Penetrating keratoplasty involves full-thickness removal and replacement of a portion of the cornea. Once healing is complete,
usually within 50 days, the visual outcome is greater than 75 percent for PK-treated horse eyes. "PK suffers from the inherent
problem of creating a vertical stromal wound that requires surface corneal sutures," Brooks explains.
The procedure produces a stroma that "never develops the structural integrity and strength of the normal corneal tissue. A
concern is that the PK graft epithelium often sloughs, leaving the graft open to infection. Besides graft failure, other complications
include aqueous leakage from wound dehiscence/suture microleaks, focal nonprogressive cataract formation, hyphema and lens
capsule rupture intraoperatively.
Photo 5: The abscess was removed by penetrating keratoplasty.
Both PLK and DLEK are useful especially in resolving deep stromal abscesses (DSA) with anterior chamber fungal invasion. With
both procedures, the poster stroma and endothelium containing the abscess are surgically removed. "The basis of these split-thickness/lamellar
surgeries is to replace only the diseased portion of the cornea, leaving the normal tissue intact, to do the least amount
of resection for the greatest amount of benefit," says Brooks.
"PLK is recommended for deep stromal abscesses in the central cornea that are 8 mm or less and have a clear-overlying anterior
stroma," Brooks explains. Abscesses larger than 8 mm may be more commonly associated with complications. This corneal transplant
procedure is "associated with a shorter surgery and treatment time than the PK," Brooks notes.
"PLK removes infectious organisms, necrotic stroma and the metabolites of degenerating leukocytes while preserving the overlying
stroma and epithelium," Denis explains. Complications of PLK include superficial suture abscesses, suture incision leaks,
flap ulcers, graft failure requiring surgical repair or replacement, flap edema and Descemet's membrane retraction from the
graft. The visual outcome for PLK is greater than 90 percent.
Photo 6: A deep stromal abscess is present under the red vascular pattern.
DLEK is recommended for DSA in the peripheral cornea that are 6 mm or less and have a clear overlying anterior stroma. As
with PLK, larger abscesses may be associated with complications. This procedure avoids superficial incisions and suturing
of the central cornea.
DLEK transfers healthy endothelium while preserving the corneal surface integrity. A fully intact epithelium with no corneal
sutures is present postoperatively.
Though similar to PLK, DLEK is associated with a shorter surgery and treatment time than PK. Complications include ulcers
near the incision, suture incision leaks, incision edema, graft failure requiring surgical repair, graft misplacement and
focal nonprogressive cataract formation. The visual outcome is greater than 90 percent.
Room for improvement
Though the visual outcomes with all corneal transplant procedures in horses are successful, "they still are evolving," Brooks
explains. Partial graft rejection and scar formation seem unavoidable. Improved technique with specialized surgical corneal
forceps and scissors, and new pharmacologic approaches to infection and graft rejection, "could technically improve the transplant
surgical procedures," Brooks suggests.
"My work is really just the beginning," Brooks says. "We've got more research, which we will start this summer to try to continue
to figure out how to help the horse eye heal." In the past, when veterinarians treated horses' eye problems, 50 percent of
the time they had to remove or enucleate the eye. "Fifty percent!" Brooks exclaims. "At the University of Florida, today,
we hardly ever have to do an enucleation, because most of the time we figure out how to treat the condition either medically
Photo 7: The deep lamellar keratoplasty (DLEK) procedure was used to remove only the deep, diseased tissue and replace it
with half-thickness donor cornea.