To finish off the series, the article we’ve all been waiting for–corneal transplants!
If you have not been following this series, be sure to check out our previous articles over different reasons someone may require a corneal transplant as treatment: Fuch’s Endothelial Dystrophy, Keratoconus, Acanthamoeba Keratitis, Trachoma, Herpes, and corneal ulcers.
As you can see, the common denominator between diseases and the need for corneal transplant is corneal destruction (often resulting in scarring or high amounts of irregular corneal astigmatism that obstruct vision) in which the appropriate treatment is to simply replace the diseased cornea with clear new donor tissue.
However, it is not as simple as it may sound. A corneal transplant is still a transplant, meaning tissue (in this case a human cornea) is removed from a donor and sewn into the patient’s eye.
There are several different ways to perform this procedure depending on exactly what the disease etiology is and the needs of the receiving patient.
Common Types of Corneal Transplants
The four most common types of corneal transplant that we will be discussing in this article are Penetrating Keratoplasty (PK), Deep Anterior Lamellar Keratoplasty (DALK), Descemet Stripping Automated Endothelial Keratoplasty (DSAEK), and Descemet Membrane Endothelial Keratoplasty (DMEK).
Corneal Anatomy Review
Before getting into the details of these transplant types, here’s a quick review of the different layers of the cornea:
Epithelium: Outermost layer. Approximately 50 microns thick.
Bowman’s Layer: A supporting layer composed of dense collagen. Approximately 10 microns thick.
Stroma: Middle most layer of the cornea. Approximately 500 microns thick.
Descemet’s Membrane: A very thin basement membrane ranging from 5 to 10 microns thick.
Endothelium: The innermost layer of the cornea containing specialized cells that pump out excess fluid, keeping the cornea in a slightly dehydrated state. Approximately 5 microns thick.
Also remember that the cornea is a very unique tissue. It requires all of its cells to be uniformly spaced without interruption or irregular cell growth, as this will cause the translucent corneal tissue to turn opaque (as in the case with corneal scarring).
Thus, not only are corneal transplants complex due to how thin the tissue is, but transplants must be extremely precise and require proper and frequent post-operative care to minimize the risk of rejection or other post-op complications.
Types of Corneal Transplants
Penetrating Keratoplasty (PK)
A penetrating keratoplasty is a full thickness corneal transplant, meaning all five layers of the host’s tissue is removed and replaced with a donor cornea.
While still complex, penetrating keratoplasties are considered the most simple type of corneal transplant, as the surgeon does not need to worry about lining up different sections of corneal tissue that are only several microns thick.
However, since a PK does involve a complete replacement of donor tissue, it consequently runs the highest risk for graft rejection.
Penetrating keratoplasties are commonly used to treat advanced keratoconus, Acanthomeboa infections, and scarred down corneas secondary to Trachoma, ulcers, and herpes infections.
Deep Anterior Lamellar Keratoplasty (DALK)
A deep anterior lamellar keratoplasty is a selective corneal transplant, replacing only the anterior cornea.
In a DALK, the corneal epithelium, Bowman’s layer, and stroma are carefully removed and replaced with donor epithelium, Bowman’s layer, and stroma. The patient’s own Descemet’s membrane and endothelium remain intact.
DALKs are performed because replacing the corneal endothelium can be very difficult. If you will recall, the endothelium contains specialized pumps used to remove excess fluid from the cornea, keeping it in a slightly dehydrated state.
Corneal endothelium cells are very fragile. If damaged, they cannot repair themselves. For this reason, many surgeons will try to avoid putting any stress on the endothelial layer of the cornea, when possible, in hopes to preserve and maximize endothelial function.
Another advantage of a DALK procedure in comparison to a full thickness penetrating keratoplasty is that less tissue is transplanted, thus reducing the risk of graft rejection.
The DALK procedure is commonly used to treat advanced keratoconus and Acanthamoeba infections.
Descemet Stripping Automated Endothelial Keratoplasty (DSAEK)
A Descemet stripping automated endothelial keratoplasty is another selective corneal transplant.
In DSAEK, the back part of the stroma, Descemet’s layer, and endothelium are removed and replaced.
DSAEK, therefore, can best be remembered and differentiated from the similar DMEK procedure based on the S in its name (S and Stroma), where only Descemet’s layer and the endothelium are replaced in DMEKs (see below).
As noted above, any procedure involving replacement of the endothelium complicates things. However, this can become necessary in circumstances in which the endothelium is non-functioning or has deteriorated to a point where it can no longer effectively pump out excess corneal fluid.
While a DSAEK is difficult to perform, it is preferred over a penetrating keratoplasty in some circumstances to reduce the risk of graft rejection, as less tissue is replaced in a DSAEK in comparison to a PK.
DSAEK procedures are most commonly used to treat advanced stage Fuch’s endothelial dystrophy or other more rare endothelial disorders.
Descemet Membrane Endothelial Keratoplasty for Fuch’s
A Descemet Membrane Endothelial Keratoplasty is also a selective corneal transplant, and is the most difficult type of cornea transplant for a surgeon to perform.
In DMEK, only the Descemet’s layer and endothelium are removed and replaced–a total thickness of less than 15 microns!
While advanced techniques are required by the surgeon, DMEK’s are beneficial for the patient because it leaves the greatest amount of host tissue intact, thus making this form of transplant debatably the safest with the least likelihood of graft rejection.
Similar to DSAEK procedures, DMEK procedures are most commonly used to treat advanced stage Fuch’s endothelial dystrophy and other endothelial disorders.
What to Expect from a Corneal Transplant
A corneal transplant is a serious surgery. The decision to undergo a corneal transplant, therefore, should not be taken lightly. Pros and cons to the surgery should be discussed between the patient and the doctor.
Next, the surgeon will want to look at the cornea prior to surgery, taking several measurements to best prepare for the surgery.
Currently, there is not a long waiting list for corneal transplants, although donor tissues will be required prior to the surgery being performed. For this reason, surgery availability can vary.
A corneal transplant is typically an out-patient procedure, meaning the patient will be able to go home the day of the surgery.
The surgery is accomplished via local anesthesia, meaning the individual will technically be awake, but likely will not remember the procedure, nor experience pain during the procedure.
Depending on the type of corneal transplant, the tissue will be removed and replaced, with the donor tissue held in place via special eye stitches. These stitches often remain in the eye for 12 or more months, less with DMEK and DSAEK procedures.
Additionally, in many corneal transplant surgeries an air or gas bubble is placed in the eye to help keep the transplant adhered firmly in the correct position. If a bubble is instilled in the eye, the patient will likely be asked to remain on his/her back for 24 hours following the procedure to ensure the bubble, and therefore transplant, is not dislodged.
Immediately after surgery, the doctor will most likely recommend wearing an eyepatch to protect the eye from outside forces. This patch is especially important when sleeping, to ensure the patient does not accidently rub the eye and dislodge the transplant.
The eye will likely be sore, irritated, light sensitive, and red for a week or two following surgery. The patient should expect to be off of work and avoid heavy lifting or strenuous activity for at least 1-2 weeks post-op.
Anti-inflammatory and antibiotic eye drops will also be prescribed to aid in the healing process. A doctor, oftentimes a post-opt doctor rather than the surgeon themselves, will have the patient come in for frequent follow up visits to ensure proper healing and watch for graft rejection.
Visual outcomes for corneal transplants are typically fairly good, although most will require some form of visual correction for optimal vision (glasses or contact lenses). Vision will not reach its optimal clarity, however, often until 12-18 months post-operation.
After a few months into the post-op period, the individual will be prescribed an anti-inflammatory eye drop to reduce risk of rejection. This eyedrop will likely be needed for life.
With any kind of transplant, there are complications associated with corneal transplants, although serious complications occur in less than 10% of individuals. These complications include, but are not limited to, graft rejection (where the host’s body attacks the donor’s tissue), infection, retinal detachment, bleeding, and possible permanent vision loss.
Generally speaking, a corneal transplant will last 10 or more years. In some circumstances, the process will need to be repeated (due to complications, age of the transplant, etc.). For this reason, regular eye exams, at least yearly, are strongly encouraged following a corneal transplant.
Interested more in corneal transplants, or if you or someone you know would be a suitable candidate for one? Make an appointment to speak with your eye doctor today!
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