Deep anterior lamellar keratoplasty (DALK) is a surgical procedure for removing the corneal stroma down to Descemet’s membrane. It is most useful for the treatment of corneal disease in the setting of a normally functioning endothelium.
A trephine of an appropriate diameter is used to make a partial-thickness incision into the patient's cornea, followed by pneumodissection or manual dissection of the anterior stroma.
This is followed by placement of a graft prepared from a full-thickness punch in which the donor endothelium-Descemet membrane complex has been removed. The intention is to preserve the patient's Descemet membrane and endothelium.
Similar to PK, the graft is secured with interrupted and/or running sutures and these are then selectively removed post-operatively.
Traditionally, penetrating keratoplasty (PK), which involves a full-thickness corneal graft, has been the treatment of choice for corneal stromal diseases. But PK can be complicated by graft rejection, irregular astigmatism and corneal opacification, thus resulting in visual impairment.
DALK offers an alternative procedure that may lessen those risks because the recipient Descemet’s membrane and endothelium are preserved. At the same time, DALK carries the potential danger of decreased visual acuity due to possible opacification at the interface layers.
DALK can be an effective treatment for any pathology of the anterior cornea (epithelium, Bowman’s layer and stroma) as long as the patient has an intact, functioning endothelium. Common indications for DALK include keratoconus and corneal scars.
Patients with keratoconus are good candidates for DALK because they are typically young and have healthy endothelium. These patients stand to lose the most from the occurrence of post-PK immunological reactions that can compromise endothelial function in up to 20 percent of cases.
Less common indications for DALK include vernal keratoconjunctivitis, corneal dystrophies and ocular surface diseases with limbal stem cell deficiency, including Stevens-Johnson syndrome, ocular cicatricial pemphigoid and chemical/thermal burns.
Knowledge of corneal ultrastructural anatomy that recognizes the presence of the pre-Descemet’s layer (PDL) brings a new understanding to deep anterior lamellar keratoplasty (DALK) and its successful completion.
The PDL, which was first described by Professor Harminder Dua and colleagues in 2013, lies immediately anterior to Descemet’s membrane (DM), beneath the posterior stroma. Although it is only 5 to 15 μm thick, the PDL has a high bursting pressure of >500 mm Hg.
Whether or not the PDL is left behind with the DM and endothelium after air injection to create the Big Bubble (BB) in DALK determines the risk of rupture into the anterior chamber.
“A BB type 1 (BB1) cleaves off or separates the posterior stroma from the PDL, leaving the PDL behind with DM and endothelium. In this scenario, there is little risk of perforation into the anterior chamber because the PDL has such a high bursting pressure.
A Big Bubble type 2 (BB2) forms between PDL and DM, leaving behind DM and endothelium. In this situation, the risk of rupture into the anterior chamber is greater because the bursting pressure of DM is only around 30 mm Hg, and this risk remains increased during the remainder of the procedure due to the fragility of DM.
By allowing surgeons to identify the location of the cannula before injecting air to create the BB, intraoperative OCT is helpful for achieving a BB1. Surgeons who do not have access to this imaging tool may be able to judge whether a BB1 or BB2 was created based on how it is formed.
In most instances, a BB1 develops from the center to the periphery whereas a BB2 develops from the periphery to the center. If there is a BB2, surgeons do not need to abort the DALK procedure and convert to penetrating keratoplasty.
In this instance, surgeons need to recognize that they are dealing with a more tenuous situation that requires patience and attention to detail in order to bring DALK to a successful completion.
Dr. Hannush offered several pearls for proceeding in the setting of a BB2. To minimize the risk of DM rupture, it is important to be watcful against touching DM with any instrument or squirting it with BSS.
Also, it is recommended to inject a cohesive viscoelastic between PDL and DM to create space and avoid inadvertent contact between the scissors and DM while removing the posterior stroma is important.
Ideally, the viscoelastic device cannula may be replaced with the same cannula used to create the BB (eg., Sarnicola, Fogla, Tan, etc.). these cannulas are preferred because they have a smooth contour and a posterior opening that will push DM away from the cannula.
It is also advised to use of rounded scissors to remove the posterior stroma/PDL after creation of BB2 and cautioned against “leaning” on DM with the posterior blade of the scissors. The Tan DALK scissors, which features a platform on the posterior blade, is designed specifically for this maneuver in cases of both BB1 and BB2.
“Before proceeding, however, ensure that the big bubble (1 or 2) extends to or beyond the trephination mark and that there are no adhesions between PDL and DM.
They can lead to rupture during removal of the posterior stroma (BB1) or the posterior stroma/PDL (BB2).