Keratoconus is a common eye disease affecting the cornea, the transparent front window of the eye covering the coloured iris.
The cornea needs to be both transparent and appropriately curved to allow good vision. Although more complex, a normal cornea can be considered like the side of a sphere (or football) or an ovoid (rugby ball). In keratoconus there is a progressive “sagging” (ectasia) which compromises good vision curvature, and later opacification (loss of transparency).
The cause of keratoconus is not fully understood, but there is clearly a genetic component. It is often found in association with atopy (asthma, eczema and hayfever) as well as some other eye or whole body diseases. It frequently occurs in isolation. The age of onset is rather variable depending on how it is looked for. Screening using corneal mapping scanners (topographers) picks up keratoconus in patients with no symptoms and at many ages. However, in terms of developing symptoms, it is a disease that typically starts in patients’ teens and twenties.
The symptoms of keratoconus are essentially increased blurred vision and a changing spectacle prescription. An increasing myopia after the age of 23 or a large and increasing astigmatism at any age should really alert an optician (optometrist) of the possible diagnosis, and delay in referral is beginning to be recognised as unacceptable.
The diagnosis is based on corneal mapping, although in the later stages there are characteristic physical changes such as Vogt’s Striae on the posterior cornea and a Fleischer ring on the anterior cornea. In more advanced disease, apical scarring or an apical pip may be observed, as well as oedema from hydrops. The well-known Munsens sign (conical deformation of the lower lid on downgaze) only occurs in advanced disease. We would want to make the diagnosis early using corneal mapping so as to allow intervention to prevent progression.
In the early stages, optical management by stronger spectacles will improve vision, but as the disease progresses, “irregular astigmatism” results from the corneal distortion (“ectasia”) and this cannot be corrected with eye-glasses. At this stage, a contact lens will usually improve vision, but it will have to be a “rigid” or gas-permeable contact lens, which patient usually refer to as a “hard” lens. Although not particularly comfortable, the improved vision is very motivational!
However, neither spectacles nor a rigid contact lens will prevent keratoconus progressing i.e. getting worse. Until about 15 years ago, there was no proven technique to prevent progression. A famous French ophthalmologist Joseph Colin described the use of intra-corneal ring segments to halt progression, but these have mostly been superseded by “collagen cross linking” usually abbreviated to C3R. This is partly because C3R is simpler, but because it is almost a “medical” treatment, although it is usually done in an operating theatre for the cleaner environment. In addition, many of these young patients want to be asleep for the procedure.
Why have C3R? As keratoconus progresses, it may become impossible to tolerate or retain contact lenses, or the scarring will reduce vision. At this stage the only treatment will be a corneal transplant (or graft) operation. Whilst deep anterior lamellar keratoplasty (DALK) or penetrating keratoplasty (PK) are safe standard procedures, they are fairly major eye operations and the risks are much greater than for C3R. As a result, patients with documented progression are usually advised to have C3R before their disease gets too advanced for this treatment. This can occur because, as the disease progresses, the cornea becomes too thin for C3R treatment to be done safely. C3R involves gently removing the superficial cells of the cornea which allows riboflavin (a B vitamin) to be applied and penetrate the deeper cornea. When the tissue is saturated, an ultraviolet light is used to induce a photochemical reaction called “cross linking”. This makes the cornea structurally stronger and stops progression in the overwhelming majority of patients, thereby avoiding the need to do a corneal transplant with the associated risks. It is important to understand that the aim of C3R is NOT to make things better, but to stop them getting worse. Nevertheless, in about 1/3 of patients, some improvement is seen, and this is of course very nice for those patients.
If a corneal transplant is needed, the choice is between DALK and PK. Each has advantages and disadvantages. A DALK is a partial thickness procedure, and surgically more difficult to the extent that 5-10% of patients have to be converted to a PK during the operation as a DALK has become impossible. It’s disadvantage is that on average the best vision is not quite as good as after a PK, and may not reach the level required for driving. However it has the big advantage that rejection is extremely rare. In contrast, a PK is easier to do (for a trained and experienced corneal surgeon!), and is more likely to give good vision, but with a greater risk of rejection. Of course, most rejection can be treated successfully if the patient seeks medical care quickly, but clearly rejection is never good. Both PK and DALK feel much the same to a patient, both need stitches which need (usually) to come out, and both take 12-18 months to settle down at which time spectacles or a contact lens is virtually always required for good vision.
In summary, the consensus amongst corneal subspecialists is that patients with keratoconus should have regular corneal mapping to look for progression, and if it occurs should be offered C3R. If you have or think you might have keratoconus, you should arrange for a corneal mapping.