donor button can be sutured in place. Advantages include the ability to expose DM over a wide surface and with a decreased risk of perforation compared with manual dissection, especially in ectatic and thinned corneas. The disadvantages include the correlation of success with surgeon experience [Fontana] as well as the risk of anterior chamber entry if DM scarring or prior perforation sites exist.

Donor preparation

After the donor button is punched, a dry Weck-cel sponge can be used to partially detach DM and endothelium from the posterior stroma. Once an intact edge is free, the DM can be peeled off. Trypan blue can be utilized to facilitate this step. It is important that the stromal surface is smooth and uniform in order to optimize the optical quality of the graft-host interface.

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Complications

Intra operative

• The most common complication during DALK is perforation of the Descemet membrane during surgery. Tears or perforations occur in approximately 10 to 30 percent of the cases. The risk is less in elderly patients, who tend to have a thicker Descemet membrane. Depending on the size of the perforation and the also the stage of the surgery at which the perforation occurred, it may or may not necessitate conversion to full-thickness corneal transplantation surgery. Perforation can occur during trephination, stromal dissection, and suturing.

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Post-operative complications

Post-DALK pseudo anterior chamber formation

• Formation of double (pseudo) anterior chamber: Usually seen in the immediate postoperative period and in cases that had an intraoperative perforation of the Decemet’s         Membrane. Although spontaneous resolution has been known to occur in long-standing cases with double anterior chamber, the usual surgical intervention consists of            injecting air into the anterior chamber in order to tamponade the detached Decemet’s membrane.
• Corneal stromal graft rejection: Uncommonly, corneal stromal graft rejection has been reported after a successful DALK surgery. This complication can present anytime after    3 months postoperatively.
• Interface haze: occurs in cases where stromal fibers have been left and is apparent in late post-operative follow-up.
• Graft dehiscence: Early suture removal may lead to graft dehiscence in some cases.
• Recurrence of the original pathology
• Decemet’s membrane folds
• Interface keratitis: interface left during DALK is a potential dead space
• Damage to the iris sphincter muscle resulting in a fixed pupil (more commonly seen in keratoconus patients) (Urrets-Zavalia syndrom)
• Pupillary block due to air/gas in the anterior chamber
• Suture-related complications: postkeratoplasty atopic sclerokeratitis (PKAS)

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Prognosis

DALK has been shown to have similar best-corrected visual acuity outcomes compared with PKP. It is thought successful baring of DM is the primary determinant of achieving maximal visual improvement because residual stromal tissue can contribute to interface opacification that limits acuity. In patients with keratoconus undergoing DALK, persistent myopia and astigmatism following complete suture removal are the most common morbidities. While some studies suggest that the incidence of postoperative myopia is similar between DALK and PKP (in a recent study, -6.1±1.8 D and-5.0±1.5 D in DALK versus PKP, respectively),others have suggested that DALK might produce higher levels of myopia as compared to PKP. Astigmatism appears to be similar in DALK and PKP. Huang et al suggests that postoperative spherical equivalent may be dependent on preoperative vitreous length.

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DALK spares transplantation of the endothelium, which eliminates the possibility of endothelial rejection. Epithelial, stromal, and mixed epithelial and stromal graft rejection, however, can occur at a rate of approximately 8 to 10%. Less than 5% of grafts fail secondary to rejection The risk of graft loss following blunt trauma is also markedly reduced as compared to PK.

DALK avoids the risk of endothelial rejection, however epithelial and stromal rejection episodes can occur which can be effectively managed using topical corticosteroids. In a recent study of outcomes using the ''big bubble'' technique, only 2 of 78 patients developed stromal rejection. This fits well with other studies reporting rejection in 3-8% of cases.

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Additional refractive procedures, such as use of Excimer lasers, or phakic IOL implantation can be performed in eyes post DALK, following complete suture removal and ensuring stable topography. Surgical management of Keratoconus has improved with advances in surgical techniques, instrumentation, and diagnostic evaluation. DALK procedures should be considered prior to PK as it helps preserves healthy host endothelium, eliminating risk of endothelial rejection, ensuring long term graft survival. Femtosecond laser technology integrated with corneal OCT is likely to improve success rates, and maximize visual outcomes of DALK in future.