Measuring and Managing Residual Refractive Error After Intraocular Lens Implantation
Scott K. McClatchey
Choosing an intraocular lens (IOL) power in children is driven by the postoperative refraction goal. Managing the residual refractive error in turn depends on the refractive trajectory until the relative stability of adult life. In contrast to adults, the initial refractive goal for a child is driven by long-term outcome, and we think that it is best to start with this goal in mind: good vision when the child grows up into an adult. The same goal applies to the clinician who manages residual refractive error after IOL implantation. This outcome goal can be divided into three parts: emmetropia in adulthood, good visual acuity as an adult, and a manageable course of refraction between IOL implantation and adulthood; this includes the need to treat amblyopia and management of the changing refractive error. Because the surgeon’s choice of initial postoperative refraction determines much of the future management of the child, this chapter builds on the discussion in Chapter 7 of this book, “Calculation and Selection of Intraocular Lens Power for Children.”
MEASUREMENT
Measurement of refractive error is well known to all ophthalmologists who examine children. Simple objective retinoscopy with or without cycloplegic drops can give an accurate assessment of the refractive error in children after cataract surgery. On the other hand, subjective refractions are difficult to perform in young children and are more prone to error in amblyopic eyes, since the child will have difficulty distinguishing the difference between alternative lenses. I do not recommend using subjective techniques until about age 7 years; even then, I prefer retinoscopy. High-power spherical IOLs will cause a greater spherical aberration than will standard-power IOLs. This can be easily seen with the retinoscope as a refraction at the peripheral pupil of a well-dilated eye that is more myopic than the central refraction; it can be measured with a wavefront machine. In comparison with retinoscopy, automated refraction can give more repeatable results.1
Refractive error can also be estimated from biometry in aphakic eyes, though this is less accurate than direct retinoscopy. This can be a useful adjunct in the few children who are strongly or violently resistant to exam in clinic; however, biometry would require anesthesia in these children, and retinoscopy will measure the refractive error directly.
The clinician should wait for 2 to 4 weeks after surgery to obtain an accurate refraction. In the early postoperative period, there are several temporary changes that resolve quickly. The young child’s eye is soft and malleable. Surgeons usually use a fine absorbable suture to close the wound in young children: this can cause several diopters (D) of astigmatism that resolves more quickly than in adults as the suture breaks or erodes through the soft corneal tissue. Residual viscoelastic, especially if it remains between the IOL and an intact posterior capsule, can displace the IOL anteriorly in the eye in the early postoperative period, giving in a brief period of myopia (or lesser hyperopia) that resolves as the viscoelastic gradually diffuses away.
The position of a corrective lens can have large optical effects when the lens power is large: the vertex distance must be precisely correct to avoid induced error. For high refractive errors (greater than about ±4 D), refraction should be measured with corrective lenses in place whenever possible. When the child is wearing his or her own glasses (or contact lenses), this corrective lens is already at the proper position with respect to the cornea; a low-power trial lens held over the glasses will allow the retinoscopist to be more precise in measurement. Because the trial lens held to do a refraction over glasses has a
lesser power, vertex distance errors will cause a much smaller error in refraction than a high-power lens.
lesser power, vertex distance errors will cause a much smaller error in refraction than a high-power lens.
Residual refractive error in aphakic and pseudophakic eyes will vary with age, and can change significantly within a few months during the 1st year of life. Because of this, it is useful to time routine follow-up examinations every 3 months up to about age 3 years, and to gradually lengthen this period up to yearly in teenage years and beyond.
MANAGEMENT
The residual error should be managed with the goal in mind: good vision in adult life. Amblyopia management is paramount to this goal in unilateral cataract patients; for all, management of refractive error should minimize the duration of blur in young children and should minimize the untoward effects of anisometropia.
If two IOLs were implanted in an eye at cataract surgery (temporary polypseudophakia2), the anterior IOL should have between 20% and 25% of the total IOL power.3
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