What are central islands?

Central islands are a major ablation defect where areas in the central cornea are not treated, or are not fully treated, by the laser.

What causes central islands and what kind of surgery are they associated with? (posted: jan 2005)

There is sometimes no clear answer for what causes central islands, which can occur - though infrequently - with any laser eye surgery. The early generation of broadbeam lasers are known to have been associated with central islands and central islands reportedly occur much less frequently on today's lasers. However, they still occur. Additionally, some cases have been attributed to uneven hydration of the stromal surface. If there is too much moisture on the stroma, it interferes with the ablation. This is most likely to be a problem with patients who had very high prescriptions, because the ablation process can be very long (a minute or more) and can force moisture out of the stroma onto the surface which then has to be wiped away to proceed safely with surgery. Finally, if the laser beam delivery is compromised in some way, it can result in uneven removal of tissue, including potentially leaving a central island.

What are the potential effects of central islands? (posted: jan 2005)

The vision impact of central islands is very serious. Central islands which have a significant elevation will typically cause multiple images and loss of best-corrected visual acuity.

How are central islands diagnosed? (Posted: jan 2005)

Central islands are diagnosed in corneal topography. They can also be detected with confocal microscopy of Artemis VHF digital ultrasound. Central islands are often not detected at all with wavefront aberrometry.

How can central islands be treated? (posted: jan 2005)

Central islands are corneal surface irregularities which cannot be corrected with glasses or soft contacts. They may, however, often be treated successfully with gas permeable contact lenses. These lenses may have to be specially designed for the patient by an optometrist with experience in fitting similar patients as the abnormal corneal shape (for example, in a myopic patient, a flatter than normal cornea but with an elevation in the middle) makes fitting a challenge. Larger lenses such as Macrolenses may be the best option as they can vault some of the cornea and perhaps have less risk of adhering to the elevation.

Unfortunately, central islands remain one of the most difficult problems to treat surgically with any reliability. Wavefront-guided procedures are not normally effective because often the wavefront aberrometer cannot even "see" a central island. Topography guided procedures may have some limited effectiveness, but probably less so than decentred ablations. Another option is PTK, which allows the surgeon to manually "hammer down" the island. This typically results in overcorrection, and so sometimes it will be combined with a conventional ablation to correct the induced refractive error.

Where can I get more information about central islands?

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