I’ve noticed that my patients who develop cortical (spoke) cataracts seem to have astigmatic changes in their refractive error while those with nuclear cataract have an increase in minus sphere. Why is this?
Research has shown that people who develop cortical (spoke) cataract can have an increase in minus for their cylindrical refractive error. The theory is that an increase in the refractive index in the material in the spokes of the cataract causes an astigmatic myopic shift which means that the optimum refractive correction needs more minus cylinder.
People who develop nuclear cataracts often exhibit a spherical myopic shift and need more minus spherical refractive correction. This is probably caused by a symmetrical increase in refractive index in the central cataractous material.
There may also be a spherical hyperopic shift with cortical cataract the cause of which is not clear to me.
Posterior subcapsular cataract does not seem to affect refractive error.
In my practice, I make use of this information in the following way.
If I have seen the patient before and have noted previously the presence of a cortical cataract, I will look for an increase in minus cylinder and positive sphere. If the patient previously had some nuclear cataract, I will look for an increase in minus sphere.
If I haven’t seen the patient before and they have symptoms (cloudy vision, glare at night time) suggestive of cortical and/or nuclear cataract I will look at the lens with my direct ophthalmoscope before the refraction to see which type of cataract they have.
This way I can focus my efforts on those parts of the subjective refraction where there is most likely to be refractive change. It helps me use my time effectively and keep my subjective refraction focused and short. This is helpful in a group of older people who may tire quickly if asked lots of questions during the subjective refraction.
Knowing where to expect change and the nature of that change is useful.