EYE ADVICE: Professional Papers

(v) Photorefraction

There has been some debate up to now as to whether one should aim to correct for the refractive, or the corneal cylinder axis and power. Some workers in the field have decided to aim for an in-between correction, whilst most aim to correct for the subjective finding.

One needs to be cautious where there is significant discrepancy between the corneal and refractive astigmatism. In those infrequent cases where the subjective astigmatism shows an ATR cylinder which is significantly higher than that of the cornea, it would be advisable to first address the possible cause of this finding. The cause most likely relates to induced ATR astigmatism as covered in section(B) above. When the cause is attended to, a trial period of two to three months with a reduced ATR correction in spectacle lenses may be considered. This would be especially important when the subjective cylinder is greater than the corneal cylinder by more than 1D ATR. If one does not address the cause and uses refractive surgery to correct for the full amount of these higher discrepancies in cylinder power, an equally high amount of 1° PMA will be exerted to achieve clear vision. Over a longer period of time, this would lead to an initial asthenopia, followed by an ATR corneal toricity change and subsequent ATR ocular astigmatism once again.

In latent astgimats, one has two options to consider. Firstly, one can correct only the manifest astigmatism(if any) and allow the PMA to persist in compensating for the remaining corneal toricity. The other option is to have a trial period where an increased WTR correction is worn full-time in order to reduce the PMA and so manifest more of the latent astigmatism. The latter option is certainly recommended in cases of higher latent astigmatism (>2D) especially if there is an associated PAA. If the LA is not corrected, the patient may later manifest significant amounts of astigmatism if there is a change in the demand on PMA such as a new job with intensive VDU work. The case of Miss V.H. above demonstrates this to a certain extent. The case of Mr G.L. also described above is relevant as well.

Tabin et al (1996) have found significant laser-induced errors for low cylinder corrections in PARK. However, one must be aware of the fact that it is also possible to have compounding post-operative changes of astigmatism which are not directly attributable to the effect of the laser. The above discussion highlights two examples of this possibility. Prescribing the astigmatic laser correction pro-actively will help to minimise the long-term post-operative astigmatism.

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