EYE ADVICE: Professional Papers

F. Further Clinical Subject Areas and Meridional Accommodation

(i) Preventing Induced Oblique Astigmatism in Children

In a study of 299 children with astigmatism > 1D at age 1, Abrahamsson et al (1988) found that the few children with significant oblique astigmatism (i.e. 1.5-2.0 D at an axis >15 degrees from the vertical or horizontal) did not emmetropise, whereas most other children did so by age four. I believe that this is most likely related to the decreased effectiveness of MA at oblique angles.

This alerts us to an important point in prescribing for astigmatism in infants. It is important to ensure that any prescribed cylinder does not induce oblique astigmatism, as this will impede the emmetropisation process. If one prescribes a cylinder with its axis being more oblique than that of the corneal cylinder, 1° PMA will be stimulated to match the inducing cylinder axis. Over a long period of time and several changes of the prescription, significant oblique lenticular (reversible) and corneal (permanent) toricity changes could be induced.

Oblique astigmatism may similarly be induced or aggravated in older children if one prescribes only on the basis of the subjective cylinder. This may be quite oblique in axis compared with the corneal cylinder. One reason for this is that the 1° PMA exerted may not be exactly at right angles to the corneal cylinder axis. A more common reason would be that 2° PMA is being exerted.

Mast A.M. Born 1980

Jan 1989 - Retinoscopy   R) +0.50/-3.00*180    L) +1.00/-4.50*180.

Jan 1989 - Subjective Rx   R) +1.00/-3.50*3    L) +1.25/-4.50*177. (not made up)

Dec 1990   Ophthalmologist. Rx   R) +0.75/-3.75*15   L) +0.50/-4.00*170.(not made up)

Dec 1990 - Subjective Rx   R) +1.50/-3.75*10   L) +1.75/-4.50*174.

Oct 1993 - Subjective Rx   R) +1.50/-4.00*10   L) +1.75/-4.75*175.

Oct 1995 - Keratometry R) 39.20*7==44.25*97  L) 39.00*170==45.00*80.

In the case of A.M. above, I purposefully prescribed the most WRT axis which would be accepted in Dec. 1990. The cylinder axis did not change significantly up to three years afterwards, even though there was substantial change without correction within the preceding eleven months.

Other than for emmetropisation, there are two further reasons why it would be advantageous for the older child to avoid development of oblique cylinders through appropriate correction. Firstly, oblique cylinders cause more distortion, especially upon changing of the prescription. Secondly, they often create more difficulty for contact lens fitting, should this be desirable at a later date. Vision through oblique cylinders has shown to be more greatly affected than in the case of ATR or WTR cylinders. This is due to the nature of meridional accommodation i.e. the cylinder power is most easily changed when the axis is not oblique.

It is also of interest to note that contrary to the trend for increasing myopia with increasing school age, Master A.M. shows a slight reduction in myopia ever since his astigmatism was corrected. This observation supports the finding that astigmatism can be a factor in myopia.

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