Polystigmatism and Astigmatism

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What is the relationship between Ocular Polystigmatism and the Astigmatism that my optometrist measures?

-- Eric Howlett (poly@world.std.com), January 26, 2002


Simple astigmatism in the eye produces a smooth blur, but, unlike the radially symmetric spherical aberration blur of simple near or far- sightedness, it has a directional component. Vertical or horizontal or diagonal lines focus differently owing to the fact that the imaging surfaces of the cornea or lens are not perfectly spherical, but have an ellipsoidal or toric shape, which can be corrected with spectacles that have a compensating slightly cylindrical shape -- a curvature that is different for two perpendicular axes. (Camera lens asigmatism is quite different -- an unfortunate duplication of terms.)

Ocular Polystigmatism (a term coined in 2001 to describe an ocular problem that a camera lens would not have) has quite different causes: it results when an eye has more than one optical axis, which can result from fragmented refracting areas on the cornea or the lens, or from extra pupils in a faulty iris, or from clear regions in a lens that is clouded by cataract. And that may not exhaust the possibilities. When there are just two axes, the term "Monocular Diplopia" is used to describe a double image visible in one eye, as distinct from ordinary "Diplopia," which is the perception of double images resulting when the two eyes do not fixate properly on a single object.

Monocular Diplopia can be diminished for a given viewing distance with spectacles even though it is not the same as myopia or hyperopia or ordinary astigmatism. But Polystigmatism is different. In general the several images do not lie on a straight line, so oculists (ophthalmologists and optometrists) who try to treat it like the single-axis refractive errors, find that spectacles don't work. In fact there is no definitive remedy using spectacles.

-- Eric Howlett (poly@world.std.com), January 26, 2002.

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