Article

PRESCRIBING FOR ASTIGMATISM

SPECTACLE RX CONSIDERATIONS FOR IRREGULAR ASTIGMATISM

Spectacle blur describes a visual phenomenon that patients experience during spectacle wear after removing their corneal contact lenses. In the 1940s, spectacle blur secondary to corneal edema became a common concern as polymethylmethacrylate (PMMA) rigid contact lenses gained popularity. PMMA lenses are impermeable to oxygen and thus cause corneal edema and warpage during lens wear (Efron, 2012). This change in corneal shape causes a temporary myopic shift, resulting in blurry vision with spectacles until the swelling has resolved. However, since the advent of GP lenses in the late 1970s, spectacle blur due to corneal edema is rare.

Spectacle blur can also result from mechanical distortion of the cornea secondary to a poorly fitted contact lens. The lens distorts the cornea and results in blurry vision after lens removal. Once the corneal GP lens is refit to better contour the cornea, spectacle blur should no longer be an issue. However, because scleral GP lenses vault over the cornea, there is minimal concern for spectacle blur due to mechanical corneal warpage.

A different form of spectacle blur can also present in patients who have irregular corneas. Corneal ectasia resulting from keratoconus, pellucid marginal degeneration, laser-assisted in situ keratomileusis (LASIK), and/or radial keratotomy cause corneal irregularities that result in decreased vision and visual fluctuations throughout the day. Given these fluctuations, there are several factors to consider when prescribing spectacles for patients who have irregular astigmatism.

Severity of Irregular Astigmatism

The level of irregular astigmatism varies according to the extent of corneal ectasia. Patients who have severe irregular astigmatism tend to have worse visual acuities with spectacle wear. These patients rely heavily on rigid GP lenses to achieve optimal vision.

Depending on mechanical stability of the cornea, patients may experience visual fluctuations. Patients who have undergone radial keratotomy often experience diurnal variations in vision and refractive error. On the contrary, you might hypothesize that keratoconus patients who have undergone corneal cross-linking have less malleable corneas, and thus more repeatable refraction results. Therefore, regardless of etiology, it is critical to perform multiple refractions on separate days and times to confirm repeatability prior to finalizing their prescription.

Time of Spectacle Wear

Next, consider the time of day that your patients plan to wear spectacles. Will they primarily wear them in the morning or evening? For how much time after lens removal do they plan to wear spectacles?

The goal is to match the prescription to their needs at the time of wear. For example, if a patient prefers to wear spectacles in the morning, refract the patient in the morning prior to lens wear. If the preference is toward the end of the day, refract the patient in the afternoon approximately 30 minutes after lens removal.

Final Considerations

After multiple refractions, assess and prescribe based on the considerations above. If the patient is young, err on the side of providing more minus; if the patient is older, prescribe based on an average of the findings.

Patient education is the most important consideration. Setting realistic expectations about the patients’ corneal irregularity and fluctuation in vision with spectacle wear is crucial. It is critical to emphasize that, despite reduced optical clarity, it is important to have spectacles in case of lost lenses, eye infection, or emergency. CLS

For references, please visit www.clspectrum.com/references and click on document #265.