Chapter 119 Contact lenses for children
Parents are the key to successfully fitting children with contact lenses. Explain the reasons why a particular lens type is chosen as well as the risks and advantages of lens wear. With the available range of lenses it is rare to find a case that cannot safely be fitted, but the tolerance of the child or the visual limitations of the parent may make it preferable to avoid or postpone lens wear.
Any type of lens can be fitted. The choice of lens depends on the condition being fitted, when the lenses are going to be worn, and the limitations of each material, design, and parameter range (Table 119.1). Rigid lenses have a name for being uncomfortable, but two drops of a local anesthetic can overcome any problems. By the time the anesthetic has worn off, the lens has usually settled.
Stress the need for strict hygiene with a simple, effective, regimen. Solutions that could cause an allergic response should be avoided. Preservative-free solutions may be preferable. The rub and rinse step prior to lens soaking greatly decreases the risk of infection and is recommended by the FDA.1
Contact lenses provide a clearer field of vision than spectacles.2 High myopes have better visual acuity with contact lenses than with spectacles, which reduce the image size. However, high hypermetropes and aphakes have better acuity in spectacles because the image size is larger.2 This can lead to disappointment when aphakes with poor acuity are fitted with contact lenses.
Contact lenses reduce aniseikonia for both unilateral axial and refractive myopia,3 but amblyopia is usually present and highly myopic eyes (over –9.00 D) do not usually respond well to patching;4 only 32% of unilateral aphakes achieve vision greater than 0.6 logMAR5 and, because binocularity is rare, children do not notice much difference when wearing a lens, making compliance poor. However, wearing a contact lens often reduces the angle of any strabismus.
The number of young aphakes requiring contact lenses has decreased in recent years as more infants have intraocular lenses (IOLs) inserted. Many of them are likely to have microphthalmos or other anomalies. Pseudophakes also frequently need to be corrected with contact lenses.
Carry out retinoscopy with the trial lens held close to the child’s eye. Back vertex powers (BVPs) are usually greater than +20.00 D (the effective power of a +20.00 D trial lens with a back vertex distance (BVD) of 16 mm is +29.41 D at the cornea). Estimate the BVD and calculate the power of the contact lens then add +3.00 D overcorrection to provide a close focus for infants. The final BVP can be more than +40.00 DS.
During the first 2 years, refractive errors reduce, K readings and corneal diameters increase. Contact lens size alters to more adult values and the refraction is changed to distance correction with bifocal spectacles for near. Bifocal or multifocal contact lenses are difficult to fit accurately in high prescriptions.
• Soft or silicone hydrogel (SiH) lens with the back optic zone radius (BOZR) approximately 0.3 mm flatter than average K and total diameter ≥2 mm larger than corneal diameter. With microcornea, larger lenses will help center the lenticular portion over the pupil. These lenses are not available from most multinational companies so order tailor-made lenses from a smaller manufacturer. Infants may rub their lenses out and lenses are prone to dry out and mislocate or fall out.7
• Silicone rubber lenses do not dry out and are not easily rubbed out. They are fitted on flattest K and assessed using fluorescein. The high Dk (oxygen permeability) means that they can be worn overnight in emergencies. The steepest lens is 7.50 mm BOZR, too flat for many aphakes.
• Rigid lenses are rubbed out less often. They can be made of hyper Dk materials. Paralimbal lenses with a decreased edge lift will lessen the risk of the lens becoming dislodged.7 The edge lift may need to be increased when the child is older.
Intraocular lenses usually give better acuity than contact lenses.8 After trauma, lens insertion can be difficult; spectacles may be preferable.
In Marfan’s syndrome, the corneas can be very flat, possibly flatter than 9.00 mm (37.5 D). One eye may be surgically rendered aphakic while the second eye might have a high degree of myopic astigmatism associated with the dislocated lens. Contact lenses provide the best chance of binocularity.
• RGP corneal lenses may give better acuity but aphakic lenses are liable to sit low because the center of gravity of the lens is in front of the cornea. This problem is exacerbated on a flat cornea (Fig. 119.2).
Blepharoconjunctivitis can lead to keratitis and corneal scarring which affects acuity, possibly resulting in amblyopia. Spectacles are unlikely to give the optimum acuity but may be the only option until the acute infection has settled when contact lenses can be fitted.