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Accuracy of intraocular lens power estimation in eyes having phacovitrectomy for macular holes

Patel, Dipali MB, ChB; Rahman, Rubina FRCOphth, FRCS; Kumarasamy, Manjula MRCOphth, FRCS

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Journal of Cataract & Refractive Surgery: October 2007 - Volume 33 - Issue 10 - p 1760-1762
doi: 10.1016/j.jcrs.2007.05.031
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Abstract

Vitrectomy with intraocular gas tamponade is an established treatment for idiopathic macular holes.1 Nuclear sclerotic cataract is a common complication of vitrectomy. Patients often require subsequent cataract surgery for correction and rehabilitation of vision.2 Combined phacoemulsification with intraocular lens (IOL) implantation and pars plana vitrectomy (PPV) (phacovitrectomy) for a macular hole is safe and effective.3–6 The combined procedure reduces costs and avoids the need for additional surgery.

The aphakic state during surgery gives excellent visibility for posterior hyaloid and internal limiting membrane peeling. It allows a more extensive vitrectomy by creating space for a larger gas bubble.7 The larger bubble exerts a tamponade effect even when the patient is upright. Internal limiting membrane peeling with trypan blue (used in this study) for macular hole repair gives good anatomical and visual outcomes.8

Accurate measurement of the axial length (AL) for accurate IOL power calculation can be difficult in eyes with macular holes because of the foveal crater. This can leave the patient with a significant refractive error postoperatively. Therefore, we performed a study to evaluate the accuracy of IOL power estimation in patients having combined phacoemulsification and vitrectomy for macular holes.

PATIENTS AND METHODS

This retrospective study comprised 40 patients who had phacovitrectomy for stage 3 or stage 4 macular holes. Preoperatively, the AL (EchoScan US-1800, Nidek) and corneal curvature were measured. Measurements were performed by experienced operators; a mean of 10 readings in each eye were taken. Keratometry readings were measured with an autokeratometer (KM500, Nidek). The IOL power was calculated using the SRK/T formula as it is accurate across a range of ALs.9 The refractive status in the fellow eye determined the refractive aim in the operated eye. The target was medium to low hyperopia in 22 eyes, emmetropia in 5 eyes, and myopia in 13 eyes.

Phacoemulsification was performed through a 2.75 mm clear corneal incision. A 3-port PPV was created, the posterior hyaloid face was detached, and trypan blue-assisted internal limiting membrane peeling was performed. This was followed by fluid–air exchange, perfluoroethane tamponade, and implantation of a foldable posterior chamber acrylic IOL in the capsular bag.

Refraction was performed 8 weeks after surgery after the gas bubble completely resolved. The achieved refraction was expressed as a spherical equivalent. The difference between the achieved refraction and the refractive aim in each eye was calculated to obtain the postoperative prediction error. The means and standard deviations of the refractive aim, achieved refraction, and postoperative prediction error were recorded. The mean absolute error (MAE) of the postoperative prediction error was also calculated. The values of the mean refractive aim and mean achieved refraction were compared using the Student t test. The percentage of eyes with an achieved refraction within ±0.50 diopter (D), ±1.00 D, and ±2.00 D of the refractive aim was used as an outcome measure.

The AL in eyes with macular holes was compared with that in fellow eyes to determine whether there was a detectable difference. The percentage of patients in whom primary hole closure was achieved was also recorded.

RESULTS

Initially, 45 patients who had phacovitrectomy for macular hole were identified. However, 5 patients were excluded because of insufficient data in the case notes.

Patients had a mean age of 68.5 years; 80% were women. Thirty percent had additional ocular comorbidity at the time of surgery. Of patients, 3.5% had clear lens extraction; the rest had various degrees of lens opacities at the time of surgery.

Forty-five percent, 67.5%, and 90.0% of eyes having phacovitrectomy achieved a refraction within ±0.50 D, ±1.00 D, and ±2.00 D, respectively, of the refractive aim; 10% of eyes were more than −2.00 D from the refractive aim. The overall postoperative prediction error ranged from +1.64 D to −2.51 D. The mean refractive aim was +0.30 ± 0.72 D and the mean achieved refraction, −0.09 ± 1.25 D. There was no clinically significant difference between the mean refractive aim and the mean achieved refraction.

The MAE of the postoperative prediction error was 0.83 D. The mean postoperative prediction error was −0.39 ± 1.01 D. This suggests that a myopic overcorrection occurred after surgery. Figure 1 shows the predicted refraction and achieved refraction in all eyes. Twenty-four patients had a myopic overcorrection and 15, a hyperopic overcorrection.

Figure 1
Figure 1:
Postoperative prediction error in eyes after phacovitrectomy.

The mean Al was 23.40 mm in the operated eyes and 23.46 mm in the fellow eyes; there was no significant difference between eyes. The success rate for primary macular hole closure was 97.5% (39 patients).

DISCUSSION

The achieved refractive results after phacovitrectomy in our study are comparable to those in studies of phacoemulsification alone. In our study, 67.5% of eyes were within ±1.00 D of the refractive aim. A study of refractive outcomes after phacoemulsification in 1871 eyes found that 72.3% were within ±1.00 D of the predicted refraction.10

We also found that in general, subgroups with poorer preoperative visual acuities had a higher percentage of patients with refractions outside ±1.00 D of the predicted value. Eighty percent of patients with a preoperative visual acuity of 6/9 were within ±1.00 D of the predicted refraction compared with 71.4% of those with a preoperative visual acuity of 6/12. This suggests that ocular comorbidity, such as a macular hole, can affect the accuracy of refractive outcomes. This effect has been noted in a previous study of the effect of visual acuity on prediction error after cataract surgery.11

The mean postoperative prediction error in our study was −0.39 D. One explanation for the myopic overcorrection is anterior displacement of the capsular bag, and thus the IOL, by the complete gas fill achieved after phacovitrectomy. Other authors have made similar observations, supporting our findings. Suzuki et al.12 report a tendency toward myopic overcorrection in a study comparing the postoperative prediction error in phacovitrectomy and in phacoemulsification. In our study, we were unable to conclude that the myopic overcorrection was solely the result of the gas fill after vitrectomy as we did not have a control group for comparison.

Another explanation for the refractive surprises could be that errors in AL or keratometry measurement affected the biometry calculations. It has been reported that a 0.2 mm error in AL measurement can result in a refractive error of ±0.92 D.13 The contribution of errors in biometry calculations could explain why the myopic shift was not seen in all operated eyes.

To our knowledge, ours is the largest study of the refractive outcomes after combined phacoemulsification and PPV for macular holes. We found that the accuracy of IOL power estimation after phacovitrectomy for a macular hole is similar to that after phacoemulsification. We also found that eyes with macular holes do not have a detectably longer AL than fellow eyes, as reported in a previous case study.14 In our study, the mean axial length in the operated eyes appeared slightly shorter (average 24.40 mm) compared with unoperated eyes (average 24.46 mm).

We suggest aiming for slight residual hyperopia in patients having phacovitrectomy for macular holes to compensate for the myopic overcorrection we observed. Any residual hyperopia would result in slight magnification of the viewed image, enhancing the visual outcomes.

REFERENCES

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© 2007 by Lippincott Williams & Wilkins, Inc.