A. Astigmatic Changes
Related to WTR Corneal Toricity
Latent astigmatism (LA) can be defined as the
part of the total potential ocular astigmatism
which is being compensated by the action of PMA.
For practical purposes the total potential
astigmatism can be considered as equal to the WTR
corneal toricity. Thus LA =
corneal toricity - subjective cylinder at the
ocular plane. LA of less than 1D
is normal, especially in non-presbyopes and is usually referred to as residual
astigmatism (RA) (see F iv).
Latent astigmats (of >1D) need to be
detected for several reasons. Firstly, LA is
associated with asthenopia,
especially in those who carry out prolonged,
constant visual work. (The case of T.Z. below is
relevant to blurred vision in LA
). This type of work can also lead to blurred vision in LA patients due
to a reduction in the amount of astigmatic
compensation for reasons of fatigue (e.g. Miss V.H.
Another reason for detecting LA is that there
are higher amounts of associated accommodative
astigmatism (see A (ii) below). A
further reason is the association of both
manifest astigmatism and LA with overaction or
spasm of parasympathetic spherical accommodation
(PSA). This may lead to increased masking
of hypermetropia. It may also contribute
to pseudo-myopia and myopia (see
case of T.Z. below). LA can easily be detected by
performing keratometry on all refractive patients.
The following case of Miss V.H.
illustrates the possibility for reduction in the
ability to compensate for WTR toricity leading to
increased manifest astigmatism
and thus blurred vision. A high
demand on meridional accommodation, such as when
an individual with LA uses computers for long
periods of time, usually manifests more
astigmatism due to fatigue of PMA.
Miss V.H. Born 1972 -
Now full time computer user, and has glare and
July 1986 - R) +0.50 L) +3.75/-1.00*180
(No K's recorded).
Nov 1996, glasses made elsewhere: R) +0.75
D.S. L) +3.75/0.75*180
K's: R) 42.25*170==44.25*80 (2D
cyl) L) 41.50*10==44.25*100 (2.75D).
Rx max subjective cylinder: R) +0.75/-0.75*172.5
L)+4.00/-1.25*3 after relaxation and repeated
The case of Miss V.H. also
demonstrates how knowledge of the keratometer
value for corneal toricity would encourage the
practitioner to see if a greater value of WTR
astigmatism can be found on subjective testing,
especially after some form of relaxation of the
eyes. Testing at the required working distance,
may also show a higher WTR cylinder (see A (ii)
below). The increased cylinder correction would
prevent fatigue arising from excessive use of PMA.
The case of Mr G.L.
illustrates the ability of latent astigmats to
adapt to higher amounts of WTR cylinder
correction. Adaptation to higher amounts of ATR
cylinder corrections from spectacles or Contact lenses in presbyopes is also possible. However,
these adaptations are usually accompanied by
changes to the refractive astigmatism and are
apparent when the corrective glasses are removed.
Mr G.L. Born 1971
Uncorrected vision: R) 6/6-
L) 6/12-3 (August 1985).
Old glasses: R) +1.50/-1.25*180
Aug 1985 - Ret: R) +1.25/-0.75*180 L) +1.50/-1.50*180
(No K's recorded).
Aug 1985 - Sub: R) +0.75/-0.75*167.5 (6/6)
L) +1.25/-1.50*2.5 (6/6).
Feb 1995 - Ret: R) +1.75/-2.50*175
Feb 1995 - Sub R) +1.00/-2.00*175
Feb 1995 - K's R) 40.75*173==44.87*83 (4.12D)
L) 40.25*176==46.00*86 (5.75D).
Mar 1995 - Re-made Rx R) +1.25/-1.50*174
Mr G.L. suffered from
headaches in 1985. He then accepted less
astigmatic correction for his glasses. In
February 1995, the retinoscopy result revealed a
lot more of his astigmatism and the keratometry
reading confirmed the LA.
The subjective result was prescribed in
February 1995, but soon after, Mr G.L. reported
"blur" upon removal of his glasses. He
was quite satisfied whilst wearing the glasses
for near work; however, it seemed that his
ability to assert purposeful PMA was weakened,
which reduced his unaided vision.
Mr G.L was not prepared to wear glasses full
time and thus the cylindrical power in his
glasses was reduced. He was accordingly advised
on visual hygiene, as prolonged close work would
lead to asthenopia, due to the
significant amount of LA.
An interesting finding in this patient, is
that in August 1985 he was probably exerting
close to 4D of PMA. The only symptom which was
possibly related was frontal headache. This
amount of compensating PMA is unusual. In my
experience, high WTR subjective cylinders are
usually associated with a reduced amount of
latent (residual) astigmatism (less than 0.50D).
Another point of interest is that despite being
able to exert a high amount of lenticular PMA,
which under normal circumstances would
automatically lead to steepening of the
horizontal corneal meridian, the transference of
force, from ciliary muscle to cornea, seems to be
lacking in this patient.
The following case demonstrates to some degree
the association of LA with overaction of
the spherical musculature.
Mrs T.Z. Born 1955 - Sewing
September 1984 complained of heaviness of eyes,
dizziness and episodes of blur, headaches and
Sep 1984 - Ret R) +0.50/-0.75*180
Sep 1984 - Sub R) -0.25/-0.25*180
Sep 1984 - Rx given : +0.75 addition.
Jul 1991 - Ret R) +0.75/-0.75*180
Jul 1991 - Rx given for sewing:
R) +1.00/-0.75*180 L) +1.00/-1.00*175.
Jul 1991 - K's R) 44.10*180==40.60*90
(2.5D) L) 43.75*180==47.00*90 (3.25D).
The asthenopia about which
this patient complained in 1984 was almost
certainly related to her LA. Had I been aware of
this condition prior to 1984, a higher cylinder
would have been prescribed to avoid fatigue.
Williams (1963) described his clinical
observations relating to LA and emphasizes that
even a small degree of corneal astigmatism of the
order of 1.25D can often be a cause of asthenopic
Mrs T.Z. shows a mild myopia
on subjective examination. Fulton et al (1981)
have shown a higher incidence of myopia in
children with astigmatism. One explanation for
this is that astigmats tend to take up a shorter
near viewing distance in order to see fine detail.
If this activity were prolonged and constant,
myopia would be expected to develop, especially
in children. To a certain degree, this would also
apply to latent astigmats as they would manifest
more astigmatism at near due to accommodative
astigmatism (see below) and also due to fatigue
of PMA with excessive close work. The greater
manifest astigmatism would necessitate a closer
working distance as described above. This would
explained why Takayama (1974) found less myopia
in children who were prescribed the full corneal
cylinder (thereby eliminating LA) compared with
children who were prescribed spectacles in the
Generally speaking, in cases of uncorrected
astigmatism (refractive or latent) one should
suspect overaction of spherical accommodation
which could cause either pseudomyopia or
increased latent hypermetropia. Accordingly, this
patient was prescribed plus lenses despite the
myopia shown by the subjective testing.