Abstract
AIM
To observe the central corneal thickness (CCT) changes in infants and young children who had been undergone bilateral congenital cataract surgery, and to compare the changes with normal control group which was selected from healthy population.
METHODS
A cross section case-control study contained 28 cases (56 eyes) of bilateral aphakia (aphakic group) due to congenital cataract surgery combining with posterior continuous curvilinear capsulorhexis and with anterior vitrectomy during 2-6 months after birth. Fourteen children (28 eyes) of age-sex matched with the aphalic group were selected as normal control group. CCT and intraocular pressure (IOP) were measured postoperatively and the results were compared between groups.
RESULTS
The mean CCT was 653.5±82.4μm in the aphakic group and 579.6±39.2μm in the control group, with a significant difference (P=0.000). The mean value of IOP in aphakic group (22.0±1.6mmHg) was greater than that of control group (16.9±2.1mmHg), P=0.023. There was a negative correlation between age and CCT in normal control group (r=-0.531, P=0.026), and there was no correlation in bilateral aphakia group (r=-0.324, P=0.165)
CONCLUSION
Aphakic children due to congenital cataract surgery have a greater CCT than normal children. It is necessary to consider CCT in evaluating IOP for children after congenital cataract surgery.
Keywords: congenital cataract surgery, aphaka, central corneal thickness, introcular pressure
INTRODUCTION
Congenital cataracts are the most common cause of treatable childhood blindness and low vision, accounting for 5% to 20% of blindness in children worldwide. As the improvement of surgical techniques of ophthalmology, it is widely accepted the ideas of early surgical involvement in these young patients to prevent them from such conditions as amblyopia and nystagmus. However, the occurrence of aphakic glaucoma following the congenital cataract surgery in early childhood tends to be increasing obviously. It was reported that the incidence of aphakic glaucoma following congenital cataract surgery within the first year of life was 6%-41%[1]-[4]. Some reported the incidence had even reached to 100% in a long term follow-up study[5]. Because the babies are not capable of cooperate a variety of the glaucomatous examinations including gonioscopy and visual field examination, the intraocular pressure (IOP) becomes the most important factor to diagnose the aphakic glaucoma. Any factor influencing the IOP measurement will be misleading the diagnosis of glaucoma. It is known to all that central corneal thickness (CCT) is a common factor that is able to influence the IOP measurement result. If a patient has a greater CCT, the IOP will be evaluated higher than normal, and vice versa. So far, it is rarely to find reports concerning the relationships between CCT and aphakic glaucoma following congenital cataract surgery. The present study used a cross section case-control study of 56 eyes (aphakic group) of bilateral aphakia due to congenital cataract surgery during 2-6 months after birth, and of age-sex matched 28 healthy eyes (control group) to compare CCT and IOP between groups.
MATERIALS AND METHODS
Materials
Twenty-eight cases (56 eyes) of congenital cataract children who underwent bilateral congenital cataract surgeries within 6 months after birth in Shengjing Hospital during Feb. 2003 to Feb. 2008, were selected as aphakic group in which, 12 children (24 eyes) were male and 16 (32 eyes) were female, whose mean ages at the last follow-up were 33.6 months (30-36 months). Fourteen healthy children volunteers whose mean ages were 32.4 months (30-36 months), including 6 males and 8 females, were also selected as control group. Except for lens opacities, other congenital ophthalmic disorders including microophthalmas and microcornea were excluded in aphakic group
Methods
Surgeries
All cataract surgeries were carried out by the first author under general anesthesia with a 2mm clear-cornea incision, anterior and posterior continuous curvilinear capsulorhexis and anterior vitrectomy. The surgical interval of the bilateral eyes was 1 week. No serious postoperative complications occurred.
Measurements
Written informed consent was obtained from the participants' parents after explanation of the nature and possible consequences of the studying in both aphakic and control groups. CCT measurements and tonometries were carried out during each follow-up. The first follow-up was 1 month after cataract surgery. The follow-up interval became 6 months thereafter, till the IOL implantation surgeries (i.e. the last follow-up). Fourteen healthy children, who were age-sex matched with those in aphakic group at last follow-up, were selected as control. All participants in aphakic group underwent CCT measurements with Tomey Pachy Meter SP-3000 and tonometry with Medtronic Tono-PEN XL. These measurements were done just before the IOL implantation surgery by an experienced ophthamic technician. The participants in control group were tranquilized by intrarectal injection of 50g/L chloral hydrate solution (1mL/1KgBw). The measurement procedures in control group were identical with those in aphakic group. The ocular hypertension criteria were defined as IOP reaches 22-35mmHg without optic nerve and optic nerve fiber layer defect.
Statistical Analysis
Statistical analysis was performed using SPSS 13.0 software. The results were considered statistically different at P < 0.05.
RESULTS
The mean value of CCT in aphakic group (653.5±82.4μm) was greater than that of control group (579.6±39.2μm), P=0.000. The mean value of IOP in aphakic group (22.0±1.6mmHg) was greater than that of control group (16.9±2.1mmHg), P=0.023 (Table 1). CCT changes as the aphakic babies growing up were shown in Figure 1. Four eyes (all unilateral) in aphakic group had persistent hypertension (23-27mmHg) postoperatively and lasted till the last follow-up. But the optic disc and visual evoke potential (VEP) examinations were kept normal, no medical practice was given except for regular follow-up.
Table 1. Comparison of CCT value between aphakic and control group.
| Group | Sex (M vs F) | Mean age (mo) | CCT(μm) | IOP(mmHg) |
| Aphakic group | 12:16 | 33.6 | 653.5±82.4 | 22.0±1.6 |
| Control group | 6:8 | 32.4 | 579.6±39.2 | 16.9±2.1 |
| P | 0.000 | 0.023 |
Figure 1. CCT changes with ages in aphakic group.
DISCUSSION
The present study reveals that the CCT of aphakic children group following congenital cataract surgery is greater than that of age-sex matched normal children group. This result is similar to that of former studies[6]-[10]. Lupinacci et al[8] reported that the CCT of aphakia following congenital cataract surgeries was greater than that of normal phakic eyes, and than that of children without cataract as well. He believed that the reason of the corneal thickness was due partly to cataract surgeries. Muir et al[10] indicated that the CCT of congenital cataract children had been similar to that of normal children, but become greater following the cataract surgeries.
In order to exclude interfering factors like age and sex from the present study, we specially selected age-sex matched healthy children as normal control. We also focused on aphakic children who had undergone cataract surgeries within 2-6 months after birth to eliminate the corneal developmental factors due to different timing of surgery. The data used to compare with control group were obtained on the operating theater under the condition of general anesthesia, just before the second-stage IOL implantation, to prohibit influencing the accuracy of CCT measurement from existence of IOL.
Central cornea is usually thicker immediately after birth. However, the cornea tends to be thinned as a baby growing and reaches nearly to the thickness of an adult at the age of three. This change is probably related to the process of liquifying, vaporizing and transparency regulation of the cornea in early childhood. Whether the cataract surgery is affecting the process of normal development of the cornea in infancy, further study is still needed. The increase of CCT after cataract surgery is what we have found in our study. We suppose that the absence of a lens barrier after cataract extraction might cause the front flow of the vitreous, which is capable of changing the microstructures of anterior eyeball and corneal physiological components. Thus, the corneal development and maturation are delayed. On the other hand, the cornea is also affected by the following factors: surgical injuries, temperature change in chambers and the release of free radicals during and after surgery. Some researchers indicated that the thickening of cornea after cataract surgery had played an important role in protecting the visual field from reduction and served as a compensatory effect to the eyeball[11],[12].
In present study, all hypertensive eyes in aphakic group had greater CCT than the average. We deduce that there must be some unknown factors leading to a subclinical corneal edema, which further destroys the function of corneal endothelium, thickens the cornea and increases the IOP.
During our follow-ups, it was almost impossible to detect the patients' visual acuity, visual field, gonoscopy and other necessary subjective examinations because of the young children' disability to cooperate these routine examinations used in adults. IOP, thus, becomes a crucial criterion in deciding whether these young patients were glaucomatous or not. Therefore, it is important to consider the CCT factor in evaluating the ocular hypertension for those who undergo congenital cataract surgery to avoid of misdiagnosis of aphakic glaucoma. It is also important to consider the instruments applied in measuring IOP, since different tonometry makes different IOP result. Ehler et al[13] found that the corrected IOP using Goldmann tonometry should be IOP value +(520-CCT)/70×5. Luan et al[14] studied IOP in rabbits and indicated that the corrected IOP using Perkins tonometry should be12.107+1.254×IOP value by Perkins-0.033 ×CCT. Foster et al[15] reported that IOP would elevate about 2mmHg if the CCT increased every 100µm. The author believes that a reasonable IOP measurement can be obtained if the influencing factors mentioned above are fully considered.
In summary, there are many reports on CCT related to open angle glaucoma, ocular hypertension and normal tension glaucoma, but rarely to aphakic glaucoma following congenital cataract surgery. The present study probably provides a clinical research data in making a proper diagnosis of aphakic glaucoma following congenital cataract surgery in early childhood. Further study is still needed to find out the relationships between CCT and the morphology of congenital cataract, and the implantation of IOL.
Footnotes
Foundation item: National Natural Science Foundation of China (No. 30973276)
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