MASTERCLASS ON KERATOCONUS

What is keratoconus?

KC is a localised abnormality of cornea shape. It consists of a "bulge" or elevation, as though someone had pushed the cornea outwards with a finger from behind. A "hill" or even "mountain" would be alternative analogies. The abnormal area generally has a round base and the cornea in this area is thin. The height of the abnormal area varies according to the severity of the KC. The location and diameter of the abnormal area vary greatly. The books say that infero-temporal is the commonest location although from my clinical perspective I would say that central/inferior/temporal are all equally likely locations.

The bulge distorts cornea shape and therefore cornea focusing properties and causes irregular astigmatism. Remember that both the anterior and posterior cornea surfaces have focusing power so that the distortion arises from both surfaces not just the anterior surface.

Aetiology

This is more a subject for the laboratory people than a clinician. At present there does not seem to be much hard information about the pathological mechanisms. There are certainly no useful, proven non-surgical approaches to treatment so the subject of the cause of KC is currently one of academic interest to ophthalmologists and optometrists, although one hopes this situation will change with time.

Usually KC is idiopathic. It is more common in patients with severe atopic keratoconjunctivitis and in those with chromosomal abnormalities such as Turners or Downs Syndromes. It only very rarely seems to be familial i.e. to run in families.

The cornea has an intricate collagen structure. Clearly this becomes abnormal in the area affected by KC. It may be that the abnormality lies in the extracellular matrix elements between the major collagen fibrils. This would weaken the cornea and allow intra ocular pressure to make it bulge forwards. The cause of this process is, as previously stated, unknown. A hot topic at the moment is the idea that it is keratocyte malfunction which stimulates this process. This concept arose when it was shown that treatments that damage the cornea epithelium eg. PRK cause keratocyte apoptosis (death) in the areas close to the treatment. The fact that keratocytes can be damaged by events on the cornea surface has therefore led to the suggestion that untoward cornea surface events such as atopy or frequent eye rubbing (as might happen in mentally sub normal patients with chromosomal disorders) leads to keratocyte death or malfunction and KC as a secondary event.

Iatrogenic keractasia i.e. doctor-induced KC following LASIK is obviously a completely different subject. In this case taking too much cornea away at the time of the ablation weakens it and leads to bulging.

Diagnosis

• Reduced aided acuity.
• Clinical signs are - conical shape on slit lamp examination, Vogt's lines (vertical striae), Fleisher ring (brown iron deposit surrounding the cone), Munson's sign with gross cones, scarring at the apex of the cone (a "proud nebula").
• Sometimes evidence of conditions associated with KC eg atopic keratoconjunctivitis, chromosomal conditions etc.
• Steep and irregular keratometry.
• Dull and scissored retinoscopy reflex.
• Corneal topography is the gold standard diagnostic method especially if the new "scanning slit" system made by Orbscan is used. This is outstandingly accurate particularly because it shows pachymetry (cornea thickness). In early cases there may be no other signs apart from a localised area of thinning.
• A gas permeable contact lens should restore normal acuity.

Differential Diagnosis

• the main trap is not to diagnose KC when there is a completely unrelated cause of vision loss eg cataract or retina problem. Remember that early KC may co-exist with any other ocular lesion.
• regular astigmatism especially if it is high and associated with steep keratometry values may cause confusion.
• other causes of cornea distortion eg trachoma, previous extracapsular cataract surgery

Other cornea thinning syndromes i.e. pellucid marginal degeneration and keratoglobus. These are rare.

Prognosis

The extent to which KC will worsen is somewhat unpredictable. There are no useful interventions so there is no point in any lifestyle modification.

The general rule is that the earlier the presentation (i.e. teens is much worse than 40's) the worst the prognosis. Similarly the worse the KC is at presentation, the more likely it is to progress further. Therefore a man in his 40's who presents with mild unilateral KC is unlikely ever to require surgery.

Apical scarring (a "proud nebula") worsens symptoms by causing glare and further reduction in acuity.

Hydrops (temporary split in Descemet's membrane) causes temporary pain and reduced acuity. If followed by scarring, acuity may be permanently reduced. Management is topical steroid but there is no evidence that this improves the long-term prognosis after hydrops.

There is little interest within the ophthalmological community in the notion that contact lenses may either improve or exacerbate KC. It does seem that pressure on the apex of the cone from the RGP contact lens might be a factor in formation of proud nebulae however.

Treatment

There have been some major improvements in KC management recently of which you should be aware.

Contact Lenses

These are of course the mainstay of management. Most patients never come to surgery. The fit and material of RGP's are improving all the time and scleral contact lenses have been a major step forward in managing advanced cases. Contact lenses should virtually always be attempted before surgery is contemplated.

Proud Nebulae

These can cause considerable photophobia and some reduction in acuity. They are usually easily removed manually by peeling them off. This is easy and in my opinion generally much better than doing a PTK. They do sometimes return however in which case the surgery can usually be repeated.

INTACS

These may well become a major factor in KC surgical management. They were originally designed for the treatment of low myopia. They consist of two plastic inserts which are placed into the cornea periphery. They reduce myopia by flattening the cornea. The same mechanism of action seems to occur in KC where the bulge is less after implantation. Only early data are available with short-term follow-up. The risks of surgery seem negligible (certainly very much less than cornea transplantation). About 80% of patients seem to benefit. There are not yet clear guidelines as to who these patients will be. Presumably those with less severe bulging and minimal scarring will benefit most. A minority of patients regain good unaided vision or are able to wear spectacles instead of contact lenses. A substantial proportion find contact lens wear much improved following surgery and so are able to avoid cornea transplantation. This approach is generating a good deal of interest amongst the ophthalmological community and you can expect to hear a good deal more about it in future.

Cornea Transplantation

Traditionally this has meant penetrating keratoplasty. Detailed discussion is outside the scope of this presentation but you are all aware of the disadvantages, namely the risk of transplant rejection and severe post-operative astigmatism. In this surgery the patient's own cornea is removed entirely and replaced with a cadaver donor.

Recently a major step forward in cornea transplantation for KC has occurred. This is the advent of deep lamellar keratoplasty. In this technique the patient's own cornea is removed except for the endothelium which is about .05mm thick. This is left behind. The donor cornea has its own endothelium removed before it is sutured in place. The major advantage of this technique is that the patient's own endothelium, which is of course not diseased, is preserved. This makes the risk of transplant rejection very much less - it is usually the endothelium which stimulates rejection not the stroma. Also fewer sutures are required to secure the donor and it is often the case that there is less post-operative astigmatism as a result. The surgery is naturally more difficult than a standard penetrating keratoplasty.

Occasionally an epikeratophakia procedure is appropriate in KC. This involves the placing of a new, donor cornea on top of the old, KC cornea without removing any of the KC tissue. This used to be a fashionable technique but the visual results are less than for the other methods because the cornea, obviously, is twice its normal thickness. It is appropriate for patients in whom there is a risk that the graft might be damaged by eye rubbing after the surgery or in whom the entire cornea is thin eg keratoglobus.