For many years, keratometry has been the measure of the magnitude of astigmatism, but we know that these devices record only the central 3mm of the cornea. This was acceptable in the days of polymethyl methacrylate (PMMA) lenses, which had diameters that typically ranged from 6.8mm to 8.8mm. Corneal GP lenses are typically larger (9.2mm to 11.8mm); therefore, the need to measure a wider diameter becomes apparent.
When the toricity is truly corneal, as opposed to internal, many patients will do well with corneal GPs. That is my preferred option, assuming that a patient’s lifestyle and adaptation tolerance permits. In cases in which the astigmatic changes are limited to the central cornea and there is a more regular midperiphery, a toric soft lens may provide adequate acuity along with comfort.
Common Instrument Use
Topographers In recent years, topography has become common in many offices that are actively involved in contact lens evaluations. Topographers that utilize a Placido disk system can accurately measure corneal curvature out to 9mm to 12mm. Thus, topographers are superior to the old keratometers in assessing the breadth of astigmatic changes to a larger corneal surface.
When astigmatism extends beyond the central 5mm (Figure 1), patients are likely to need a more sophisticated contact lens. If you decide to use a GP lens, a bitoric design with two unique base curves is an excellent option. In the event that a soft toric lens is the better choice, a custom soft toric allows you to select both the overall diameter (OAD) and the base curve.
Another advantage of topographers is the ability to accurately measure horizontal visible iris diameter (HVID). HVID gives a reasonable “guesstimate” of overall corneal elevation. Elevation is helpful in determining the optimal sagittal depth of a soft lens, especially in the larger lens diameters typical of custom soft toric lenses (e.g., 15mm) (van der Worp and Mertz, 2015).
Sagittal depth in a soft lens is a function of the relationship between the base curve and the OAD. This is not a new idea. One previous lens came in four different “vaults” (I, II, III, IV) rather than in the more common parameter of base curve in millimeters.
Tomographers Tomography is a procedure in which near-infrared radiation is used to create high-definition images. Anterior segment optical coherence tomography (AS-OCT) typically utilizes a Scheimpflug camera system to penetrate tissue to render a two-dimensional image from solid three-dimensional tissue. The advantage for practitioners is the ability to view and precisely measure the posterior corneal surface. The Scheimpflug strategy is able to differentiate between normal eyes and those that have signs of early keratoconus. Studies have shown that posterior elevation is the major criterion for subclinical detection (Miháltz et al, 2009; Fernández Peréz et al, 2014).
Even without access to a tomographer, clinicians should be suspicious if traditional topography shows that astigmatism is increasing, the axes of astigmatism are skewed (not at 90º), inferior-superior (I-S value) corneal asymmetry is detected, and the patient is between 12 and 40 years of age (Fernández Peréz et al, 2014; Rahman et al, 2020). CLS
The author wishes to acknowledge Dr. Joe Janes for his contributions in the preparation of this article.
For references, please visit www.clspectrum.com/references and click on document #309.