EYE ADVICE: Professional Papers

(ii)    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.

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