When patients are diagnosed with astigmatism, they may struggle to spell the word let alone understand its definition. As clinicians, we are familiar with the definition of astigmatism as it relates to the shape of ocular structures, specifically the cornea and the crystalline lens.

Astigmatism produces two radii of curvature, or major meridians, referred to as the horizontal/vertical meridians or the steep/flat meridians. Astigmatism can be categorized as regular or irregular. Regular astigmatism is defined as a difference in major meridians 90º apart, whereas having meridians that are less than or greater than 90º apart is defined as irregular astigmatism. With increasing availability of ocular imaging technology such as corneal topography, corneal tomography, and scleral topography, it is important to recognize and understand the different types of astigmatism.

Which Kind Is It?

Regardless of whether a patient is being fit with contact lenses, it is important to understand the shape of the cornea. Corneal shape can be defined by keratometry values, which can be measured using a keratometer or auto-keratometer. However, these devices alone do not provide a complete picture, and they sample only a few millimeters centrally. Corneal topographers provide a greater, more comprehensive picture of the cornea.

One type of astigmatism is central astigmatism (Figure 1A), which means that the steep/flat meridians are localized to the central 3mm to 6mm of the cornea, and the peripheral cornea is relatively spherical (Caroline and André, 2009). With limbus-to-limbus astigmatism (Figure 1B), the steep/flat meridians extend to the limbus area.

Distinguishing between central or limbus-to-limbus astigmatism is important when fitting corneal GP lenses. Corneas with central astigmatism ≥ 2.50D may benefit from a back-surface toric base curve, whereas corneas with 2.50D of limbus-to-limbus astigmatism may benefit from both a toric base curve and a toric peripheral curve.

Importance of Scleral Shape

Practitioners should also consider the topographical shape of the sclera. Similar to the cornea, the sclera can have steep/flat meridians, which may influence the fit and comfort of scleral lenses. DeNaeyer et al (2017) determined that 40 (28.6%) of the 140 eyes measured were classified as having a primarily regular toric shape. Individuals who have a regular toric scleral shape (Figure 1C) may benefit from a toric peripheral curve, as such designs have been shown to provide improved comfort and stability (Visser et al, 2006).

As contact lens designs become more customized and lens manufacturing becomes more precise, it is important that practitioners have a complete understanding of a patient’s corneal and/or scleral topography. Utilizing instruments such as corneal or scleral topographers can provide clinicians with additional data, which is used to design contact lenses that are more comfortable and more stable. CLS

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