Laser in situ keratomileusis (LASIK) is routinely used for the correction of refractive error. The aim of this procedure is to achieve the best visual outcome without using spectacles or contact lenses. When operating on a normal healthy cornea with excellent corrected visual acuity, any possible complication cannot be accepted by the patient. Myopic regression and late complications of LASIK such as astigmatism and keratectasia1–7 can occur even in the absence of apparent preoperative risk factors.
Early investigations have shown that the number of myopes and myopic power increase with age.8 Myopic power reaches its maximum at age 24.9 Hyperopia has been reported to exist in most newborn babies (males +2.33±0.24 diopters [D], females +2.96±0.19 D).10 Hyperopia, however, tends to move to emmetropia with age. At age 14, the refractive power was found to be +0.93±0.3 D for male children and +0.62±0.26 D for female children.10 Increase of the axial length of the globe was found to be mainly responsible for this emmetropization of the eye.9,11 These results suggest that the optical system as well as the growth of the globe are dependent from a coordination mechanism.11 Incorrect coordination is partially responsible for the development of refractive errors. High myopia and high hyperopia are considered outside the scope of this mechanism.11,12 Previous investigations have shown that significant correlation exists between the corneal refractive power and axial length of the globe.13 The correlation shows the following: a) in hyperopia, an increase in corneal refractive power occurs in parallel with an increase in axial length; b) in myopia, an increase in axial length is actively emmetropized by a decrease in corneal refractive power, with both of the aforementioned conditions aiming at achieving emmetropia; and c) patients with high axial myopia either react in the form of condition b—state of active emmetropization13—or show an increase in corneal refractive power in association with an increase in axial length—state of decomposition.13 This confirms previously published data,14–17 which indicate that an emmetropic state can be derived from a steep cornea coupled with a relatively short axial length, or a flat cornea combined with a relatively long axial length, which is evidence for coordinated eye growth and the existence of an active emmetropization process.
The aim of this study is to determine the relationship between refractive error, axial length, and corneal power as well as determine the likelihood of changes in these relationships with age. In addition, suitable age ranges for performing refractive surgery based on the potential for regression of refractive error are examined.
Patients and Methods
During the past 10 years, 46,384 consecutive patients (77,124 eyes) were examined at an outpatient eye clinic in Amman, Jordan, to determine levels of refractive error as a function of age. Biometry measurements were also performed in 2296 patients (4240 eyes) using A-scan ultrasonography. The age and sex distribution of examined patients is shown in Table 1. The age of examined patients ranged from 3 to 82 years, and the refractive errors ranged from +8.00 to −22.00 D.
Table 1: Prevalence of Refractive Errors in 46,384 Patients
Refractive errors were found in 45.1% of patients. The prevalence of myopia was 23.8%, with the prevalence of high myopia (>6.00 D) being 3.8% and the prevalence of hyperopia being 17.5%. The mean value was −0.03 D. The refractive powers of the cornea and lens as well as the axial length of the eye are shown in Table 2. Changes in the level of refractive error with age were noted. The lower end of the age range for refractive surgery is typically ~20 years old. Ideally, the refractive error of these young patients should be stable to reduce the likelihood of progression of the refractive error following refractive surgery. However, Figure 1 illustrates that depending on the level of myopia at the time of surgery, refractive error can continue to progress up to age 30. Patients must be educated prior to surgery regarding the potential for progression based on their required correction so they can adequately determine the risks associated with the procedure.
Table 2: Mean and Standard Deviation of Biometric Measurements of 4240 Eyes
Figure 1. Prevalence of Myopia for Each Age Group and Refraction.
A comparison between the prevalence of refractive error independent of age at the first eye examination is shown in Figure 2. In low to moderate myopia, the prevalence increases up to age 20, and then a relative decrease with age is noted. However, in high myopia, the prevalence increases up to age 30 and remains at the same level. Young hyperopic patients can have 20/20 vision with and without corrective lenses. Therefore, many hyperopic patients can be missed during a routine eye examination. These patients complain of asthenopia and blurring of vision before the presbyopic age and usually ask for reading glasses before age 40. This effect is illustrated in Figure 2, as the prevalence of hyperopia increases after age 40 years. The prevalence of astigmatism is 26.7%, and is usually associated with myopia or hyperopia.
Figure 2. Correlation Between Age and Refractive Error.
The correlation between corneal refractive power and axial length has been described previously.13 In myopia, an increase in axial length is actively emmetropized by a decrease in corneal refractive power, aiming at achieving emmetropia. Patients with axial myopia of >26 mm can react in the same fashion as low myopia (active emmetropization) or many cases can experience deficiency of the active emmetropization (decomposition). These cases show an increase in corneal refractive power and axial length. This fact is confirmed in the present study.
Figure 3 shows the correlation between corneal refractive power and axial length. Of all myopic patients, 92.6% had an axial length <25 mm, which follows the natural regulation mechanism aimed at achieving emmetropia. The remaining 7.4% show an increase of axial length that parallels the increase in corneal refractive power. Patients in which the axial length of the eye exceeds 26 mm in conjunction with higher corneal powers are likely in a state of decomposition and are at risk of marked progression of refractive error following refractive surgery.
Figure 3. Correlation Between Corneal Refractive Power and Axial Length in 4240 Eyes.
Laser in situ keratomileusis is generally performed after age 20 when the refractive power of the eye becomes stable. Clinical studies have shown that LASIK retreatment was performed in 14% of cases18 and 10-year follow-up of LASIK for high myopia shows that 27% of operated eyes underwent retreatments attributable to undercorrection and/or regression.19 However, enhancements and retreatments are commonplace and are applied to manage refractive instability and myopic regression that occurs because of ocular changes in the axial length/corneal curvature as mentioned over time. Thus, the premise that there is a 20-year-old cutoff for myopes or an 18-year-old cutoff for low myopes is intrinsically flawed, because the correction of refractive errors over an adult lifetime may need multiple interventions.
The population results of this study suggest that the lower age boundary for LASIK could be reduced to 18 years for patients with <1.00 D of myopia at age 10 and <3.00 D of myopia at the time of surgery. This age reduction is due to the stabilization of refraction in this subset of patients. In patients with myopia >3.25 D at age 10, the magnitude of refractive error increases up to age 30 in 0.6% of patients with moderate myopia and 0.9% of patients with high myopia. Because LASIK can be performed in these groups while refractive error is potentially unstable, patients should be informed of the likelihood of progression. With this information, patients can make informed decisions on whether the risk of progression outweighs the benefits of refractive surgery.
This population study also demonstrated that hyperopes tend to progress after age 30. Patients in this group should be informed as well. Finally, 7.4% of patients with myopia >6.00 D and axial length >26 mm are possibly in a state of decomposition, where the corneal power and axial length are progressively increasing. These patients may be at increased risk for complications following refractive surgery. Under these conditions, the corneal stroma may be less stable with increased risk of keratectasia following refractive surgery. Refractive surgeons should further assess these risks in this population subset.
- Chen CL, Tai MC, Chen JT, Chang CJ, Lu DW. Acute corneal hydrops with perforation after LASIK. Clin Experiment Ophthalmol. 2007;35:62–65. doi:10.1111/j.1442-9071.2007.01370.x [CrossRef]
- Kim TH, Lee D, Lee HI. The safety of 250 micron residual stromal bed in preventing keratectasia after laser in situ keratomileusis (LASIK). J Korean Med Sci. 2007;22:142–145. doi:10.3346/jkms.2007.22.1.142 [CrossRef]
- Randleman JB. Post-laser in situ keratomileusis ectasia: current understanding and future directions. Curr Opin Ophthalmol. 2006;17:406–412. doi:10.1097/01.icu.0000233963.26628.f0 [CrossRef]
- Javadi MA, Mohammadpour M, Rabei HM. Keratectasia after LASIK but not after PRK in one patient. J Refract Surg. 2006;22:817–820.
- Tabbara KF, Kotb AA. Risk factors for corneal ectasia after LASIK. Ophthalmology. 2006;113:1618–1622. doi:10.1016/j.ophtha.2006.03.045 [CrossRef]
- Lifshitz T, Levy J, Klemperer I, Levinger S. Late bilateral keratectasia after LASIK in a low myopic patient. J Refract Surg. 2005;21:494–496.
- Klein SR, Epstein RJ, Randleman JB, Stulting RD. Corneal ectasia after laser in situ keratomileusis in patients without apparent preoperative risk factors. Cornea. 2006;25:388–403. doi:10.1097/01.ico.0000222479.68242.77 [CrossRef]
- Blegvad O. Über die progression der myopie. Klin Monatsbl Augenheilkd. 1928;60:1955–1981.
- Steiger A. Die Entstehung der Sphärischen Refraktion des Menschlichen Auges. Berlin, Germany: Verlag Karger; 1913.
- Betsch A. Über die menschliche refraktionskurve. Klin Monatsbl Augenheilkd. 1929;82:265–379.
- Kettesy A. The stabilization of the refraction and its role in the formation of ametropia. Br J Ophthalmol. 1949;33:39–47. doi:10.1136/bjo.33.1.39 [CrossRef]
- Benjamin B, Davey JB, Sheridan M, Sorsby A, Tanner JM. Emmetropia and its aberrations: a study in the correlation of the optical components of the eye. Special Report Series Medical Research Council (Great Britain). 1957;11:1–69.
- Sayegh FN. The correlation of corneal refractive power, axial length, and the refractive power of the emmetropizing intraocular lens in cataractous eyes. Ger J Ophthalmol. 1997;5:328–331.
- Bruckner A. Optische konstanten, refraktion, akkomodation. In: Tabulae Biologicae XXII. Hague, Netherlands: Dr W. Junk; 1963:120–126.
- Stenström S. Untersuchungen über die variation und kovariation des optischen elements des menschlichen auges. Acta Ophthalmol XXVI. Upsala, Sweden: Verlag Munksgaard; 1946.
- Sorsby A, Leary GA. A longitudinal study of refraction and its components during growth. Special Report Series of the Medical Research Council (Great Britain). 1969;309:1–41.
- Sorsby A, Benjamin B, Sheridan M, Stone J, Leary GA. Refraction and its components during the growth of the eye from the age of three. Medical Research Council Memorandum. 1961;301:1–67.
- Netto MV, Wilson SE. Flap lift for LASIK retreatment in eyes with myopia. Ophthalmology. 2004;111:1362–1367. doi:10.1016/j.ophtha.2003.11.009 [CrossRef]
- Alió JL, Muftuoglu O, Ortiz D, Pérez-Santonja JJ, Artola A, Ayala MJ, Garcia MJ, de Luna GC. Ten-year follow-up of laser in situ keratomileusis for myopia of up to −10 diopters. Am J Ophthalmol. 2008;145:46–54. doi:10.1016/j.ajo.2007.09.010 [CrossRef]
Prevalence of Refractive Errors in 46,384 Patients*
|Age (y)||No. of Patients (%)||Prevalence (%)†|
|Myopia (n=11,039)||High Myopia (n=1763)||Hyperopia (n=8117)||Astigmatism (n=12,384)|
|0 to 5||3582 (7.7)||0.9/0.4||1.3||1.0/0.3||1.3||9.4/8.4||17.8||5.6/4.7||10.3|
|6 to 10||3665 (7.9)||8.4/8.1||16.5||0.5/0.5||1.0||9.9/8.7||18.6||11.5/11.3||22.8|
|11 to 15||3588 (7.7)||14.2/18.1||32.3||1.0/1.0||2.0||6.7/5.7||12.4||12.7/13.9||26.6|
|16 to 20||3550 (7.7)||14.1/22.8||36.9||1.4/2.3||3.7||4.3/4.3||8.6||13.9/16.3||30.2|
|21 to 30||7795 (16.8)||14.2/19.8||34.0||2.2/2.7||4.9||4.1/3.4||7.5||13.7/16.0||29.7|
|31 to 40||6047 (13.0)||13.5/14.3||27.8||2.3/2.2||4.5||5.0/5.1||10.1||15.0/14.9||29.9|
|41 to 50||5746 (12.4)||12.7/10.8||23.5||2.2/2.0||4.2||9.4/12.0||21.4||15.9/14.2||30.1|
|51 to 60||5555 (12.0)||10.4/8.2||18.6||2.3/2.4||4.7||13.1/19.3||32.4||14.4/13.4||27.8|
|61 to 70||4500 (9.7)||8.8/6.8||15.6||2.2/3.0||5.2||13.6/15.8||29.4||14.3/12.4||26.7|
|71 to 80||1897 (4.1)||8.5/7.2||15.7||2.1/2.3||4.4||10.1/11.1||21.2||13.6/10.7||24.3|
Mean and Standard Deviation of Biometric Measurements of 4240 Eyes
|Refractive Power (D)||Axial Length (mm)|