Laser Eye Surgery - Specific Circumstances

Certain issues specific to each patient need to be considered.  Here are some common examples.

Keratoconus and abnormal corneal topography

The most common condition identified through topography is keratoconus or its subtle precursor condition forme fruste keratoconus. This relatively uncommon condition involves increased corneal thinning and steepening. Proceeding with laser vision correction in this condition can result in further thinning and steepening of the central cornea, a condition called ectasia. It occurs in approximately one in 2000 cases of LASIK.

It is important to note that most cases of this condition occurred in an era when the criteria on corneal topography for diagnosing forme fruste keratoconus were less stringent. The Pentacam corneal topographer has the Belin Ambrosio Index, a very objective and accurate predictor or keratoconus and hence forme fruste keratoconus. This index uses a database of patients who have developed ectasia. Their corneal topography has been compared statistically with the many patients who have had an uneventful recovery and who did not develop ectasia. The Belin Ambrosio is a relatively objective predictor of a given patient developing ectasia. Most patients have normal corneal topography and their ectasia risk is low. Pentacam corneal topographers are used at all our clinics and are state-of-the-art technology.

Thin corneas

Corneal strength is, in part, due to its thickness. The laser ablation does result in some thinning; the higher the amount of short-sightedness treated, the greater the amount of cornea ablated and the greater the thinning. This thinning is still small in relative terms, typically less than 10 to 20 per cent of total corneal thickness. Although rare, this thinning of the cornea can result in ectasia.

Corneal thinning from the ablation is less significant when performed at a deeper level in the cornea. LASIK is performed under a flap of cornea, typically 110 to 120 micrometres thick. PRK (phototherapeutic keratectomy), on the other hand, is performed at a more superficial level under the epithelium, which is only 50┬Ám thick. The risk of ectasia, although very low, is slightly greater in relative terms after LASIK compared with PRK.

There are generally accepted safety margins regarding how much cornea should be ablated (or how much short-sightedness should be treated) for different corneal thicknesses. For thinner corneas, consideration should be given to PRK in preference to LASIK. For particularly thin corneas, PRK should be performed.

It is also worth noting that other factors are taken into account when considering whether PRK or LASIK is the best treatment for patients with thin corneas.

Increased risk of eye trauma

Some patients, by virtue of their occupation or recreational pursuits, have an increased risk of eye trauma. Following LASIK, there is a relatively rare incidence of the flap being dislodged or damaged as a consequence of a knock to the eye. Consideration should therefore be given to PRK in preference to LASIK in individuals who play contact sports frequently or are tradespeople.