Knowledge is key for successful DALK procedure


Understanding corneal anatomy can minimize risk of Descemet’s membrane rupture

Knowledge of the presence and characteristics of the pre-Descemet’s layer provides a foundation for the understanding of tissue separation during deep anterior lamellar keratoplasty that will minimize the risk of rupture with Big Bubble creation and during the remainder of the procedure.

Reviewed by Sadeer B. Hannush, MD

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, said Sadeer B. Hannush, MD.

Dr. Hannush, attending surgeon, cornea service, Wills Eye Hospital, and professor of ophthalmology, Sidney Kimmel Medical College, Thomas Jefferson University, Philadelphia, explained that the PDL, which was first described by Professor Harminder Dua and colleagues in 2013,1 lies immediately anterior to 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,” he said. Dr. Hannush also noted that 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,” he explained.

Dr. Hannush said that 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.

RELATED: The OSD opportunity: Hard facts and a protocol for cataract patients“In most instances, a BB1 develops from the center to the periphery whereas a BB2 develops from the periphery to the center,” he said. If there is a BB2, surgeons do not need to abort the DALK procedure and convert to penetrating keratoplasty. In this instance,

Dr. Hannush noted that 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, he cautioned against touching DM with any instrument or squirting it with BSS. Dr. Hannush also recommended injecting a cohesive viscoelastic between PDL and DM to create space and avoid inadvertent contact between the scissors and DM while removing the posterior stroma.

“Ideally, the viscoelastic device cannula may be replaced with the same cannula used to create the BB (eg., Sarnicola, Fogla, Tan, etc.),” he said, noting these cannulas are preferred because they have a smooth contour and a posterior opening that will push DM away from the cannula.

Dr. Hannush also advised 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,” Dr. Hannush concluded. “They can lead to rupture during removal of the posterior stroma (BB1) or the posterior stroma/PDL (BB2).”

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Sadeer B. Hannush, MD
This article was adapted from Dr. Hannush’s presentation during Cornea Day at the 2019 meeting of the American Society of Cataract and Refractive Surgery. Dr. Hannush has no relevant financial interests to disclose.


1. Dua HS, et al. Ophthalmology. 2013;120:1778-1785.

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