BLOG: Making sense of corneal surgery alphabet soup
The topic of corneal transplant surgery can quickly evolve to alphabet soup with all of the acronyms involved.
Our next few blog entries will address corneal transplants, so this is a brief outline of the relevant terminology.
Penetrating keratoplasty (PK) has historically been the only option but is now used primarily for full-thickness keratopathies. Indications would include corneal trauma, ectasias (such as keratoconus with hydrops or deep scarring) and other deep or full-thickness scarring.
Thankfully, with the advent of corneal cross-linking and deep anterior lamellar keratoplasty (DALK), we will be seeing less PK treatment of keratoconus (KCN) in the future. After several failed PK attempts, keratoprosthesis may become the only option.
DALK is a partial thickness surgery that replaces the stroma, leaving Descemet’s and the endothelium intact. Opacification at the interface layers may occur, and subsequent visual acuity may be lower than a fully healed and corrected PK.
DALK is a possible treatment for KCN that is advanced but without hydrops, corneal scars that are more anterior but still too deep for phototherapeutic keratectomy and stromal dystrophies.
If accidental perforation occurs during surgery, a full PK transplant may become necessary.
Descemet’s stripping automated endothelial keratoplasty (DSAEK) is a graft of at least 50 microns thick that includes the endothelium, Descemet’s and some stroma so it holds its shape and is less susceptible to scrolling up. It is easier to manipulate than Descemet’s membrane endothelial keratoplasty (DMEK) and can be a better choice for eyes with more difficult anatomy. It can be used to treat endothelial failure (such as Fuchs and posterior polymorphous corneal dystrophy.
DMEK is a very thin graft that only replaces Descemet’s and the endothelium. It is thought to have a superior visual outcome to DSAEK (about one line better due to lack of host-donor stroma-stroma interface). DMEK treats the same conditions as DSAEK.
Both endothelial keratoplasties (DMEK and DSAEK) are less invasive, have shorter surgery time, shorter healing time and leave the eye less susceptible to injury than PK. These transplants enjoy greater structural integrity because the anterior cornea is left largely intact, making these eyes more resistant to perforation or rupture, ultimately inducing less astigmatism and with a lower risk of infection than PK. Additionally, this is usually a sutureless procedure, reducing or eliminating suture-related complications such as broken suture, neovascularization, infection and subsequent rejection. (Because it is not a full-thickness procedure, in DALK the resultant wound is also stronger than that of a PK).
As you can see from the accompanying figure, the most appropriate corneal transplant depends heavily on the layers affected.
There are many possible indications for corneal transplantation, including corneal anomalies, corneal degenerations and corneal dystrophies.
Corneal anomalies can include trauma or infection that can lead to scarring and irregular astigmatism as well as idiopathic problems such as pseudophakic bullous keratopathy (chronic post-cataract extraction corneal edema) and decompensation of donor tissue, leading to the need for repeat transplant.
Corneal degenerations are metabolic and age-related and, therefore, occur later in life. They can be bilateral, but are often asymmetric. They first affect the peripheral cornea, then move centrally and have associated inflammation and vascularization. Examples include Salzmann’s nodular degeneration and band keratopathy. Corneal degenerations may call for surgical intervention but rarely corneal transplantation.
Corneal dystrophies are hereditary, with early onset, usually in the first or second decade of life. They are bilateral and symmetric and start centrally; therefore, they are often more visually significant than degenerations. Corneal dystrophies include congenital corneal ectasias like KCN and pellucid degeneration.
Join us next month as we review preoperative considerations for corneal transplantation.