prescribing for astigmatism
Toric Soft Lenses and the Eyelids
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO
Rotational stability is generally the key to good vision through toric soft lenses. Excellent stability, both in terms of minimal blink-induced rotation and consistent rotational position, allows you to optimize the patient's vision by lining up the lens' correcting cylinder with the patient's refractive astigmatism axis. The amount of lens "misalignment" from the 6 o'clock base down position or from other lens rotational markings (i.e. rotation) is not critical to achieving good vision, but stability is.
You can compensate for a lens that consistently locates away from the 6 o'clock position by changing the prescribed lens axis relative to the patient's refractive axis by the amount in degrees of "misalignment" using LARS (Left Add, Right Subtract) or resultant cross-cylinder calculations.
In this column, rotation will refer to where the base of a prism-ballasted toric soft lens typically locates or orients relative to the 6 o'clock position on the cornea. Rotational stability will refer to lens movement away from its typical orientation during a blink, during change in gaze direction or when reapplied to the cornea. You might not be able to generalize the study results presented to dual thin-zone toric soft designs.
Several researchers have investigated the effect of lid aperture size and eyelid angles on lens rotation. Young, Hunt and Covey (2002) using a prism-ballast toric design found better lens rotational stability for patients who have smaller lid apertures.
It's been widely postulated that the prism base of a toric soft lens should generally rotate to an inferior-nasal position because of the commonly held belief that the lower eyelid moves in a nasal direction during a blink. This should result in the lens rotating in the same direction. However, research has not substantiated that toric soft lenses tend to rotate in an inferior-nasal direction.
Young et al (2002) reported that the outer canthus was higher in 80 percent of their subjects' eyes. Not surprisingly, they also found the temporal upper eyelid higher than the nasal upper lid. These findings were associated with the lens prism base generally locating in an inferior-temporal position. However, keep in mind that the amount of rotation from the base down position was less than 10 degrees for the majority of subjects. They did not find the same relationship for the lower eyelid angle. This implies that the upper eyelid usually has more effect on lens orientation than the lower eyelid. This is consistent with the watermelon seed principal published by Hanks (1983).
What's a Practitioner to Do?
The ideal clinical situation is to be able to apply diagnostic toric soft contact lenses and after lens equilibration observe rotation and rotational stability during and between blinks. Also, have the patient change fixation from straight ahead to other directions of gaze to evaluate if stability is altered. Remember, downgaze rotational instability could adversely affect your patient's reading vision.
If appropriate diagnostic lenses are not available, or if you prefer to order empirically, you may make a small adjustment based on the tightness and orientation of the patient's eyelid angle. If the upper eyelid angle is mild, an adjustment to the axis is not indicated; if the angle is more pronounced, perhaps consider adjusting your diagnostic lens axis order by 5-to-10 degrees.
Rotational stability is more important than the amount that a lens base rotates away from the 6 o'clock location. No matter how much a rotationally stable lens is "misaligned," using LARS or cross-cylinder results to prescribe your axis should result in the correcting cylinder aligning optimally with the patient's refractive astigmatism. CLS
For references please visit www.clspectrum.com/references.asp and click on document #159.
Dr. Edrington is a professor at the Southern California College of Optometry. He has also worked as an advisor to B&L. E-mail him at email@example.com.