This article is more than 5 years old. Information may no longer be current.
Does your patient need a toric lens? Look at the corneal, not the manifest astigmatism
Click Here to Manage Email Alerts
Case presentation: A 67-year-old patient complained of glare while driving at night and problems reading fine print. The refraction was: -0.50 D -0.50 D x 167, 20/50 OD and -0.25 D, -0.25 D x 016 20/60 OS. Examination revealed nuclear and cortical cataracts consistent with this patient’s level of vision. Fundus examination, though limited by media opacity, was unremarkable.
Work-up: Work-up for this patient included keratometry readings to determine corneal astigmatism as well as ultrasound and optical biomicroscopy for axial length measurement. These measurements are essential for selecting an IOL, which is based upon the axial length of the eye and effective corneal power. Newer-generation formulas for lens power selection also factor in anterior chamber depth and individual factors specific to both the lens implant being used and the surgeon’s outcomes with that particular lens.
Management: Of particular interest in providing spectacle independence for this patient is neutralizing all astigmatism. However, the preoperative refraction, which shows relatively little astigmatism, is of little importance in planning cataract surgery because manifest astigmatism is a sum of both corneal and lenticular astigmatism, and lenticular astigmatism will be eliminated when the crystalline lens is removed and replaced with an implant. Instead, we must rely upon the corneal astigmatism as measured by keratometry, topography or some other method to guide surgical enhancement. Another factor that must be considered in planning surgical correction of astigmatism is the surgically induced astigmatism, that is, astigmatism induced by the corneal wounds created during surgery. These typically cause about 0.5 D of flattening at the horizontal axis.
Take-home point: Patients desiring spectacle freedom after cataract surgery should be counseled about methods for astigmatism correction, and it is corneal rather than manifest astigmatism that determines the need for limbal relaxing incisions or a toric implant when surgery is performed.