Refractive errors

• Aphakia (absent lens)
• Anisometropia
• Astigmatism (blurring of vision)
• Hypermetropia (far sightedness)
• Myopia (near or short sightedness)
• The correction of refractive errors

Light entering the eye is focused (refracted) by the cornea and lens. Under normal circumstances the light rays are focused exactly on the retina resulting in a clear image. A refractive error occurs when the light is not focused accurately on the retina, resulting in a blurred image.

Accommodation is the process by which the eye changes optical power to maintain a clear image (focus) on an object as its distance changes. As an object moves closer, the ciliary (eyelid) muscles in the eye contract, the lens becomes fatter and its refracting power greater so that light rays are focused onto the retina. In addition, the eyes normally turn inwards (converge) when focusing on a near object. Accommodation and convergence are linked within the control centres in the brain and occur in relation to each other to maintain clarity of vision and appropriate focus.

Aphakia (absent lens)

Aphakia is the condition where the lens has been removed because of a congenital (born with) or developmental cataract or the lens is lost through trauma or is absent. Children with this condition either need contact lenses, spectacles with a high power convex lens or the insertion of a replacement lens in the eye (intraocular lens).

Anisometropia

In this condition one or both eyes have a significantly different error (e.g. high hypermetropia in one eye and a normal other eye). This may lead to asymmetric (unequal) development of vision with the visual cortex ‘favouring’ the normal eye or the eye with the least error and poor vision in the affected eye.

Astigmatism (blurred vision)

Astigmatism occurs when the cornea is irregularly shaped so that light is not focused regularly onto the retina and vision is blurred at all distances. Cylindrical lenses are prescribed for constant use.

The child may have a combination of astigmatism with hypermetropia/myopia.

Hypermetropia (far sightedness)

In hypermetropia, the eyeball is too short or the refractive power of the cornea and lens are too weak, and light from a near object is focused behind the retina. Children with hypermetropia have difficulty seeing near objects clearly, but if the refractive error is mild they can accommodate to focus light from near objects onto the retina. If correction is required, a convex lens which causes light to converge before hitting the cornea must be used. Again spectacles are usually prescribed for constant use.

Myopia (near or short sightedness)

In myopia, the eyeball is too long or the refractive power of the cornea and lens are too great, and light from a distant object is focused in front of the retina. Myopic children can only see near objects clearly. To correct this, a concave lens which causes light to diverge before hitting the cornea must be used. In children the spectacles are prescribed for constant use.

The correction of refractive errors

Hypermetropia (a shorter eye) is the normal condition at birth and there is a high incidence of astigmatism. Subsequent eye growth normalises the refractive state of the eye. Therefore, spectacles are only prescribed if the error is beyond what would be expected and/or there is an associated eye condition such as squint (misalignment of eyes).

If the error is beyond the norm, correction may be achieved by using spectacles or contact lenses (hard or soft) or by surgery. “Laser Surgery” is at present not recommended for children in routine clinical practice

Mild refractive errors may not need correction if the child is functioning well.

Severe refractive errors may require correction to improve vision and because they can cause amblyopia (lazy eye). In addition, if there is a significant difference in refractive error between the two eyes, this requires correction to avoid amblyopia developing in the eye that is poorly focused. In children with other disabilities, even small refractive errors may need to be corrected to optimise performance.

Spectacles are also used to treat squint, usually when the child has hypermetropia and the eyes turn in (esotropia) or less commonly if the child has myopia and the eyes turn out (exotropia). Spectacles are necessary and form an important part of the treatment if misaligned eyes are straight with spectacles. Surgery to correct misalignment is not then indicated. If the squint is only partially corrected then surgery to align the eyes is undertaken to correct the degree of squint present with spectacles.

Spectacles can be prescribed for the youngest infant and for the child with multiple disabilities because the refractive error can be determined objectively using lenses of varying power.

Prognosis: If the refractive error is detected early and corrected with good compliance, the outlook is good.