Astigmatic Refractive Errors
Astigmatic refractive errors of the human eye are not uncommon. Corneal astigmatism is most common, but lenticular astigmatism is also found. In each case, we can consider the net refractive surface as equivalent to a spherical component with a concurrent plus cylinder astigmatic component (corresponding to the forward bulging of the cornea into the surrounding air). For example, the most common pattern of corneal astigmatism seen in younger patients, known as “with-the-rule” astigmatism, suggests a compression of the cornea by the pressure of the eyelids. This compression deforms the cornea, steepening the vertical meridian and flattening the horizontal meridian. This acts as a plus cylinder lens with axis horizontal (Figure I-19).
The net astigmatic error is corrected by placing a cylinder of equal power and opposite sign over the eye, with the same axis as the intrinsic cylindrical error. For example, with-the-rule astigmatism can be corrected with a minus cylinder correcting lens placed with axis horizontal (Figure I-20).
Equivalently, such a refractive error can be corrected by placing a plus cylinder lens with axis vertical, and then compensating with a corresponding reduction in the power of the spherical component.
Figure I-19 Schematic drawing of an eye that has “with-the-rule” astigmatism, with horizontal flattest corneal meridian.
(Illustration developed by Scott E. Brodie, MD, PhD; original illustration by Mark Miller.)
Figure I-20 Schematic illustration of correction of with the rule astigmatism with a minus cylinder lens placed with axis horizontal, to compensate for excess forward bulging of cornea with steepest axis vertical.
(Illustration developed by Scott E. Brodie, MD.)
Thus, correction of an eye with an astigmatic refractive error requires the determination of 3 quantities: the power and axis of the correcting cylindrical component, and the power of the remaining spherical component. In practice, this task is accomplished as follows:
Attempt to correct the eye with spherical lenses only, or at least to determine the optimal purely spherical correction.
Determine whether there is an astigmatic component to the refractive error.
If astigmatism is present, determine the correcting cylinder axis. This can be done even when the optimal cylindrical correcting power is not yet known.
Determine the optimal cylindrical correcting power.
Determine the optimal power to correct any residual spherical error.
Step-by-step instructions for this process are provided in Part 2 of this Quick-Start Guide.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.