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.