B. Reflex or 2° PMA and Induced
ATR Astigmatic changes
Throughout my practicing career, I have observed ATR lenticular
changes preceding ATR corneal changes in under or uncorrected presbyopes.
I have also observed similar ATR lenticular and corneal changes
in some younger people who actively accommodate. An example of this
latter finding is reported by Weisz (1978).
I initially considered these astigmatic changes to be due to incidental
spread of firing to the PMA subnucleus from the anatomically adjacent
PSA nucleus. After becoming familiar with the phenomenon of primary
accommodative astigmatism (PAA) as discussed above, it soon dawned
on me that reflex PMA is an integral component
of the normal visual system. It appears that increasing accommodation
proportionately decreases the leverage of tonic PMA in compensating
for WTR corneal astigmatism. Reflex PMA aims to supply the additional
PMA that is required to overcome the adverse effects of PSA and
thus prevent manifesting of astigmatism (PAA).
One may refer to reflex PMA as 2° PMA to distinguish
it from 1° PMA that is initiated by a purposeful response to meridional
blur. As 2° PMA is stimulated reflexly in response to PSA, it also
forms part of the ocular motor near reflex, serving
to counteract PAA. The amount of 2° PMA stimulated is proportional
to the amount of PSA innervation. Thus one can talk of a 2°
PMA/PSA ratio in the way that one talks of an AC/A ratio.
There is thus a tetrad, not a triad of motor functions involved in the ocular motor near reflex.
Those with low 2° PMA/PSA are prone to PAA, while those with normal 2° PMA/PSA are not, despite the magnitude of the L.A.
On the other hand, a relatively high 2° PMA/PSA ratio accounts for the induced ATR ocular astigmatism
that is sometimes noted in younger people (non-presbyopes) who actively
In order to prove that any observed ATR ocular astigmatism in non-presbyopes
is related to active accommodation and consequent induced ATR lenticular and
corneal changes due to a high 2° PMA/PSA ratio, one must be able
to demonstrate an increase in ATR ocular astigmatism with increasing
accommodation. A documented increase in ATR ocular astigmatism associated
with active accommodating in non-presbyopes strongly suggests a
relatively high PMA/PSA ratio. Pro-active prescribing
is then necessary for those who actively engage in much close work
to alleviate the associated eyestrain and permanent ATR ocular astigmatic
changes that can increasingly impair distance vision. The most effective
intervention is the use of spherical plus lenses in a form that
does not blur distance vision. This will reduce the effects of 2° PMA. Any ATR changes that are permanent will still remain after
several months of wearing these glasses, at which time an astigmatic
component may be incorporated in the lenses.
The ATR corneal toricity and ocular astigmatism changes in presbyopes
are widely known. Their cause has not previously been well understood.
ATR changes do not affect all presbyopes to the same extent. If
it were possible to have presbyopes continuously wearing their full
correction so that there are no periods of time where they are slightly
under-corrected (e.g. prior to changing their lenses), these presbyopes
would not normally undergo any ATR astigmatic changes.
These changes mainly occur in under-corrected or uncorrected
presbyopes. In these cases of insufficient lenticular accommodation,
the striving for clear vision would necessitate near-maximal activity
of the sub-nucleus responsible for PSA. This reflexly stimulates
the sub-nucleus for PMA producing 2° PMA and consequent ATR astigmatic
changes. Most insufficiently corrected presbyopes are prone to these
changes as even those who have normal 2° PMA/PSA ratios will have
relatively high amounts of reflex PMA innervation due to the high
demand on PSA.
Over longer periods of time, 2° PMA causes an apparent spasm
or over-action of PMA which shows up as ATR astigmatism on distance
refraction. In time, ATR corneal changes are induced through attachments
of the PMA muscle fibres to the temporal corneo-scleral junction.
This applies to both to the insufficiently corrected presbyopes
as well as the younger people with high 2° PMA/PSA ratios.