Corneal Disease: Keratoconus, Fuchs' Dystrophy and Keratitis
This is a very broad heading. Common corneal diseases include Keratoconus, Fuchs' dystrophy, and scarring from keratitis.
What is Keratoconus?
Keratoconus is a common corneal disease which has a complex but partly genetic cause and which in some populations can affect 1 in 200 people. In this condition, the cornea which is the transparent membrane over the coloured iris of the eye (and which should be shaped like a dome, i.e. equally curved in all directions) sags to become like a cone with a "point" or apex. This is regarded as a progressive disease i.e. it gets worse with time. Initially this causes astigmatism which can be corrected by spectacles, but later patients need rigid (hard or gas-permeable) contact lenses to be able to see. Finally, many patients need surgery in the form of a corneal graft or transplant.
Fortunately, there is now technology available called "Collagen Cross Linking" or C3R which allows us to "freeze" the cornea before the disease has advanced to a stage where surgery might be necessary. This is a wonderful advance, and is indicated when there is progression i.e. the disease is getting worse.. C3R is a treatment and not an operation (although it is usually done in an operating theatre for the clean environment), and can be done either with the patient awake or asleep. As a rule, it does not need to be repeated, and although the purpose is to prevent progression, some patients do see an improvement in the months after treatment. Some patients may be able to have laser vision correction after collagen cross-linking, although there is still controversy about this.
If cross-linking is not possible because the disease is too advanced, then a corneal graft operation (DALK or PK) is required to restore vision. These two operations are similar and indeed usually feel the same to the patient, but have individual strengths and weaknesses.
A DALK (Deep Anterior Lamellar Keratoplasty) is a partial thickness corneal transplant. The innermost layer (the endothelium) of the patient's endothelium is left, and as it belongs to the patient it cannot be rejected. Since this layer keeps the cornea transparent, this is an obvious advantage. However, not all patients get driving vision after a DALK, and in general the final vision is not quite as good as for a PK. It should be noted that this is technically extremely demanding surgery, and in approximately 10% of cases it is not possible to achieve a DALK in which case the operation is converted to a PK.
In contrast, a PK (Penetrating Keratoplasty) is a full-thickness corneal transplant. This means that there is a risk of rejection involving the endothelium, with loss of corneal clarity. Fortunately, with rapid intervention, the prognosis is relatively good, which is to say that a rejection does not always proceed to graft failure. In general, if a patient presents within 3 days of developing symptoms, the prognosis is good. What are the symptoms? They can be remembered as "RSVP" which in this context are abbreviations for Redness, Soreness, Vision (worse) and Pain. However, the overall results for PK are very good, and a patient is more likely to achieve driving vision with a PK than with a DALK.
It should be noted that patients who have a PK or DALK virtually always need spectacles or a contact lens for the best vision, although some may be suitable for refractive surgery (LASIK, Refractive Lens Exchange or a Supplemental IOL) when everything has settled post-operatively.
What is Fuchs' Dystrophy?
Another common corneal disease is Fuchs' endothelial dystrophy, named after Dr. Fuchs, an Austrian ophthalmologist. In this common condition, the cells on the inner surface of the cornea fail, and the cornea becomes cloudy with loss of vision. This is because these cells, called "the endothelium" keep the cornea transparent by pumping water out so that it is partially dehydrated. As they fail, the cornea becomes water-logged, and this produces symptoms of blurry vision on wakening. Why on wakening? This is because some water evaporates from our corneas and this contributes to the cornea being clear. When we sleep and our eyes are closed, this cannot happen, and with a compromised (diseased) endothelium, this can tip the balance and cause corneal hydration and loss of clarity. Typically, the period of morning blurring becomes progressively longer until it lasts all day.
Traditionally treated with a penetrating keratoplasty operation (PK – discussed above), this operation, whilst highly successful has a number of limitations. Firstly it typically takes 12-18 months to settle down before vision can be obtained, and then often only with difficult spectacles. Secondly, sutures are required which can loosen causing inflammation and rejection or predisposing to infection. Since a full thickness incision is required, any serious trauma to the eye tends to cause the wound to burst with fairly catastrophic consequences. Fortunately, we now have operations available called DSEK (Descemet's Stripping Endothelial Keratoplasty) and DMEK (Descemet's Membrane Endothelial Keratoplasty) where only the inner layer of endothelial cells is replaced, although with DSEK there is some stromal "carrier tissue". Not only do patients typically get their vision in a few weeks with normal or no glasses, but the eye is not structurally weakened and rejection appears to be very much rarer. DMEK is now my preferred operation for Fuchs' dystrophy. This is usually done with a full anaesthetic but can be done as a day-case.
What is Keratitis / Corneal Scarring?
Keratitis is caused by infections caused by bacteria or viruses. Keratitis can be related to contact lens wear. Keratitis often results in corneal scarring with loss of vision.
This is usually surgical, although occasionally spectacles or contact lenses can give some improvement. If the scar is very superficial, it may be possible to laser it off with an excimer laser photo-therapeutic keratectomy (PTK), but most are too deep, and a partial thickness (DALK or deep anterior lamellar keratoplasty) or full thickness (PK or penetrating keratoplasty) corneal graft operation is necessary to restore vision. This surgery is usually done under a general anaesthetic although it is normal to be discharged the same day if the patient wishes. It is not usually painful, but does take several months to settle down, and to get the best vision spectacles or contact lenses are almost always required. Since this is a form of transplantation surgery, albeit very successful, it is terribly important to follow instructions about post-operative medication and to attend all planned follow-up visits.