22.2 Eye disorders
Examination of the eyes should be included in all general medical paediatric checks because it is only through timeliness of diagnosis of ophthalmic pathology that the best vision can be achieved.
Development of vision
At birth, an infant has a visual acuity of approximately 6/120 and by 12 months this has improved to about 6/12. This rapid development is the result of retinal maturation, myelination of the visual pathways, the ability to accommodate (change the focal length of the eye) and maturation within the visual cortex. The maturation of vision occurs probably until 14 years of age, with the most rapid phase being in the first 2 years, and subtle changes occurring after 8 years of age.
Measurement of vision in children
Asking a parent ‘Does your child see well?’ or ‘How well do you think your child sees?’ often gives useful information about an infant’s visual function. If a parent expresses concern about an infant’s vision, take note, as this concern is often well founded.
An understanding of normal visual behaviour is vital to estimating visual function in infancy. At birth, when alert, an infant should be able to fix on a face briefly. By 6 weeks of age most infants smile in a visually responsive fashion to a face. At this age the infant will also be able to follow a face or light. By 6 months of age an infant can actively follow objects in the visual environment. Comments on an infant’s ability to ‘fix’ and/or ‘follow’ are very useful qualitative measures of vision. Forced preferential looking tests are used to measure vision quantitatively.
Picture-naming tests can be done by children between 2 and 3 years of age, and single letter-matching tests are within the abilities of most 3–4-year-olds. The standard Snellen chart test is generally not performed well until the child is between 5 and 6 years of age.
Children with specific language delay or intellectual delay will have difficulty with some tests of visual acuity, and forced preferential looking tests may be more appropriate.
The vision should be tested for each eye individually.
Repeat the test on another occasion if the test results seem inaccurate.
The notation for documenting visual acuity is often based on the Snellen fraction (e.g. 6/6). Most visual acuity tests use standard distances of 3 or 6 m between subject and chart. The numerator of the Snellen fraction is the distance from the chart, whereas the denominator indicates which line on the chart was the smallest to be seen. If the vision is poor, the subject should be brought closer to the chart. The vision then may be recorded as 2/18 or 1/60, etc., depending on how close the subject is to the chart and which line is read.
What level of vision is abnormal?
An infant who is not fixing and following must be examined further and investigated.
In an older child, a vision of worse than 6/9 is a reasonable cut-off for referral. In addition, a difference in visual acuity between the two eyes of two or more lines indicates the need for further assessment.
Assessment of a child with a possible eye problem
History
Prematurity, perinatal difficulties (e.g. birth asphyxia), significant syndromes (e.g. Down syndrome) and other sensory impairment (e.g. deafness) are all associated with an increased risk of eye disease. Common childhood eye problems such as strabismus and refractive errors have a familial tendency, although the precise genetics are not well understood. Finally, the parents’ perception of a child’s visual function is important, particularly if there is concern that the vision is poor.
Examination
As with any paediatric medical examination, observation of the child in the environment of the waiting room, walking towards your clinical room and in the clinical room is generally the key to diagnosis. Observe whether the child smiles at a face, looks around the room or follows moving objects.
Systematic examination of the eye involves dividing the areas into three: external (eyelids, eyelashes and periorbital region), anterior segment (cornea, pupil, iris and lens) and posterior segment (vitreous cavity, optic nerve and retina).
Most eyelid, eyelash and ocular surface abnormalities can be detected by observation. Many intraocular abnormalities can be detected by examination of the ‘red reflex’. This is the red to orange colour seen within the pupil when the line of illumination and observation are approximately coaxial (that is, the same). This situation is most easily obtained by observing the child’s eye with a direct ophthalmoscope from a distance of about 1 m. The light reflex for each eye can be compared. A dull or absent red reflex indicates an opacity, such as a cataract, in the normally clear media of the eye. A white reflex results from an abnormally pale reflecting surface within the eye, such as a white retinal tumour (retinoblastoma; Fig. 22.2.1) Although these intraocular disorders are rare, they are important in terms of the severe effect on vision or threat to life.
Misalignment of the eyes
Strabismus or squint occurs in 3–4% of children. Observation will confirm the presence of a large-angle strabismus. However, a broad nasal bridge or prominent epicanthic folds will mimic milder degrees of strabismus, especially in younger infants. This condition is known as pseudostrabismus (Fig. 22.2.2). The epicanthic folds cover the sclera on the medial aspect of the globe, while the lateral sclera is easily visible. This creates the appearance of misalignment, particularly when the child looks laterally. Examination of the symmetry of corneal light reflections will aid in determining whether there is an esotropia (in-turning of the eyes) or only pseudostrabismus.
Fig. 22.2.2 This infant has prominent epicanthic folds, giving rise to the appearance of misaligned eyes. This is pseudostrabismus. Note that the corneal light reflections are symmetrical. Cover testing failed to reveal misalignment of either eye.
A cover test is a reliable method of detecting strabismus. The cover test is done by first getting the child to fix on an object while the observer determines which eye appears to be misaligned. The eye that appears to be fixing on the object (and not misaligned) is then covered while the apparently misaligned eye is observed. If strabismus is present, a corrective movement of the misaligned eye will be seen as this eye takes up fixation on the object of regard (Fig. 22.2.3). If no movement is seen, the eye is uncovered.
Fig. 22.2.3 Cover test. First the child’s attention is attracted with a toy (top). Then the eye that appears to be looking directly at the toy is covered and the other eye is observed for a refixation movement (bottom). In convergent squint there will be an outward movement of the uncovered eye (pictured), and in divergent squint there will be an inward movement of the eye. If no movement is detected, the test should be repeated but covering the other eye first.
The cover test is then repeated, covering the other eye this time; the eye that is not covered is again observed for a corrective movement and, if present, strabismus is confirmed. The test can be repeated as many times as necessary. If no movement is seen following repeated covering of either eye, then strabismus is not present. Care must be taken to allow the child to fix with both eyes open before covering either eye, otherwise normal binocular control may be prevented and a small latent squint (phoria) may be detected. Latent squints are normal variants and are of no significance.
Common eye problems in childhood
Amblyopia
Amblyopia, the cortical response to abnormal input from the eyes and manifest as reduced visual acuity in one or both eyes, results from refractive (spectacle) error (in one or both eyes), strabismus or from deprivation (e.g. cataracts). Amblyopia usually responds to treatment if detected in a timely fashion. Detection of amblyopia is one of the major reasons for routine visual screening in childhood as the earlier it is detected the more chance there is that treatment will be successful.
Refractive errors cause a poorly focused image to be transmitted from the retina to the cortex. Such input does not stimulate normal cortical development and amblyopia results.
Strabismic (misaligned) eyes each send a different view of the world to the cortex. If the brain ‘paid attention’ to the image from each eye, diplopia would ensue. However, the immature visual cortex is capable of ignoring the image from one eye. Eventually the cortex may suppress the input from a deviating eye, with resulting amblyopia.
Treatment of amblyopia involves the correction of any focusing errors with appropriate spectacles and forcing the brain to use the amblyopic eye by depriving the brain of clear input from the better-seeing eye. This is commonly done with a patch. Unfortunately, realigning strabismic eyes is not enough to overcome amblyopia secondary to strabismus.
Strabismus
A squint or misaligned eye is frequently associated with amblyopia. Childhood strabismus is often the result of failure of binocular control at a cortical level. Less commonly it is the result of cranial nerve lesions or extraocular muscle disease. In most children, strabismus is not associated with neurological or intellectual problems. However, children with widespread central nervous system abnormalities have an increased risk of developing strabismus. Down syndrome is a good example of this, with an approximately 10-fold increased risk of developing strabismus.
The following is a brief description of frequent patterns of strabismus seen in childhood and an outline of their management.
Infantile esotropia
This is a large-angle convergent squint seen before 6 months of age. Strabismic amblyopia is common in infantile esotropia, but refractive errors are rare. Occlusion therapy and surgery are the most common treatments. Children with infantile esotropia need to be followed up throughout childhood: approximately one-third require further surgery, and amblyopia can occur despite adequate eye alignment.
Intermittent divergent strabismus (exotropia)
While exotropia can occur in infancy, it is more common from 18 months of age. It is often more noticeable on distance fixation and may be associated with monocular closure in bright light. Amblyopia is less commonly associated with exotropia than with esotropia because the deviation is intermittent and, presumably, when the eyes are straight normal visual development proceeds. In some cases the divergence becomes more constant and surgery may be undertaken to improve alignment.
Accommodative esotropia
This occurs in children who are excessively ‘long-sighted’ (hypermetropic). To overcome hypermetropia and focus a clear image on the retina, accommodative effort is used. Accommodation consists of the combination of changing focal length of the lens together with convergence of the eyes (so that both are directed at the near object of regard). Thus, in children with excessive hypermetropia there is increased focusing and at times excessive convergence; a convergent squint (esotropia) appears as a result of the increased accommodative effort used by these children. Accommodative esotropia can be completely or partially corrected by prescribing glasses that compensate for the appropriate amount of hypermetropia. Amblyopia, sometimes in both eyes, is often seen in association with an accommodative esotropia. Occlusion therapy may be required. If glasses only partly correct the esotropia, surgery may be indicated to obtain optimal alignment.
Clinical exampleA 3-year-old girl presented with a history of a worsening inward turn of her left eye over 4 months. The cover test confirmed a left convergent squint and the red reflex was normal in each eye. Subsequent assessment by an ophthalmologist confirmed the findings and the girl’s visual acuities were 3/3 in the right eye and 3/9 in the left, with refraction showing that she was hypermetropic. When seen 4 months later she was wearing glasses and had been patching her right eye for 2 hours a day. Her eyes were straight to cover test, her left eye vision was improving and strabismus surgery was less likely to be appropriate.
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