CASE 1-6
Poor Vision with a Toric Soft Lens
Figure 1-6
History
Patient DS, a 38-year-old Caucasian female, presents with complaints of unstable vision in her right eye. She reports no problems in her left eye. She is a toric soft contact lens (CL) wearer for 3 years. She states that she never had a problem with her vision until she tore her right lens and ordered a replacement 1 month ago. Ever since then, her right eye has been unable to achieve stable, clear vision, despite two lens reorders. She notes that her visual acuity is clear for long periods, followed by stretches of blurry vision later in the day. At that point, manually rotating the lens clears her vision.
She is here to get a second opinion because her previous optometrist told her that her vision is clear and he can do nothing more. Rigid gas-permeable (RGP) lenses were discussed, but she
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30 Clinical Cases in Contact Lenses
rejected that option. Her average wearing time is 12 hours per day. She uses AOSept to clean and disinfect her lenses. Her ocular and medical histories are unremarkable. She takes multivitamins and has no allergies.
Symptoms
Unstable vision OD with toric soft lens, especially later in the day
Clinical Findings
• VA with contact lenses: OD 20/20–
OS 20/20
•Current wearing time is 3 hours, and she reports that her vision currently is clear
•Over-refraction:
OD +0.25 –0.50 × 140, 20/20 OS Plano sphere, 20/20
• Keratometry (after CL removal):
OD 43.25 / 43.50 @ 090, mires clear OS 43.50 / 43.75 @ 080, mires clear
• Subjective refraction without CL:
OD –2.00 –1.75 × 180, 20/20
OS –2.25 –1.50 × 170, 20/20
• Contact lens parameters:
OD: Optima Toric / 8.6 / –2.00 –1.75 × 10 / 14.0 OS: Optima Toric / 8.6 / –2.25 –1.25 × 170 / 14.0
•Biomicroscopy: No abnormalities noted OU
•Contact lens fit: Central position, 0.5 mm movement on blink, 1 mm lag movement, toric lens markings at 6:00 OD and OS, slow rotation with blinks OD, stable rotation OS (see Figure 1-6)
Based on the clinical data, develop your list of differential diagnoses. Then, determine your final diagnosis and develop your treatment plan.
Differential Diagnoses
•Unstable toric lens rotation
•Steep-fitting lens
•Flat-fitting lens
Poor Vision with a Toric Soft Lens 31
•Incorrect lens power
•Surface deposits
•Refractive shift OD
Diagnosis
Unstable toric lens rotation
Management
The patient’s symptoms suggest a lens that starts out fitting well but later tightens such that the lens rotates and remains off axis. Loosening the fit may alleviate symptoms by preventing the lens from tightening up off axis. New contact lens parameters:
OD Optima Toric / 8.9 / –2.00 –1.75 × 010 / 14.0
If she reports similar symptoms with this new lens, a refit to a soft toric lens with larger diameter or increased prism ballast, or to an RGP lens, would be indicated.
Discussion
Soft toric contact lens fitting can be frustrating because of the difficulty in obtaining clear, stable vision in some patients. The lack of precision in fit characteristics that are sometimes encountered can be further exacerbated by imprecision in lens manufacturing. However, soft toric lens manufacturing has improved considerably to where accuracy and reproducibility generally are very good. Factors that contribute to blurry or unstable vision include the following1:
•Inaccurate axis location
•Excessive rotational movement on blink or eye movement
•Slow return to static position after blink
•Flat fit (poor stability of movement and rotation)
•Steep fit (poor draping)
Each of these factors must be ruled out to determine the actual cause of the blurry vision:
•Inaccurate axis location. This can be due to rotation of the lens on the eye, inaccurate marking of the lens by the manufacturer, or inaccurate assessment of rotation.1–4 Because it is very difficult to accurately verify the soft toric lens cylinder axis in office, other
32 Clinical Cases in Contact Lenses
possible causes of axis inaccuracy must be ruled out. Rotation can be assessed in the slit lamp by several methods: reticule with protractor, slit beam rotation (on slit lamps with rotational markings), or estimation using lens markings or clock hours. The first two methods are more accurate, although an experienced observer can accurately determine rotation by estimation. A laterally decentered lens must be carefully assessed because the indicator marks on the lens are laterally displaced as well, giving the appearance that the lens is rotated when it is not. Once the amount of rotation is determined, it can be used to adjust the lens axis so that it aligns with the cylinder axis of the eye. LARS or crosscylinder calculations (via calculators) can be used to determine the final lens axis.5
•Excessive rotation. Some patients’ upper eyelids cause a soft toric lens to rotate on blinks because they move obliquely to the axis of the cylinder power or the prism ballast. In these cases, the patient may notice transient blur, especially as the cylinder power becomes higher.1–4,6 Tightening the fit by steepening the base curve or increasing lens diameter helps reduce rotation. Increasing the prism ballast by switching to a different lens brand also may help. Finally, refitting to a lens brand with more uniform meridional lens thickness may reduce blink-induced rotation.
•Slow post-blink return. Transient lens rotation is often tolerated by patients as long as the lens returns to its static position quickly. Lenses fit too tightly may cause a slow return, which causes noticeable blur.1,2 Although tighter-fitting soft toric lenses often improve rotational stability, the base curve should not be made so steep that the lens cannot restabilize quickly. Also, complications induced by tight lenses must be detected early and avoided.
•Flat fit. Too flat a base curve results in a lens that has less stable movement and rotation.1,2,4,6 The patient may also note discomfort from excessive movement or edge lift. A loose or unstable lens should be reordered with a steeper base curve or larger diameter.
•Steep fit. In addition to causing a slow post-blink return, a steep fit may cause blur via poor draping that results in distortion of the central portion of the lens.1,2 Blinking may improve clarity but only transiently. A distorted retinoscopy reflex that clears with blinks will help diagnose this problem. Refitting to a flatter base curve helps alleviate this problem.
Poor Vision with a Toric Soft Lens 33
Clinical Pearls
•A soft toric lens that is fit too steeply may result in the lens tightening up off axis, with resulting blurry vision.
•A blurry retinoscopy reflex also may point to a steep fitting soft lens.
•Flattening the base curve of a soft toric lens will eliminate blurry vision in a steep-fitting lens by allowing it to drape better on the corneal surface.
References
1.Mandell RB. Hydrogel lenses for astigmatism. In: Mandell RB (ed). Contact Lens Practice, 4th ed. Springfield, IL: Charles C Thomas, 1988:659–680.
2.Watanabe RK. Managing the astigmat with contact lenses. Contact Lens Spectrum. 1999;14(8):42–47.
3.Lindsay RG. Troubleshooting toric soft lenses. Contact Lens Spectrum. 2000;15(9):27–33.
4.Becherer PD. Toric lenses, then and now, some timeless pearls. Eyequest. 1991;1(1):14.
5.Lindsay RG, Bruce AS, Brennan NA, Pianta MJ. Determining axis misalignment and power errors of toric soft lenses. Int Contact Lens Clin. 1997;24(3):101–106.
6.Tomlinson A, Ridder WH, Watanabe R. Blink-induced variations in visual performance with toric soft contact lenses. Optom Vis Sci. 1994;71(9):545–549.