On modeling the causes of presbyopia

On modeling the causes of presbyopia Weale (2000) presents a theoretical analysis to support his contention for a multifactorial explanation for presbyopia. He suggests that the model supports the notion that the accelerated loss of accommodation relative to other physiological indicators of age is due to the combined effect of normal age related changes in the component elements of the accommodative apparatus. Weale's model considers age changes in the ratio of Young's modulus of elasticity of the lens and capsule, capsular thickness, the ratio of the lens major and minor axes and the anterior lens radius of curvature. These parameters are used to compute lens/capsular stress and thereby changes in lens anterior surface curvature and accommodation. Weale (2000) suggests his model predicts the progression of presbyopia shown by Brü ckner, Batschelet, and Hugenschmidt (1987) and argues that it demonstrates that lens hardening (or any other single factor) cannot be considered as a predominant cause of presbyopia. Glasser and Campbell (1998) contend that an analytical approach is only as good as the assumptions used and that the true utility of an analytical approach is demonstrated when it is shown to match empirical measurements. Weale (2000) has elected an analytical approach that necessarily requires assumptions and data that come from the work of others. Very different conclusions about the utility of this model, than those drawn by Weale (2000), can be made with a closer examination of the data employed. Weale (2000) mentions equatorial growth of the un-accommodated lens as a factor in presbyopia. Interestingly , this is also considered as a predominant factor in a controversial theory of presbyopia which serves as the basis for which scleral expansion surgery is offered as a ironically all cite the same study in which the diameter of isolated human crystalline lenses were measured after excising them from enucleated eyes (Smith, 1883). Weale (1982) also graphs data from a less accessible source (Johansen, 1947: cf Weale, 1982) together with the data from Smith (1883) to show age changes in lens diameter. Since the data from Johansen (1947) is plotted by Weale (1982) together with the data from Smith (1883) and shows the same relationship, it is assumed that lens diameters were measured in the same way in the two studies, i.e. on isolated lenses. This is a reasonable assumption since, until recently, this would have been the only way to measure lens diameter, i.e. in …