Corneal Anatomy

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.

Endothelial failure

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.

DMEK

Descemet’s membrane endothelial keratoplasty is a development of DSEK surgery (see separate page) in which the donor tissue is an ultra-thin strip of endothelium supported only by the Descemet’s membrane. This layer of tissue (0.02mm thick) is the same as the unhealthy tissue that is removed from the eye in DSEK or DMEK surgery, thus achieving true ‘like-for-like’ replacement of tissue. The result is even faster and more complete visual recovery than with DSEK surgery. The procedure may not be suitable for eyes with more complex causes of corneal failure and it does require 48 hours of face-up lying down posturing after surgery to ensure that the donor tissue adheres correctly. Usually the first 24 hours are in hospital and then the patient can be discharged and complete the process at home.

Peripheral Iridotomy

Around 2 weeks before surgery it is necessary to make a tiny hole in the iris (coloured part of the eye, around the pupil), to prevent eye-pressure problems during surgery. The laser rarely causes problems, but occasionally there is post-laser inflammation of elevated eye pressure that requires treatment. Bleeding from the laser site is uncommon, and usually minor/self-limiting. Larger bleeds and damage to the eye or vision are very rare.

Cataract Surgery

If cataract surgery is required this may be carried out prior to DMEK or at the same time. If the cornea is too cloudy for cataract surgery it may be necessary to clear the cornea first with a transplant (DMEK, DSEK or PK) and then carry out the cataract operation.

The Operation

Anaesthetic for DMEK

DMEK surgery can be performed under local anaesthetic (the patient awake, but the eye numb to prevent pain and reduce vision during the operation) or under general anaesthetic (the patient asleep). It requires 1 night stay on the day of surgery.

Surgery

The donor will be prepared first. If donor preparation fails then the operation cannot go ahead, and will need to be postponed.

Placing the donor graft into the eye, unrolling and positioning takes around 1 hour.

You will be examined on the ward on the day of surgery, and then at 1 week post-op in the outpatient clinic. You will be prescribed eye drops to prevent infection and inflammation. Further eye drop treatment will be explained in clinic, but it is most important not to stop the steroid drops until advised that it is safe to do so, as otherwise corneal graft rejection may occur.

Risks of DMEK

Dislocation of Graft

The endothelial graft may come away from the patient’s cornea in the first few days after surgery.

If the graft is completely detached the result is marked corneal oedema (water-logging) causing reduced vision and often painful blisters on the corneal surface. This is seen in up to 20% of cases.

Partial detachment will often resolve spontaneously, but major or complete separation requires a return to the operating theatre for ‘rebubbling’. In this operation an air bubble is injected into the eye to push the donor material up against the patient’s cornea. The graft may need to be repositioned, and the air bubble kept in place for up to 60 minutes.

In some cases the donor will not adhere, or adheres but does not function. In either situation a replacement graft will be required. It may be necessary to use an alternative endothelial approach (DSEK), or a penetrating keratoplasty (PK).

If a ‘wait-and-see’ approach is adopted for partial dislocation, in most cases the remainder of the graft will attach in a few weeks. In some cases this fails and repositioning or replacement of the graft is needed.

Complications during surgery

Damage during donor preparation – preparation of the donor occasionally results in damage to the tissue such that it is not safe to proceed with the surgery. The patient would be brought back on another day when replacement donor material was available. This is identified before starting the operation on the patient and does not lead to harm to the eye.

If the donor material has been perforated during preparation but is otherwise intact then the operation may proceed, but with a slightly higher risk of graft dislocation.

Damage to the donor during surgery  – difficulty unfolding or positioning the donor occasionally damage the tissue such that the graft fails, or only lasts a short time before needing to be replaced.

Damage to the iris and/or lens – it is sometimes necessary to make small apertures in the iris to manage the intraocular air-bubble during surgery. Visual effects are uncommon, but double-vision and glare may occur.  Lens damage may cause cataract, and require cataract surgery at a later date.

Intraocular bleeding – major bleeds are rare (approximately 1:500), but may result in significant loss of vision.

Early post-operative complications

Excessive inflammation after surgery – inflammation is normal and usually controlled by the use of steroid drops alone. Severe inflammation is uncommon and unpredictable, but controllable with immunosuppressive therapy by mouth. Although this type of treatment commonly has short term side effects the treatment is usually only necessary for 3 – 4 months and is usually successful.

Infection – infection is rare following DMEK, and is probably similar to that after cataract surgery (~1:1000). Infection within the eye is serious, and may result in significant loss of vision.

Late complications

Corneal graft rejection

Rejection occurs less often than with other forms of corneal graft, but may be seen in around 1% of cases. It is important not to stop post-operative steroid eye-drops unless instructed to do so (and to obtain a prescription from your GP before you run out).

Rejection requires prompt therapy and patients need to contact the Ophthalmic A&E Department at the Royal Berkshire Hospital, the Oxford eye Hospital or their local eye unit within 24 hours. Most rejection episodes can be treated with steroid drops alone. In a few cases oral steroids may be needed.

Glaucoma

Raised eye pressure may result from corneal graft surgery itself, or from the need to use steroid eye drops. In some cases glaucoma medicines, or surgery are required to control the eye pressure, and prevent damage to the eye.

Graft Failure

Some grafts may work beautifully for years and then fail, usually gradually. Sometimes late failure results from further eye surgery such as cataract surgery. If this occurs then the failed donor may be removed, and replaced with a fresh donor graft.

Alternatives to DMEK

If your vision remains reasonable, it may not be necessary to have any surgery at all. There are however currently no effective medical (non-surgical) treatments for endothelial failure. This may well change in the future, with experimental eyedrop treatment (Rho-kinase inhibitors) looking hopeful, but this is very much a treatment for the future, and is not available at present.

Descemet’s stripping endothelial keratoplasty (DSEK)

This operation is similar to DMEK, but uses a slightly thicker, and smaller diameter piece of donor tissue. It may be advised if for example it is necessary to combine the draft surgery with a cataract operation, or if the recipient cornea has a lot of opacity.

Penetrating keratoplasty (PK)

Full-thickness corneal transplantation may be a better option for a small proportion of eyes, particularly in complex eyes with multiple previous operations. PK is also sometimes necessary in the case of repeated failure of DMEK or DSEK, for example due to non-adhesion of a DMEK or DSEK graft.