A baby who is emmetropic or myopic at birth is likely to become progressively more myopic during childhood and adolescence.
Practical advice
Eighty-three per cent of neonates will follow a face but not a white light. A smile in response to a silent smile should be present at 6 weeks of age and indicates good central vision.
2.2 Assessment of visual function
Assessment of the visual acuity of a neonate or a small baby requires patience, attention to detail and a modifi cation of the techniques used for adults. The examiner should be prepared to spend a substantial amount of time on the examination. At birth the full-term neonate can see colours and faces at arm’s length, and fi xation should be present. In the term infant, fi xation is the most reliable clinical test of visual function. The type of fi xation target used, however, is of paramount importance.
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29 Visually directed reaching, although possible at 3–4 months, is usually not a useful clinical test until approximately 6 months of age. Other important signs in a baby who is suspected of being visually impaired include the presence of abnormal ocular move- ments, abnormal pupillary reactions (although often diffi cult to illicit in the neonate), the presence of eye rubbing or poking, lack of facial mobility or expression, and the absence of optokinetic nystagmus.
Practical advice
Abnormal eye movements and visual inattention are the most common signs of poor vision. The more uncoordinated the movements, the more impaired the visual acuity.
The parents are often the fi rst to sense that there may be a visual problem with a young baby. Experienced mothers tend to present their children earlier. It is important not to dismiss parental con- cerns, as this may backfi re. A useful adage is: ‘If in doubt, believe the mother and either re-examine or refer’. Even if the eyes look normal, the parents may complain of the child not fi xing or reacting appropriately to visual stimuli. There may be an obvious abnormal- ity such as a white pupillary refl ex ‘leucocoria’ or a squint. Occa- sionally family snapshots using fl ash photography show up the absence of a red refl ex and bring this to the parent’s attention.
Practical advice
If in doubt, believe the mother and either re-examine or refer.
On examination, the position and steadiness of the eyes in primary gaze is important. The presence of a persistent squint may indicate that there is an opacity of the media. It must be remembered that transient losses of binocular fi xation can be normal for the fi rst 3–4 months of life; after this age, most infants demonstrate con- sistent binocular ocular alignment over a range of stimulus dis- tances.4 Fusion in infants develops between 4.5 and 6 months of life. Any abnormal ocular movements such as nystagmus, or more rare abnormalities such as saccadomania or ‘dancing eyes’, may be indicative of a midline cerebral tumour.
The presence of leucocoria (white refl ex) points to the diagnosis of cataract, primary hyperplastic vitreous or retinoblastoma. Enlargement of the globe may indicate congenital glaucoma. If there is no opacity of the media, detailed examination of the fundus, and in particular of the optic discs, is possible. Optokinetic nystag- mus is not a test that is specifi c for visual acuity, but one that is representative of the integrity of the visual system. It is normally absent in blind and severely brain-damaged children. In children with a profound visual problem, the parents often notice that the baby appears to be unresponsive to visual stimuli.
2.2.1 Electrophysiological testing
Electrophysiological testing is extremely important in the assess- ment of poor vision in neonates and children. Experience and patience are required for the acquisition of good quality results in the very young. These tests not only help to secure specifi c diag- noses, but by systematic assessment of function along the visual pathways can also localise the problem underlying the visual defects. Among children, development as well as disease can affect electrophysiological parameters (Fig. 2.1); therefore, the diagnosis of abnormality depends critically on knowledge of the normal responses for age.5 A brief description of the most common elec- trophysiology tests and their function is given below.
• Visually evoked potential (VEP) detects dysfunction of the visual
pathways from the optic nerve to the visual cortex
• Electroretinography (ERG) assesses the function of the neuro-
retina. Different stimuli can be used to assess the function of the rod and cone photoreceptors separately. The ERG waveform can also be used to differentiate between dysfunction in different layers of the neuroretina and is an indicator of whether the problem lies within the macular area or the optic nerve. The P50 component is representative of macular function and the N95 component of optic nerve function, although an abnormality in one may infl uence the other
• Electro-oculography (EOG) assesses the integrity of the retinal
pigment epithelial (RPE) cells
The interpretation of electrophysiological data is a complex issue and must always be made with reference to the clinical fi ndings. Examples of expected electrophysiology abnormalities in inherited disorders are given below (see Table 2.2, p. 39)
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31 Figure 2.1 A, Normal waveforms for standard electrophysiology tests
(a–f). The ERG represents the combined electrical activity within the retina. The ‘a’ wave represents the activity of the photoreceptors; the ‘b’ wave has its origin in the Muller (glial) cells. (a) The pattern ERG demonstrates good macular function (P50) peak and normal optic nerve function (N95) trough. Normal cone function is shown by a normal cone ERG (using a bright red fl ash) (b) and a normal 30 Hz fl icker ERG (c). Normal rod function is illustrated by a normal rod ERG tested under conditions that preferentially stimulate rod function (dim blue fl ash) (d). The maximal ERG response gives an indication of total photoreceptor function (e). The EOG waveform, which refl ects retinal pigment epithelial function, indicates a normal ‘dark to light’ rise which refl ects the comparison of amplitudes under dark and light adapted states (f). 5 0 OD OS –5 0 50
P50 max (ms) N95 max (ms) b wave max (ms)
a wave max (ms) b wave max (ms) a wave max (ms) b wave max (ms)
b wave max (ms) 100 Pattern ERG 40’ check (a) (d) (b) (e) (c) (f) Rod ERG
Dim blue flash (subject dark adapted)
Cone ERG Bright red flash
Maximal ERG Bright white flash
30-Hz Flicker ERG Moderate white flashes
EOG Light peak/Dark trough %
Dark adaptation/Light adaptation (min)
150 200 0 50 100 150 200 0 20 40 60 80 100 0 50 100 0 50 100 150 200 0 5 10 15 20 25 30 uV 200 0 OD OS –200 uV 500 0 0 OD OD OS OS –500 200 uV 500 0 OD OS –500 uV 500 0 OD OS –500 uV –200 Oscillatory potentials P50 N95 a wave b wave b wave b wave b wave Light peak Dark trough a wave A Ch002-H1815.indd 31 9/15/2006 12:06:54 PM
Figure 2.1 B, Electrodiagnostic tests from a patient with retinitis
pigmentosa showing ‘fl at’ tracings for the maximal ERG (e), cone ERG (b) and rod ERG (d), indicating profound loss of photoreceptor function. There is also absence of the ‘light rise’ on the EOG (f). Interestingly, the patient’s central vision remains 6/9 [Log MAR 0.2] in either eye, indicating some preservation of central photoreceptors. This is confi rmed by the pattern ERG (a), which shows an abnormal but relatively preserved waveform.
5 0 RE LE –5 0 50
P50 max (ms) N95 max (ms) b wave max (ms)
a wave max (ms) b wave max (ms) a wave max (ms) b wave max (ms)
b wave max (ms) 100 Pattern ERG 40’ check (a) (d) (b) (e) (c) (f) Rod ERG
Cone ERG Maximal ERG
30-Hz Flicker ERG EOG
Dark adaptation/Light adaptation (min)
150 200 0 50 100 150 200 0 20 40 60 80 100 0 50 100 0 50 100 150 200 0 5 10 15 20 25 30 uV 200 0 RE LE –200 uV 200 0 0 RE RE LE LE –200 50 50 uV 500 0 RE LE –500 uV 333 0 RE LE –333 uV – B
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