Early visual symptoms of keratoconus prompt a patient to complain of a reduction in visual quality, which is not completely corrected, even with the use of new glasses. This causes frequent optical visits and changes in prescription – a result of the increasing irregular astigmatic changes underlying the disease, that are causing higher order aberrations which glasses will not correct. Undiagnosed patients at this stage often present for laser surgery consultation, seeking to improve their vision in another manner, where topographical screening typically shows posterior or inferior steepening and irregular astigmatism. Optometric diagnosis can be made using a retinoscope, where the typical ‘reflex’ seen becomes split, swirling, or distorted, and very difficult to objectively gauge a prescription. Moderate keratoconus may be identified using a slit lamp bio-microscope, where Vogt’s striae and prominent corneal nerves can be observed in the central cornea. In more severe cases, corneal thinning and cone protrusion are observed, and an area of iron deposition may be seen around the base of the cone. Advanced cases display Munson’s sign – with the lower lid protruding as the patient looks downward, and in some cases the biomechanical endothelial stress causes an influx of fluid into the corneal stroma, causing swelling, pain and loss of transparently and visual quality.
In the early stages, vision is corrected using glasses or contact lenses. In advanced cases, treatment was typically a corneal graft, known as penetrating keratoplasty, where the central 7-8mm of the cornea is removed and replaced by a healthy donor cornea. Recurrence of the condition in the healthy cornea is very rare. More recently, a technique called lamellar keratoplasty has become popular, in which only the front layers of the cornea are removed and replaced by layers from a donor cornea, leaving the deeper layers of the cornea in place. Other techniques include intra-stromal rings. In this procedure, ring segments made from PMMA are inserted into the cornea through small incisions. By careful positioning of the ring segments, the steepness of the cone can be reduced. The technique is safe, though somewhat unpredictable, and has the advantage that it is reversible – the segments can be removed or repositioned if necessary. Cross-linking the cornea is a novel treatment that aims to mimic the natural ageing/strengthening process by artificially inducing molecular cross-links to stabilise the cornea. It can be used in combination with intra-stromal rings or alone.