Primary Accommodative Astigmatism (PAA)
From my clinical observations, and those of
others (eg. Beau-Seigneur 1946, Ukai and
Ichihashi 1991, Kabe 1968) there appears to be a
tendency in some individuals for increasing
WTR astigmatism with increasing
accommodation. I have found that this occurs in
some individuals who have normal accommodation (thus
excluding presbyopes) and appreciable amounts of latent
astigmatism. I have termed this primary
accommodative astigmatism, to
distinguish it from less common types of
accommodative astigmatism which may be of
pathological aetiology and are atypical in their
axis. These are described in some detail by Beau-Seigneur.
Larger amounts of PAA are associated with a
higher degree of LA. However, not all cases of LA
are associated with PAA. Some cases of higher
degree LA (>2D) may not show evidence of PAA.
In contrast, some accommodators with mild LA (1D
to 1.5D) may show evidence of PAA, the value of
which is a proportion of that of LA.
One would logically ask the question as to why
some latent astigmats maintain their ability to
compensate for their WTR corneal astigmatism with
increasing accommodation (PSA) and some manifest
part of their LA as PAA. My hypothesis
is that PSA causes changes within the eye that
proportionately increase the demand on PMA. To
overcome this problem that is imposed by PSA, the
visual system is designed to reflexly activate
PMA in response to PSA. Where reflex PMA
(see section B below) is lacking, PAA is expected
to result. On the other hand, if the visual
system is innately wired such that there is an
excess of PMA per unit of PSA, this leads to induced
against-the-rule (ATR) astigmatic changes
of the lens and cornea. These are described in
section B below.
Brzezinski distinguishes true accommodative
astigmatism from induced spectacle "accommodative"
astigmatism which is related only to the
influence of spectacle lens effectivity on
astigmatism at near. The latter phenomenon is
important only in the higher cylindrical and
spherical powers and usually changes the required
correcting cylinder at near by less than 10% of
the distance cylinder finding.
An example of 1° accommodative astigmatism:
Miss V.M. age 17
K's: R) 41.87*170==44.00*80 (2.13D)
L) 41.50*10==43.25*100 (1.75D).
Subjective refraction yielded: R)
pl/-0.50*160 L) -0.25/-0.25*15.
Amplitude of accommodation 10D (R&L). The
power of the subjective cylinder found at near
varied according to the viewing distance. At 30cm,
the cylinder finding was R) -1.25 L)-0.75.
At 15cm it became R) -1.75 L) -1.25.
Generally speaking if a correction is to be given for close work
only, then the subjective cylinder and axis should be checked again
at the required working distance. Borish (p.779, 1970), describes
several techniques for near refraction. The near cylinder finding
may be given almost in full if a half-eye frame can be accepted
by the patient. If the glasses are to be used for distance viewing
as well, a reduced correction that can be tolerated for distance
viewing would be necessary.