What is a small optical zone? (Posted: jan 2005)

In a laser eye surgery patient, a small optical zone is when the fully corrected portion of cornea is too small in diameter for the patient's optical needs based upon the size of their pupil in good and/or poor lighting. This may occur because the programmed optical zone was too small, or because the programmed optical zone, though correct, was not achieved.

Why are some patients given a smaller programmed optical zone than their dark-adapted pupil size? (Posted: Jan 2005)

The single most frequently occurring reason, based upon our experience with reviewing patients' cases, is mismeasurement of the dark-adapted pupil size. This means, for example, that the patient was measured as having a 6mm dark pupil. They underwent surgery and were given a 6mm treatment (programmed optical zone). Subsequently, they found that their pupils were in fact 7.5mm in dim light. If that had been apparent before surgery, they could have (if corneal thickness permitted) had a larger programmed optical zone, or could have been informed of the limitations and risks based upon their pupil size.

Why are some patients' effective optical zones smaller than the programmed optical zone? (posted: jan 2005)

There are several different possible explanations for this and we will address just a few here:

1) In some lasers, and particularly (but not exclusively) the older ones, the programmed optical zone "on paper" may or may not be what the patient is actually given, depending on their prescription. Specifically, the higher their prescription, the smaller the delivered optical zone becomes in relation to the theoretical programming - for example, a 6mm treatment might become a 5.5mm treatment for someone with a prescription of X, and a 5.0mm treatment for someone with a prescription of X+3D. This is thought to be due, in part, to internal transition zones.

2) A laser may simply fail to deliver the programmed optical zone, for unknown reasons. There is a documented case study about this (see systematic underablation for details).

3) There is an effect known as shrinkage which can, and to a certain extent may be expected, to occur after surgery - a sort of smoothing effect which may shrink the optical zone by 1/2 or even 1 dioptre.

What does it mean for the patient? (posted: jan 2005)

In case of the programmed optical zone being too small, the classic problems are night vision disturbance, reduced contrast sensitivity and ghosting. This is because in dim light or dark when the pupil enlarges, it becomes larger than the treated area of the cornea and light is passing through both treated and untreated cornea, causing scattering which prevents proper focus of light on the retina.

In case of something like systematic underablation, symptoms may be present even in good lighting, because the effective optical zone may be just a few millimeters in diameter.

How is it diagnosed? (posted: jan 2005)

Corneal topography. However, very few have the skills to diagnose this, as topography is underutilised and poorely understood in refractive surgery.

What are the treatment options? (posted: jan 2005)

Like all serious ablation defects, small optical zones have no easy, quick, reliable fix. The best option at present remains gas permeable contact lenses. Progress is being made with therapeutic wavefront guided surgeries, but there are still several hurdles. For example, increasing an optical zone of a patient who has a relatively large (6mm or greater) effective zone seems to be easier and less risky than increasing an optical zone of a patient whose paracentral corneal is underablated. Additionally, most lasers have problems controlling the refractive error when attempting this kind of correction.

Where can I learn more about small optical zones? (posted: jan 2005)

Click here for Encyclopedia entry on effective optical zones, which may have additional articles and links.