EYE ADVICE: Professional Papers

A. Astigmatic Changes Related to WTR Corneal Toricity

(i)     Latent Astigmatism

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. described below).

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 fatigue.

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 testing.

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   L) +1.75/-1.75*10

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   L) +2.75/-4.00*175.

Feb  1995 - Sub R) +1.00/-2.00*175   L) +2.00/-3.25*180.

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   L) +2.00/-2.50*180

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 machinist

September 1984 complained of heaviness of eyes, dizziness and episodes of blur, headaches and neurotic symptoms.

Sep 1984 - Ret  R) +0.50/-0.75*180   L) +0.50/-1.25*175.

Sep 1984 - Sub  R) -0.25/-0.25*180   L) -0.25/-0.50*165.

Sep 1984 - Rx given : +0.75 addition.

Jul   1991 - Ret  R) +0.75/-0.75*180   L) +0.75/-1.00*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 symptoms.

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 usual manner.

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.

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