Visual Impairment in the Young PDF

2 CHAPTER Visual impairment in the young Giuliana Silvestri Visual loss in childhood and the early teens is predominantly congenital or hereditary in nature.1 In this chapter the various conditions causing visual impairment in this age group are dis- cussed, with particular emphasis on the diagnos...

2 CHAPTER Visual impairment in the young Giuliana Silvestri Visual loss in childhood and the early teens is predominantly congenital or hereditary in nature.1 In this chapter the various conditions causing visual impairment in this age group are dis- cussed, with particular emphasis on the diagnostic features that are helpful in making the correct diagnosis. The three most common clinical scenarios presenting to the ophthalmologist are discussed in detail: The neonate or young baby who appears to be visually impaired The visually impaired baby with no obvious ophthalmic abnormality The child with visual difficulty. Recent advances in the understanding of the molecular genetics of many of these conditions have been truly exciting, and a brief review of the recent molecular discoveries is given where relevant. 2.1 Visual development The refractive state of the normal neonate is 2–3 dioptres of hyper- metropia, accompanied by a high prevalence of astigmatism. At birth the globe measures 16.5 mm in diameter and over the first year the diameter increases rapidly to reach an almost adult 27  Ophthalmology for low vision diameter of 24.5 mm.2 The spherical equivalent refractive error of full-term newborn infants is normally distributed about a mean of +2 dioptres with a standard deviation of 2 dioptres.3 The retina is well developed at term; however, the foveal region is immature, not reaching adult maturity until 4 months of age. The neural pathways are also immature at term, with cells in the lateral genic- ulate ganglion reaching adult size at age 2 years and the optic nerve becoming fully myelinated at 7 months. Coupled with increasing maturation of the visual system, the hypermetropic state reduces steadily and visual acuity improves. By the age of 20–30 months the visual acuity is estimated to be 6/6 [LogMAR 0.0]. In young children with a refractive error, the standard devia- tion of the error is greater the earlier the onset of the visual impair- ment; for example, the refractive error in tyrosinase-negative albinism would be much greater than that in Stargardt’s disease where near-normal variability is expected. Practical advice A baby who is emmetropic or myopic at birth is likely to become progressively more myopic during childhood and adolescence. 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 modification 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 fixation should be present. In the term infant, fixation is the most reliable clinical test of visual function. The type of fixation target used, however, is of paramount importance. 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. 28  Visual impairment in the young 2 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 difficult 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 first 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 backfire. 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 fixing or reacting appropriately to visual stimuli. There may be an obvious abnormal- ity such as a white pupillary reflex ‘leucocoria’ or a squint. Occa- sionally family snapshots using flash photography show up the absence of a red reflex 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 fixation can be normal for the first 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. 29  Ophthalmology for low vision The presence of leucocoria (white reflex) 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 specific 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 specific 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 influence 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 findings. Examples of expected electrophysiology abnormalities in inherited disorders are given below (see Table 2.2, p. 39) 30  Visual impairment in the young 2 (a) Pattern ERG (d) Rod ERG 40’ check Dim blue flash (subject dark adapted) 5 P50 b wave 200 OD OD uV uV 0 N95 0 OS OS –5 –200 0 50 100 150 200 0 50 100 150 200 P50 max (ms) N95 max (ms) b wave max (ms) (b) Cone ERG (e) Maximal ERG Bright red flash Bright white flash 500 b wave 500 b wave OD OD Oscillatory a wave potentials uV uV 0 0 a wave OS OS –500 –500 0 20 40 60 80 100 0 50 100 a wave max (ms) b wave max (ms) a wave max (ms) b wave max (ms) (c) 30-Hz Flicker ERG (f) EOG Moderate white flashes Light peak/Dark trough % 500 200 b wave OD OD Dark trough Light peak uV 0 0 OS OS –200 –500 0 50 100 150 200 0 5 10 15 20 25 30 b wave max (ms) Dark adaptation/Light adaptation (min) A 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 flash) (b) and a normal 30 Hz flicker ERG (c). Normal rod function is illustrated by a normal rod ERG tested under conditions that preferentially stimulate rod function (dim blue flash) (d). The maximal ERG response gives an indication of total photoreceptor function (e). The EOG waveform, which reflects retinal pigment epithelial function, indicates a normal ‘dark to light’ rise which reflects the comparison of amplitudes under dark and light adapted states (f). 31  Ophthalmology for low vision (a) Pattern ERG (d) Rod ERG 40’ check 5 200 RE RE uV uV 0 0 LE LE –5 –200 0 50 100 150 200 0 50 100 150 200 P50 max (ms) N95 max (ms) b wave max (ms) (b) (e) Cone ERG Maximal ERG 200 500 RE RE uV uV 0 0 LE LE –200 –500 0 20 40 60 80 100 0 50 100 a wave max (ms) b wave max (ms) a wave max (ms) b wave max (ms) (c) 30-Hz Flicker ERG (f) EOG 333 50 RE RE uV 0 0 LE LE – 50 –333 0 50 100 150 200 0 5 10 15 20 25 30 b wave max (ms) Dark adaptation/Light adaptation (min) B Figure 2.1 B, Electrodiagnostic tests from a patient with retinitis pigmentosa showing ‘flat’ 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 confirmed by the pattern ERG (a), which shows an abnormal but relatively preserved waveform. 32  Visual impairment in the young 2 2.3 Clinical scenarios 2.3.1 The neonate or young baby who appears to be visually impaired When a baby with a suspected visual problem is presented to the ophthalmologist, important clinical signs to look for are those detailed in Section 2.2. If there appears to be a visual problem, the working differential diagnosis is as follows (this list is given in alphabetical order and is not all inclusive, as many other rare conditions exist): Albinism Cerebral blindness Congenital cataract Congenital glaucoma Congenital idiopathic nystagmus High refractive error Leber’s congenital amaurosis Macular colobomata Optic atrophy or hypoplasia Primary hyperplastic vitreous Retinoblastoma Retinopathy of prematurity. Retinopathy of prematurity (ROP) is unlikely in a full-term infant, but if the baby was of low birthweight or pre-term (born earlier than 37 weeks) ROP should be considered. Babies with a birthweight of less than 1500 g are at risk of ROP and should be screened rou- tinely. Total visual loss from ROP has been reported in 2–4% of babies who weigh less than 940 g.6 Examination of the optic discs may reveal the diagnostic double halo of hypoplastic discs or the normal-sized but pale discs of optic atrophy. If an intracranial tumour is present, papilloedema may be seen. The heredofamilial optic atrophies are characterised by their patterns of inheritance, and hereditary optic atrophy can present in several different ways. The clinical findings and age of onset are different in each group. A simplified list of the spectrum of the inherited optic atrophies is shown in Table 2.1. The presence of leucocoria is often reported by the parents and, although the most common cause of leucocoria is cataract, retino- blastoma must always be excluded promptly. Retinoblastoma is a rare life-threatening tumour with complex genetic inheritance. 33  Ophthalmology for low vision Those affected demonstrate a predisposition to retinoblastoma, which can be either heritable or non-heritable. Childhood cataract, although surgically treatable, causes significant visual morbidity because of associated amblyopia, even with early surgical treat- ment. Cataracts that are present at birth or become apparent in the first year of life account for just under 20% of all causes of blind registration in children under the age of 15 years in England and Wales. Genetically determined cataracts are a hetero- geneous group of disorders.7 Approximately 25% of congenital cataract is inherited in an autosomal dominant manner; autosomal recessive cataract is uncommon in Britain and is more prevalent in communities where consanguinity is common. X-linked cata- racts are rare. They usually occur in isolation, although congenital cataract can be associated with other ophthalmic manifestations such as microphthalmos, glaucoma and coloboma. Congenital cataract can also be part of a systemic syndrome, of which there are many. Congenital glaucoma occurs in 1 in 10 000 births in Western countries. Although several modes of inheritance have been documented, in most families the disorder is transmitted in an autosomal dominant manner with full penetrance. A full account of the molecular genetic details can be found at the Online Mendelian Inheritance in Man (OMIM) website (http://www.ncbi.nlm.nih.gov/omim).8 Even though the ocular globe is often enlarged at birth in congenital glaucoma owing to raised intraocular pressure in utero, an early diagnosis is advanta- geous in limiting field loss and preventing increasing buphthal- mos. Evidence also exists indicating that, owing to the plasticity of the visual system, young babies may have some potential for optic nerve recovery if treated early. In the absence of a positive family history, the diagnosis can sometimes be overlooked in the early stages. Congenital glaucoma often presents with a ‘watery eye’. The crucial question on history taking is to determine whether the ‘epiphora’ was present at birth or developed later. Epiphora from nasolacrimal duct obstruction is usually not present at birth but begins when tear production becomes well established. A watery eye at birth should flag up the diagnosis of congenital glaucoma. Congenital idiopathic nystagmus and macular colobomata are rare causes of poor central vision, whereas primary hyperplastic vitreous presents as an opacity in the media. High refractive errors should always be kept in mind in these cases. 34  Table 2.1 Clinical Features of the Inherited Optic Atrophies Juvenile Congenital Behr’s Optic atrophy Leber’s optic (infantile) (simple) syndrome with diabetes neuropathy ± deafness Pattern of Dominant Recessive Recessive Recessive Mitochondrial inheritance inheritance Age of onset 4–8 3–4 1–9 6–14 11–30 (years) Visual loss Mild/moderate Severe Moderate Severe Moderate to severe, depending on the mutation (see section 2.5.1) Final visual 6/12 [LogMAR 0.3] 6/60 [LogMAR 1.0] 6/60 [LogMAR 1.0] 3/60 [LogMAR 1.3] 6/60 [LogMAR 1.0] acuity to 6/60 to hand motion to 1/60 to 1/60 [LogMAR 1.0] [LogMAR 1.8] [LogMAR 1.8] Colour vision Blue/yellow defect. Severe Moderately severe Severe Dense central Note: if acquired dyschromatopsia dyschromatopsia dyschromatopsia scotoma for colours red/green defect – wrong diagnosis Nystagmus Rare Usual In 50% No No Optic discs Mild temporal Marked diffuse Mild temporal Marked diffuse Disc swelling and pallor pallor pallor pallor telangiectatic vessels Visual impairment in the young in acute stage. Moderately diffuse pallor 35 2  Ophthalmology for low vision Practical advice As epiphora from nasolacrimal duct obstruction is usually not present at birth but begins when tear production becomes well established, a watery eye at birth should flag up a diagnosis of congenital glaucoma. Children with multiple handicaps Children who suffer from congenital conditions such as rubella, cytomegalovirus, toxoplasmosis or fetal alcohol syndrome often have multiple handicaps that can involve hearing as well as vision. Recent studies have indicated that almost half of visually impaired children have cerebral palsy and the majority will exhibit behavioural problems (63%), learning disabilities (50%) and other sensory deficit (hearing impairment 18%) early in childhood.9 2.3.2 The visually impaired baby with no obvious ophthalmic abnormality If the baby is visually impaired but no obvious ophthalmological problem is detectable clinically, the following diagnoses should be strongly reconsidered and the baby re-examined. These condi- tions can be difficult to detect in the early stages: Albinism – oculocutaneous (tyrosinase positive) and ocular albinism Cerebral blindness Delayed visual development Leber’s congenital amaurosis Optic atrophy Optic disc hypoplasia. In the early stages, Leber’s congenital amaurosis often shows no abnormality on fundoscopy; the optic discs, however, can some- times appear slightly pale, and early thinning of the arterioles is an important early sign in this condition. The pupillary reflexes, although sometimes normal, are often sluggish or absent. The baby is often photophobic. Electrophysiological testing is helpful for diagnostic purposes. Recently, Leber’s congenital amaurosis has been mapped to five genetic loci, indicating a spectrum of disease manifestation or phenotypic variation. Exact details can 36  Visual impairment in the young 2 be located on the OMIM website.8,10 Leber’s amaurosis is listed as disorder no. 204000. Optic disc hypoplasia can be subtle and is easily missed unless specifically looked for with the direct ophthalmoscope. Compari- son of the optic disc size with the large aperture on the ophthal- moscope can be useful in diagnosing optic disc hypoplasia. Indirect ophthalmoscopy alone is not sufficiently sensitive to pick up hypoplasia of the optic disc. Other helpful signs include the presence of nystagmus, sluggish pupillary reflexes and the char- acteristic optic disc double ring sign. Practical advice The large target on the Welch Allyn direct ophthalmoscope is the same size as an average optic disc. Cerebral blindness manifests with normal ophthalmic findings, including the presence of normal pupillary reflexes. In cerebral blindness, the electroencephalogram (EEG) is abnormal; the baby often has other signs of developmental delay and may have midline defects including cleft lip or palate. Albinism can be very evident and a family history may be present, although the signs can be subtle in tyrosinase-positive ocular albi- nism. The pupillary reflexes are normal, as are the optic discs, but the baby may be photophobic. Iris transillumination is present, but may be difficult to elicit in a young baby using the slit lamp. The fundus will appear albinotic or blond.11 Practical advice An easy way of detecting iris transillumination in a baby is to place the transilluminator from the ophthalmoscope on the lower lid in a darkened room. Iris defects will be seen easily. Delayed visual development involves no ophthalmological or electro- physiological abnormality. The baby is often premature or small for dates (smaller than the expected weight for age). Except in children with multiple handicaps, it is unusual for delay to persist 37  Ophthalmology for low vision beyond the age of 4 months.12 Although the prognosis for vision is generally good, a small proportion of patients are left with a resid- ual deficit. The electrophysiological findings in the conditions described above are summarised in Table 2.2. 2.3.3 The child with visual difficulty History and examination These children have usually been normally sighted in infancy and have thus had a relatively normal early educational history. Prob- lems usually become apparent late in the first decade. During the early years of primary school, these children begin to have pro- gressive problems with seeing the blackboard and subsequently with small print and low contrast material. In this age group a history of the visual symptoms and a positive family history of inherited conditions may also be present. Children with decreased distance acuity with good near acuity may simply be myopic. The normally sighted myope will, however, use a conventional near working distance, whereas the visually impaired myopic child will tend to hold things closer than expected. Other details on history taking that can be helpful include the following. Children suffering from optic atrophy complain of reducing vision but usually have no other specific symptoms. Those suffering from cone dystrophies, however, characteristically complain of intense photophobia and difficulty with colour vision. Practical advice Interestingly, patients with cone dystrophy may remark that their visual acuity is better in dim lighting. Visual acuity can increase by two to three lines in mesopic conditions. The parents of children with early-onset retinitis pigmentosa may report night blindness or restriction of fields, which is often mani- fested as clumsiness. Parents who suffer from retinitis pigmentosa are often acutely sensitive to the presence of night blindness in their children. In retrospect, when asked, people with retinitis pigmentosa will admit to having been night blind from as far back as they can remember. 38  Table 2.2 Inheritance Patterns, Visual Prognosis, Visual Field Loss and Electrophysiological Findings in Inherited Diseases Inheritance Visual VER ERG EOG Visual Visual pattern prognosis field aids defects Leber’s AR Very poor Low amplitude Unrecordable Usually Complete loss Require vision amaurosis or = 20/20 in 74%, 20/25 to 20/40 in 18%,