Congenital cataract is one of the leading causes of form deprivation amblyopia in children. 1,2 Surgical correction is essential at an early age to prevent amblyopia, strabismus, or poor fusion and should be followed by immediate and permanent optical correction of the resulting aphakia. Preferably, treatment should start within the critical period of the visual system, 17 weeks after birth. 3–6
Spectacles have the disadvantage of distorted peripheral images, constriction of the visual field, and problems of low compliance. Intraocular lenses and epikeratophakia are difficult to use because of the subsequent growth of the infant eye, resulting in changes of necessary refractive power. Furthermore, the long-term effects of intraocular lenses in infant eyes are not known. Early contact lens fitting seems to be the most practical solution for correction of infant aphakia. 7,8
Fitting contact lenses in infant aphakia involves several characteristic problems. Measuring the eye parameters of such young children is not easy, and assessment of the fit on the eye with a routine slit lamp examination is practically impossible. Usually, keratometry and evaluation of the lens fit need to be done under sedation or general anesthesia, in the operating room. 9 The necessary, high plus refractive power leads to lenses with remarkable center thickness. To avoid decentration (especially low riding) or lens loss, the lens diameter must often be taken as large as the cornea, and a relatively steep fit is indicated. The insertion and removal of the lenses on a daily-wear basis may be a stressful event for both parents and child. Because the young aphakic eye must be corrected during all sporadic waking hours, 10 extended wear is a key demand for successful treatment, at least up to an age of 3 to 4 years old.
For these reasons, the use of rigid contact lenses has not been successful. The high risk of lens loss and the chance of lens binding on the cornea are further complications. High-water content soft contact lenses, although comfortable to wear and easy to fit, show a high rate of complications due mainly to reduced oxygen transmission in the high-plus thick lens modality. 9 Therefore, contact lenses made of silicone elastomer with high oxygen permeability have the potential to meet the high demands in the correction of young aphakic children.
Silicone contact lenses have had a long and troublesome history of development. 11–14 Since the experiments of Becker, in 1956, the material, lens design, production, and reproducibility improved, but only slowly. In September 1984, the Silsoft (Bausch & Lomb, Rochester, NY) lens, manufactured from a silicone elastomer, was the first lens that reached U.S. Food and Drug Administration (FDA) approval as a 30-day extended-wear lens for aphakia. 13 The basic material is hydrophobic, and, therefore, a surface treatment (with the formation of hydroxyl groups on the lens surface) is required. This process gives the lens a low contact angle without making it hydrophilic. 15 This means that there is no dehydration in low humidity, and the more stable lens usually gives a better visual acuity than hydroxyethyl methacrylate (HEMA) lenses. 16 The material is soft but stable enough to assure good optical quality and high resistance to tearing; and it has a good thermal conductivity. 15–17
The most important feature of silicone elastomer is its high oxygen permeability. Equivalent oxygen percentage (EOP) is a measure of the amount of oxygen that is available to the cornea. The baseline of the EOP scale is 21% oxygen available from air, without contact lens wear. For safe, permanent wear (including closed-eye conditions), a minimal EOP value of 18% has been indicated. Hill 18 showed that Silsoft lenses have an average EOP of 18.7% and 18.4% in lenses that had been worn more than 12 hours per day during an 8 to 10 month period. Barr 15 demonstrated a graph from the work of Fizgerald that yields approximately the same EOP value results but also shows that the EOP value is independent of lens thickness.
Oxygen transmissibility of a contact lens can also be given as the Dk/L value. The oxygen permeability of the material is Dk, and L is the lens thickness. The Dk value of Silsoft lenses is stated by the manufacturer as 340 × 10−11 cm2 mL O2/sec mL mm Hg. Glasser and Weissman 19 found a value of only 50, whereas Weissman, Fatt, and Pham 20 reported a value of 190 using a single-chamber polarographic technique. The high standard deviation (SD = 79) found in the latter study led the authors to suggest that better methods should be developed to evaluate oxygen performance of high-Dk materials. According to the same study, lens thickness does correlate with oxygen transmissibility, but, even with a very thick lens, oxygen transmissibility is high enough to meet the demand for permanent wear (Dk/L = 90).
The aim of this long-term prospective study was to investigate the performance of Silsoft contact lenses under permanent wear conditions in children with aphakia after congenital cataract.
SUBJECTS AND METHODS
An important patient-intake criterion in this study was an early diagnosis to ensure that the cataract was congenital. 21 All patients in this study underwent surgery at the Academic Medical Center, University of Amsterdam, The Netherlands. Normal standards were followed to establish a multidisciplinary diagnosis, prognosis, pre- and postoperative care, and informed consent.
All aphakic eyes were fitted with Silsoft contact lenses (Table I) as soon as possible after surgery. The fitting of the lenses was performed without sedation in the consulting room, and the fitting procedure did not involve keratometry. In all cases, a lens with a BOZR of 7.50 mm and a total diameter (TD) of 11.3 mm was inserted as first choice. Using a penlight, a magnifying lens (+10 D), and an ophthalmoscope or hand-held slitlamp, the motility and centering of the lens were evaluated 30 minutes after insertion. If the lens moved too much or decentered, the TD was increased to 12.5 mm. In cases of insufficient motility, a lens with a flatter BOZR was used and evaluated again. Lens power was calculated from retinoscopy. An addition of +4 D (counted from birth) was given to assure optimal focussing at short distance. This addition was decreased by 1 D each year.
Parents were directed to routinely check the eyes of their child. They were instructed to look for a clear reflex of the front surface because this indicates the wettability of the lens. If dull areas were present, parents were to apply a saline solution containing no preservative. Parents were also directed to routinely insert one drop of saline (no preservatives) when the child awakened. Parents were taught how to remove, clean, and insert the lens and were advised that cleaning should only be done in case re-wetting failed. As long as the surface was wet and the eye remained clear, the lenses were worn permanently. If the eyes became red or there was any doubt about the lens or eye condition, the parents were instructed to consult the clinic immediately.
Follow-up visits were scheduled at 1 night, 1 week, 1 month, and every 3 months thereafter. During follow-up visits, an ophthalmologic examination was performed that included assessment of movement, centration, and condition of the lens. The refractive power of the lens was checked using retinoscopy with and without the lens in situ. Lenses were routinely exchanged every 3 months. To make sure no wearing time was lost due to delay in ordering of lenses, spare lenses were stocked in the clinic for all children.
Seventeen children completed this study. Eight (2 male, 6 female) patients suffered from unilateral congenital cataract, and nine (4 male, 5 female) patients suffered from bilateral congenital cataract (Table 2). A total of 26 eyes were treated. The confirmed etiology of the congenital cataract was hereditary (3 cases), associated with Down syndrome (3 cases), associated with rubella infection and microphthalmos (2 cases), and associated with encephalopathy (1 case). Typically, in the remaining 8 cases (47%), the etiology was idiopathic. In 3 eyes, persistent hyperplastic primary vitreous (PHPV) was present. Surgical technique consisted of lensectomy with anterior vitrectomy in all eyes.
The time period between birth and the start of contact lens wear averaged 20.3 weeks and ranged from 8.6 to 41 weeks. This wide spread in time period could already be found in the time interval between birth and diagnosis, averaging 11.2 weeks and ranging from 1 to 22 weeks. Surgery was performed at an average of 6.7 weeks after the diagnosis (range 1–26 weeks). Contact lens wear was started at an average of 2.4 weeks (range 1 day–9.5 weeks) after surgery.
Snellen visual acuity at the age of 4 years is listed per eye and per type of aphakia in Table 3. The typical discrepancy between visual acuity in bilateral and unilateral cases can be seen. Visual acuity in bilateral cases was significantly better (chi squared P < 0.05) than that of the treated eye in unilateral aphakia.
Few complications occurred until patients reached the age of 3 years (cumulative wearing time 72 years). In 4 eyes (2 patients), a bacterial conjunctivitis caused by Haemophilus influenzae occurred. Both cases resolved without sequelae. In one case an acute red eye was associated with lens binding, and a flatter lens solved the problem. In 4 eyes, decentration of the lens was better corrected using a 12.5-mm lens compared to a 11.3-mm lens with steeper base curve.
Lens power at 3 months of age showed a large range (+26 – +32 D) that was equal for both unilateral and bilateral aphakia. In Figure 1, the distribution of mean ± SD of lens power, from 3 to 36 months old, is given for bilateral and unilateral aphakia. The mean ± SD contact lens power at 3 months of age for the total group (+28.40 D ± 2.76), the bilateral group (+28.00 D ± 3.10), and the unilateral group (+29.00 D ± 2.45) were not statistically different (P > 0.01). With increasing age, the power spread in bilateral cases generally channels into a lower range of +15 to +18 D (mean +16.44, ± 1.24) at 3 years of age. This is not so much the case in the group of unilateral aphakia, with a range of +16 to +29 D (mean + 19.63, ± 5.01). The effect on the SD is entirely due to the two cases of unilateral aphakia with the etiology of Rubella-associated microphthalmos. Excluding these two cases from the analysis in the unilateral group reveals a power range of +26 to +29 D (mean 27.50 D, ± 2.68) at 3 months of age and +16 to +18 D (mean +17 D, ± 1.1) at 3 years of age. The latter figures are not statistically different (P > 0.1) from the bilateral group.
The average change in power per month of age for the total group, the bilateral group, and the unilateral group were 0.32 D, 0.35 D, and 0.28 D, respectively. If the two cases of Rubella associated with microphthalmos are excluded from the unilateral group, the change in power per month of age is 0.31 D. The individual cases of Rubella showed power changes of 0.18 D and 0 D per month of age.
BOZR changes follow a remarkable line (Table 4). Until the age of 1.5 years, almost all eyes needed a BOZR of 7.50 mm. Within the next 2 years, significant changes are made to a BOZR of 7.70 and 7.90 mm.
At around the age of 3 years, deposit formation increased rapidly and patients were refitted with hydrophilic or HGP lenses under daily wear conditions (see Table 4). A further analysis of the age relation for the total of 60 extra replacements for deposits revealed a significant positive correlation with age (r = 0.89 at P < 0.05).
Table 5 shows the number of lenses needed up to the age of 4 years (cumulative wearing time 90 years) and the reasons for lens replacements. Although the standard procedure included replacement of lenses every 3 months, this turned out to be lower than planned (3.2 lenses/eye/year) because routine replacements were combined with lenses replaced for other, specific reasons. Extra lens replacements per eye per year were made, in order of importance, because of deposits (0.7), power (0.7), BOZR (0.4), lens loss (0.4), defects (0.2) and diameter (0.04). The total average number of lenses used per eye/year was 5.6. A dramatic spread in lens usage per child was found. Some children needed only the routine 4 lenses per eye/year, whereas others used many more lenses; results showed that replacement also was often needed for the same reason per child.
Generally, the results on the fitting procedure with Silsoft contact lenses in aphakia after congenital cataract are in accordance with data found by Lightman and Marshall. 22 It was also found that, up to 6 months of age, a BOZR of 7.50 mm and a diameter of 11.3 mm are good starting points for successful fitting without keratometry. In some cases, increasing the diameter was a better method for preventing lens decentration than fitting with a steeper BOZR. In our study, the crucial period in which patients needed changes in BOZR also started around the age of 18 months. The shift in BOZR to 7.70 and 7.90 mm takes place between 1.5 and 4 years of age. At age 4 years, 77% of the eyes were wearing a base curve 7.90 mm.
Comparing the average powers needed at 6, 12, 24, and 36 months to those of Lightman and Marshall, 22 our study used, on average, lenses of lower power (1.8 D); however, the rate of power change in our study (0.32 D per month) is practically the same as they reported (0.35 D per month).
The two cases of microphthalmia showed a different pattern in both BOZR and power change, but there were also differences between the cases. In one case, both BOZR and power remained constant up to the age of 6 years, which would indicate no dimensional development of the eye. The other case showed a change of BOZR from 7.50 mm to 7.70 mm at an age of 2 years and a change in power from +32.00 to +26.00 from 3 to 36 months of age, indicating a change in the usual direction but less than normal.
Results of visual acuity data in unilateral cases turned out lower than in bilateral cases. Because there was no selection concerning visual acuity prognoses in this study, this result is not surprising. This study shows that, although successful permanent optical correction is feasible in infant aphakia, it is not the only answer to visual development; this is in accordance with conclusions by Moore. 23 The question remains whether it is worth the risk of surgery and the necessary intensive treatment in case of unilateral congenital cataract if visual acuity remains low. Opinions in the literature are diverse on this issue. Some studies are negative, 24 others show the possibility of stimulating visual acuity but with variable results 25,26 or as possible only for a select group of patients. 27 Although binocularity remains a problem, there are also reports of relatively good results. 28–30 With the advancements in surgical techniques, the risk of operating on infant eyes is greatly reduced, and as shown in our study, permanent optical correction is possible. The remaining issue, particularly in unilateral cases, seems to be the development of optimal orthoptic treatment schemes 23 to reduce the unequal competition between the eyes. 31
Some studies report difficulties in removing deposit formation, 28,32 but these studies are not specific on the age of onset. Because of the higher risk of deposits associated with permanent wear, Lightman and Marshall 22 recommended daily wear. In this study, deposit formation was found to become disturbing around the age of 3 years. Prior to that age, deposit formation was not a significant problem in permanent wear when combined with a routine wetting procedure. However, strict follow-ups, replacement of the lenses whenever needed, intensive instruction to the parents and parental cooperation, together with a clinical setting that provides for emergencies on a 24-hour basis are essential requirements for providing optimal care in this patient group. Also, a lens usage of 5.6 lenses per eye per year seems to be necessary to successfully correct aphakic children after congenital cataract, with silicone contact lenses. It seems that, from the age of 4 years, silicone lenses are no longer compatible with tear composition and should, in most cases, be replaced by daily-wear high-water content soft lenses, or, at a later age, replaced with HGP lenses.
With this publication we honor our dearest friend and the initiator of this study, Dr. Jan H.C. Kok who died at a much too early age on November 21, 1998. We would also like to thank V.S. Gangaram Panday, M.D. for his help in processing the data.
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