Anisometropia is the condition in which there is a difference in refractive power between two eyes of a patient. The disparity between image magnification on two retinas is known as aniseikonia. Generally, spectacle wearers become symptomatic when approximately 3 D of difference exists in spherical or cylindrical correction. Anisometropia can compromise the patient’s binocular function. It may result in headache, photophobia, blurring, tearing, diplopia and blurred vision.
Methods of correction
Among the many methods available to correct anisometropia are correction with spectacles or contact lenses. When spectacles are used, the difference in image formed by either eye prevents perfect fusion of two images, causing loss of binocular vision and usually amblyopia in the affected eye. Many of these patients cannot tolerate full correction with spectacles because of aniseikonia. Undercorrection becomes mandatory to decrease these types of symptoms.
Contact lenses are the most realistic method for correcting significant anisometropia. They yield more satisfactory visual results than spectacles. Both soft and gas-permeable contact lenses are extremely effective and remain the primary technique of visual rehabilitation for patients who cannot tolerate spectacles.
But contact lens correction is not always possible in patients requiring visual correction. In many cases, lens intolerance and inability to adapt the lens to the eyes’ condition lead to the failure of therapy and development of amblyopia. Contact lens intolerance may be caused by ocular, occupational and systemic factors. Patients with dry eyes, blepharitis, lid abnormalities and corneal neovascularization may not tolerate contact lenses.
Occupational concerns include exposure to environmental factors such as wind, water, smoke and poor sanitary conditions. Unmotivated, unwilling or unable patients cannot tolerate contact lenses. In these patients, refractive surgery may be the best alternative.
In recent years many surgical procedures such as radial keratotomy (RK), photorefractive keratectomy (PRK), LASIK, intracorneal rings, phakic lens, clear lens extraction and IOL implantation have been used to treat myopia.
LASIK is a potential alternative treatment for anisometropia. We present our results assessing the efficacy, stability and safety of LASIK in treating myopic and astigmatic anisometropia.
Patients and methods
Twenty-five anisometropic patients (18 women and 7 men) underwent LASIK between July 1996 and February 2000 at Hong Kong Sanatorium and Hospital, Department of Ophthalmology, Refractive Surgery Center. Mean patient age was 34.12 ± 8.33 (range 19 to 55 years). Nineteen patients wore contact lenses for 8.79 ± 5.22 years (range 2 to 20 years). They stopped wearing them at least 1 week before surgery. All patients hoped to not wear spectacles or contact lenses after surgery.
Preoperative examination included uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA) by manifest refraction, cycloplegic refraction, slit-lamp microscopy, applanation tonometry, dilated funduscopy and corneal topography and pachymetry.
Forty operative eyes of 25 patients (uniocular 10, binocular 15) were myopic with or without astigmatism. Mean preop spherical refraction was –11.7 ± 4.78 D (range –3.75 to –22 D). The amount of preop plus astigmatism ranged from 0 to 6 D with a mean of 1.98 ± 1.48 D.
Mean preop spherical and cylindrical refractive difference between two eyes of 25 patients was 7.47 ± 3.76 D (range 2.5 to 16.5 D) and 1.21 ± 1.44 D (range 0 to 5.75 D), respectively.
Mean preop corneal thickness was 544.23 ± 34.72 mm (range from 478 to 613 mm). UCVA ranged from hand movement, counting finger (CF) to 20/200. BCVA ranged from CF to 20/20, with a median of 20/30 preoperatively. Sixteen eyes achieved 20/20 (four eyes) or 20/25 (12 eyes). Twenty-four eyes had BCVA of 20/30 (eight eyes), 20/40 (nine eyes) or less than 20/40 (seven eyes).
Amblyopia was diagnosed if BCVA was 20/30 or worse in the amblyopic eye and was not attributable directly to any underlying structural abnormality of the eye or visual pathway. Excluding two extra high myopia patients with macular atrophy whose BCVA was CF and 20/30, there were 22 amblyopic eyes diagnosed. The mean preop spherical refraction and astigmatism of these eyes was –13.36 ± 3.48 D (range –4.25 to –22 D) and 2.5 ± 1.29 D (range 0.75 to 6), respectively.
The same surgeon performed all the surgeries. After using the Automated Corneal Shaper (Bausch & Lomb) to create a flap, laser ablation was performed in the stromal corneal bed using the Bausch & Lomb Technolas 217 excimer laser. Plate size was 130 µm (20 eyes) or 160 µm (20 eyes). Mean ablation zone was 5.09 ± 0.54 mm (range 4 to 6 mm). Mean central ablation depth was 172.45 ± 41.38 µm (range 64 to 241 µm).
After completing the laser ablation, the surgeon irrigated the corneal bed surface, placed back the flap to dry for 3 to 4 minutes, tested adequate adherence and removed the speculum. A plastic eye shield was applied postoperatively for 1 day. The patients were instructed to avoid rubbing the operated eye.
Postoperative treatment included the use of tobramycin four times a day for 4 weeks. Nonpreserved artificial tears were used as needed for a prolonged period.
Follow-up examinations were scheduled at 1 day, 1 week, 1 month, 3 months, 6 months, 1 year and 2 years. At each routine examination, manifest refraction, UCVA and BCVA, Goldmann tonometry and slit-lamp biomicroscopy were performed. Corneal topography was examined at 6 months after surgery.
All P values were obtained from the Student’s t test.
There were no intraoperative complications. Patients were followed for 3 to 36 months (mean 15.6 ± 8.52 months). Three eyes with undercorrection underwent enhancement (two at 10 months and one at 6 months) after surgery. Thirty-four eyes had a mean sphere of –1.03 ± 1.76 D at 3 months, 32 eyes had a mean sphere of –1.23 ± 2.14 D at 6 months, 26 eyes had a mean sphere of –1.41 ± 2.51 D at 1 year and 16 eyes had a mean sphere of –1.88 ± 2.8 D at 2 years. There was no statistical difference in the postop sphere at these four times (P > .01).
Mean postop spherical refraction was –1.24 ± 2.02 D (range 1.5 to –11 D). Twenty eyes had a postop spherical manifest refraction within ±1 D. Mean postop cylindrical refraction was 0.58 ± 0.62 D (range 0 to 2.25 D). The mean changes in spherical and cylindrical refraction of surgical eyes was 10.34 ± 3.85 D (range 3.0 to 19.0 D) and 1.43 ± 1.5 D (range –1.25 to 5.52 D), respectively. The changes were statistically significant (P < .001) (table 1).
Mean postop spherical and cylindrical refractive difference between two eyes was 1.49 ± 1.36 D (range 0 to 5.57 D) and 0.65 ± 0.71 D (range 0 to 2.25 D), respectively, statistically significantly lower than the preop value (P < .001) (table 2).
After surgery, UCVA ranged from CF to 20/20, and 24 eyes achieved a significant improvement in UCVA (20/40 or better). BCVA ranged from CF to 20/20 with a median of 20/22.5 postop. There was a statistically significant difference in BCVA between preop and postop eyes (P < .001).
BCVA remained the same in eight eyes, increased by one to seven lines in 31 eyes and decreased by one line in one eye, because of macular disease (table 3).
Among 22 amblyopic eyes, 14 eyes achieved 20/25 or 20/20, and only eight eyes were 20/30 (four eyes) or less than 20/30 (20/40 in three eyes, 20/70 in one eye).
After LASIK, the central corneal thickness was reduced to a mean of 440.97 ± 43.9 µm (range 333 to 544 µm) (P < .001)
The preop intraocular pressure (IOP) ranged from 10 to 20 mm Hg with a mean of 14.86 ± 3.22 mm Hg. After LASIK, IOP was reduced to a mean of 13.19 ± 2.32 mm Hg (range 10 to 17 mm Hg, P > .01). None of the eyes developed a secondary rise in IOP due to topical corticosteroids.
Transient trace haze had been found in four eyes around 4 weeks after surgery, was treated with FML three times a day for 2 months and became clear. There was no flap complication such as epithelial ingrowth, flap striae, free flap or flap melt.
Examination of the corneal topography showed proper centration of laser ablation in all eyes. There were no cases of irregular astigmatism or central islands.
All patients reported some relief of asthenopic complaints such as tearing, headache and blurred vision. Twenty-two patients were happy with the results.
Three less-satisfied patients included one patient with loss of one line vision because of macular disease, one patient who was undercorrected because of extremely high myopia (–21.75 D) and one patient with double vision because of exotropia who needed muscle surgery.
Two patients reported a little transient double vision, one had monovision, and another had binocular vision.
Traditionally, patients with anisometropic myopia and astigmatism are fitted with contact lenses to decrease aniseikonia. In recent years, refractive surgical procedures such as RK, PRK and LASIK have been used to treat myopia and astigmatism, especially in patients with contact lens and spectacle intolerance.
Ahmet reported the results of unilateral RK in 20 eyes of 20 patients between 18 and 60 years of age. Fewer reported the results of PRK in children with amblyopia resulting from anisometropia in which conventional therapy was unsuccessful and the results in adult patients with anisometropia induced by previous retinal detachment surgery.
PRK, especially in high myopia, may result in the development of corneal haze with regression of refractive result and reduction of BCVA.
LASIK maintains the integrity of Bowman’s layer and gives more predictable and stable refractive results than PRK. Many studies reported that LASIK for myopia and astigmatism was safe, effective, stable and predictable. To date, few studies reported the results of LASIK in anisometropic patients. Khaled reported the results of LASIK in pediatric patients with myopic anisometropia and amblyopia. LASIK in these patients yielded good results with no significant complications.
Donnenfeld reported the results of LASIK in 23 eyes of 22 patients after penetrating keratoplasty. At present, there is no report of LASIK for anisometropia in adult patients without other corneal surgery who could not tolerate spectacles and contact lenses.
In our study, the patients had myopic anisometropia with or without astigmatism and amblyopia. We attempted to decrease anisometropia by performing LASIK on uniocular or binocular and tried to improve the BCVA of amblyopic patients.
The results of LASIK in these patients showed the procedure is safe, effective and predictable.
The mean spherical manifest refraction was reduced from –11.7 ± 4.78 D preop to –1.24 ± 2.02 D after LASIK. Twenty eyes had a postop spherical manifest refraction within ±1 D. The amount of refractive astigmatism was reduced from a mean of 1.98 ± 1.48 D preop to a mean of 0.58 ± 0.62 D after surgery.
Even though there is a small amount of residual postop myopia in some patients, decreasing the difference in refractive error between the two eyes allowed the patients to wear spectacles more comfortably and improved the visual acuity in the operated eye. The refractive results in this study showed relative stability at 3 months after LASIK.
One of the main criteria of success in LASIK for anisometropia is decrease of refraction difference between two eyes. The mean preop spherical and cylindrical refraction difference between two eyes was 7.47 ± 3.76 D and 1.21 ± 1.44 D, and decreased to 1.49 ± 1.36 D and 0.65 ± 0.71 D after surgery, respectively.
When considering the common conception that up to 3 D of difference is tolerated by most people, LASIK for anisometropia can be recommended for 3 to 14.5 D of myopic anisometropia. This result is a good indicator of the effectiveness of LASIK for the treatment of anisometropia.
Surprisingly, 31 eyes of our patients had an increase in BCVA, by one line in 17 eyes, two lines in 10 eyes and more than two lines in four eyes. Twenty-four eyes achieved a significant improvement in UCVA (20/40 or better). This improvement is because LASIK acts the same as a contact lens placed directly on the corneal plane, avoiding optical aberrations caused by spectacles.
Among 22 amblyopic eyes, 14 eyes achieved 20/25 or 20/20 of BCVA. So, LASIK is also helpful for improving amblyopia.
Based on the therapeutic goal, LASIK for anisometropia results in good patient satisfaction with improvement in anisometropia. We conclude that in patients with refractive anisometropia, it is reasonable to perform LASIK to equalize refraction with the clinical goal of decreasing the anisometropia, especially in spectacle- and contact lens-intolerant patients.
For Your Information:
- Haili Li, MD, PhD, can be reached at the department of ophthalmology, The First Teaching Hospital, Medical College of Peking University, Xi Shiku Street No. 8, Beijing, 100034 China; +(86) 10-66171122-2748; fax: +(86) 10-66176450; e-mail: firstname.lastname@example.org.
- Guy Chan, MD, FACS, can be reached at The Refractive Surgery Center, Hong Kong Sanatorium and Hospital, Hong Kong, China.
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