contact lens case reports

Not All Astigmatism is Created Equal

BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO

For more than 200 years we have used the term astigmatism to describe a wide range of refractive and/or topographic anomalies. Today, modern corneal mapping techniques have allowed us to better understand the subtleties to this condition especially as it relates to fitting GP contact lenses.

Types of Astigmatism

John Mountford, Dip. App. Sc., FAAO, FVCO, FCLS, has described two forms of corneal astigmatism: incomplete/central astigmatism and limbus-to-limbus astigmatism.

Incomplete/central astigmatism is a toricity isolated to the central 5mm to 6mm of the cornea and surrounded by nearly 360 degrees of spherical cornea in the midperiphery (Figure 1A).

Figure 1. Incomplete/central (left) versus limbus-to-limbus astigmatism (right).

When any GP contact lens is placed on this eye (spherical, aspherical, or orthokeratology), the lens will "land" on the cornea with nearly 360 degrees of midperipheral bearing at 3 o'clock and 9 o'clock and, most importantly, also at 12 o'clock and 6 o'clock. The resulting fitting relationship obstructs lens movement along the vertical meridian and can result in subjective fatigue or discomfort especially toward the end of the day.

Figure 2. Fitting relationship of spherical GP lenses on the two forms of astigmatism.

Because of the fluid dynamics required to make ortho-k "work," this semi-closed, fluid-filled system is ideal for ortho-k. It optimizes the pressure forces beneath the lens for maximum corneal reshaping effects (Figure 3A).

Figure 3. Fitting relationship of corneal reshaping GP lenses on the two forms of astigmatism.

Limbus-to-limbus astigmatism extends throughout a significant area of the cornea (Figure 1B). Any GP contact lens will "land" first at 3 o'clock and 9 o'clock, which "locks" the lens along the horizontal meridian. Along the vertical meridian the lens will exhibit varying degrees of clearance (depending on the amount of midperipheral corneal toricity). A low amount (i.e., 0.50D to 1.50D) of limbus-to-limbus toricity is ideal for a spherical GP contact lens because it permits unobstructed movement of the lens along the vertical meridian in the case of with-the-rule astigmatism (Figure 2B).

For ortho-k, a limbus-to-limbus astigmatism is less desirable because it permits the escape of the fluid forces that make ortho-k "work." In this situation, an ortho-k lens with a toric periphery may be the appropriate choice (Figure 3B). CLS

Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.