E. Refractive Techniques and
MA
(i) Balancing
Meridional Accommodation
MA requires balancing just as it is required
for spherical accommodation. For MA this is quite
time-consuming, as we have the added difficulty
of fluctuations in axis as well as power. As it
is not ordinarily possible to measure refractive
astigmatism under binocular viewing conditions, I
usually try to relax the focussing as much as
possible in both eyes to get the best
possible MA balance.
We normally wish to measure the astigmatic
error (power and axis) when the visual system (and
thus MA) is most at rest. As with spherical
errors, we should not
prescribe re-actively by correcting for any over-action
of accommodation (in this case PMA). This
will only serve to stimulate more 1° PMA and
create asthenopic symptoms.
Several aids can be employed to achieve a more
relaxed visual system thus facilitating
measurement of cylinder power, axis and MA
balance at both distance and near:
- Advising patients to avoid prolonged
near work close to examination time.
- Resting the accommodation by having
the patient close his/her eyes intermittently for
a few seconds before important measurements are
taken.
- Using a trial frame instead of
refractor head, would generally relax the patient
and thus reduce neuromuscular unrest.
Laurence and Marquez have suggested ways of
relaxing PMA. Laurence (1920): who is quoted by
Fletcher, advocated the fogging method of
refracting for astigmatism. This would minimise 1°
and to a lesser extent 2° PMA due to reduction in
meridional blur stimulus and reduction in PSA.
Marquez (1942) applied a trial lens to
neutralise the corneal cylinder. He then
performed an over-refraction with separate lenses
and calculated the resulting cylinder and total
sphere power. It appears that this method would prevent
PMA stimulation in WRT astigmats when the
trial WRT cylinder is removed.
Keratometry readings can serve as a check
for MA balance. Hofstetter and Baldwin (1957)
have found that the RA between the two eyes is
most often equal. As there are a few exceptions
to this rule, one cannot use the inter-ocular
corneal cylinder difference (IOC) as a definitive
measure of the inter-ocular difference in
refractive astigmatism (IOR). However, I believe
that in most cases, any difference between IOC
and IOR would be related to overreaction or
underaction of PMA.
Near retinoscopy could be used to check
cylinder power and MA balance for near
prescriptions.
(ii)
Binocular Test of Cylinder Power.
A binocular test of best cylinder power can be attempted after
testing for monocular cylinder power/axes and MA balance at both
distance and near. The final power used would depend on the individual's
needs. E.g. for prescriptions that are mainly for near work one
should aim for full WRT or minimum ATR cylinder correction to prevent
fatigue.
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