The cornea is a layered, or lamellar structure, with different layers providing different functions. The surface layer is called the epithelium. It protects the cornea and provides a smooth surface for focusing light. The middle layer of the cornea, the stroma, provides strength. The deepest layer, facing the inside of the eye, is the endothelium. This is a single layer of special cells that work as a pump, draining fluid out of the stroma. Without the endothelial pump the stroma would become water-logged and cloudy.
If the corneal endothelium fails the cornea becomes waterlogged and opaque, reducing vision. In the early stages this may only be apparent on waking, as eyelid closure overnight reduces endothelial function and allows the cornea to thicken. Blurred vision then wears off after 1-2 hours of eye opening as the cornea clears. With more severe endothelial failure the cornea remains cloudy all the time. In the worst cases fluid blisters appear on the corneal surface, causing severe pain when they burst, and laying the eye open to infection.
Cause of endothelial failure
Endothelial failure most commonly arises as a result of previous eye surgery, e.g. cataract or glaucoma operations. It may also occur spontaneously in a condition called Fuch’s Endothelial Dystrophy. This is a genetic condition, although not always inherited, whereby patients do not have enough cells to last lifelong. Patients develop symptoms between the ages of 40 and 60, often in both eyes.
Over the past 10 years, surgery for endothelial failure has improved dramatically, with the advent of selective endothelial replacement. Previously, the standard technique for endothelial failure was to provide a new sheet of endothelial cells from a donor eye as part of penetrating keratoplasty, or PK. PK is an established and effective operation, and is still useful in some cases, e.g. where the cornea has become scarred as a result of severe and prolonged corneal clouding. The majority of patients however are suitable for Descemet’s Stripping Endothelial Keratoplasty (DSEK). In this procedure the endothelium alone is replaced, leaving the healthy majority of the cornea in place. The donor cornea heals onto the patient’s cornea and pumps fluid out of it, clearing the vision.
In DSEK a very fine layer of endothelium and supporting tissues approximately 0.01 mm thick is removed from a donor cornea. The unhealthy endothelium is then stripped from the patient’s eye. The donor endothelium is gently rolled and inserted into the eye, where it unrolls and is floated into position. It sits in place without stitches because its fluid pumping action sucks it into position. The operation may be performed in eyes with or without previous cataract surgery. In some case DSEK may be combined with cataract surgery and lens insertion.
DSEK surgery may be performed under general anaesthetic or local anaesthetic (usually sub-Tenon’s, rather than eyedrops alone), and usually as a day-case procedure.
Donor endothelium is prone to rejection by the patient’s immune system; this may happen after any kind of graft procedure. Steroid eye-drops are prescribed post-operatively and may need to be continued for a year or sometimes long-term depending on the individual case. If at any time after a graft operation, even years later, if the eye becomes red, painful or blurred then urgent ophthalmic advice must be sought. Rejection can usually be reversed as long as treatment starts within a few days of onset.
Advantages of DSEK
The advantages of DSEK over penetrating keratoplasty result from the much smaller incision. The eye recovers much more quickly, with good vision usually achieved after 1 to 2 months, but with further improvement (another 10-20%) occurring gradually over several months. Refractive error, i.e. the need for glasses, contact lenses or further corrective surgery is greatly reduced. Because the incision is much smaller, the risks of wound leaks or infection are less, and the eye is much less vulnerable to injury than after a PK.
In some cases it may not be possible to complete the operation, and full-thickness grafting (PK) would instead be needed. The donor endothelium may scar where it adheres to the patient’s cornea, creating a haze between the donor and patient’s tissues which may reduce final visual acuity.
The most common complication after DSEK is donor dislocation or separation, where the donor graft fails to adhere properly. This is seen in up to 20% of cases and requires an additional operation ‘rebubble’ to reposition the donor. In some cases adhesion or satisfactory endothelial function is not achieved and the graft will need to be replaced.
In most cases of DSEK failure, replacement with another DSEK graft is the best option. Depending on the cause of failure it may be better to switch to penetrating keratoplasty (PK) as an alternative approach.