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Editor,—The goal of cataract surgery is the rapid attainment of good visual acuity, ideally unaided for the intended focal distance and this is dependent on accurate intraocular lens (IOL) power calculation. Refractive prediction errors, if substantial, can be problematic for the patient.
If the problem is one of symmetrical ametropia, spectacle correction is usually successful. Contact lens fitting may be necessary for those patients with anisometropia in whom spectacle wear would induce significant and intolerable aniseikonia. IOL exchange as a secondary procedure may have to be considered in cases refractory to these measures.
In our study, we aimed to ascertain whether objective refraction at the end of surgery was feasible and if so, to determine how this refraction related to the final refraction at 4–7 weeks postoperatively with a view to suggesting guidelines for immediate exchange of implant in cases of gross refractive prediction errors.
Consecutive patients undergoing phacoemulsification cataract surgery after continuous curvilinear capsulorrhexis with capsular bag implantation of Chiron C10UB injectable IOLs were refracted at the end of surgery on the table objectively with a streak retinoscope, and at 4–7 weeks postoperatively when objective refractions were refined subjectively for best visual acuity. Viscoelastic used was Provisc (sodium hyaluronate 1%). The immediate postoperative refraction results were not available to the ophthalmologist performing the final refractions. Patients with glaucoma, diabetic retinopathy, macular degeneration, peroperative, and/or postoperative complications and patients who failed to attain 6/9 or better Snellen visual acuity at final refraction were excluded.
Twenty six eyes of 26 patients were included (six males and 20 females, 16 right eyes and 10 left eyes). Ages ranged from 62 to 86 years with a mean of 78.9. Twenty four patients had topical anaesthesia and two had peribulbar injections followed by application of Honan balloon. IOL powers ranged from 16 D to 27 D with a median of 22.5 D.
Data were analysed using statistical software on Macintosh computer.
We were unable to refract two patients owing to the presence of corneal epithelial haze.
Figure 1 shows a few values clustering around the neutral—that is, “no change” line, and a single significant myopic change but the predominant feature is the shift towards hypermetropia. The single significant myopic change occurred in a patient who had topical anaesthesia.
The mean change in refraction was 1.11 D hypermetropia with a standard deviation of plus or minus 0.94 D. The range was from 1.63 D myopic change to 2.75 D hypermetropic change. The 95% confidence interval was 0.73 D, 1.48 D. Paired two tailed Student’st test performed on the immediate postoperative and final refraction results gave a p value of <0.001.
Høvding et al reported that about a third of the 188 patients in whom a PC-IOL was implanted after extracapsular cataract extraction ended up with more than plus or minus 1.0 D deviation from the predicted postoperative refraction. In about a tenth deviations of more than plus or minus 2.0 D from the calculated value were found.1
The earliest reported IOL exchange was performed on the first postoperative day.2 In other studies the average interval between the primary surgery and the implant exchange ranged from 3.5 years to over 5 years.3 4
We found that following phacoemulsification surgery, the preservation of corneal clarity together with the minimally induced surgical astigmatism allowed us to perform immediate postoperative objective refractions satisfactorily in most cases.
There was a statistically significant hypermetropic shift. We think that the most likely explanation lies in the reduced effectivity of the IOL as it settles back in the postoperative period. After phacoemulsification cataract surgery with continuous curvilinear capsulorrhexis, the depth of anterior chamber has been found to increase gradually, the anterior capsular size to narrow, and the refraction to tend towards hypermetropia.5 The IOL used in our study was a Chiron C10UB—a plate haptic lens with no angulation and a predicted final anterior chamber depth of 5.59 mm.
Various factors which can affect the accuracy of immediate postoperative refraction should be considered. Corneal haze from any cause or presence of a posterior capsular plaque can interfere with retinoscopic reflexes. Viscoelastic should be removed completely at the end of surgery especially from behind the IOL. We made sure that the globe was firm, neither too soft nor too hard, before we refracted. If a gross refractive error is detected, we would advise a fundal examination be performed to exclude any posterior pole lesions hitherto undetected.
In conclusion, objective refraction immediately following phacoemulsification surgery is feasible; if a gross refractive prediction error is found, immediate implant exchange may be considered, taking into account the mean hypermetropic shift of about 1 dioptre from immediate postoperative objective refraction to the final refraction in the postoperative period in cases where the Chiron C10UB lens has been implanted.
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