Cerebral Visual Impairment
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Cerebral visual impairment (CVI), is defined as vision loss resulting from damage to retrogeniculate pathway, in the absence of any ocular pathology, with increased survival of preterm babies and better perinatal care, CVI has emerged as one of the leading cause of visual impairment. Other causes of visual impairment reported in children with multiple disabilities may be secondary to ocular defects such as uncorrected refractive errors, cataract, nystagmus, retinopathy of prematurity (ROP), optic nerve atrophy and delayed visual maturation (DVM). The prevalence of CVI in childhood has been steadily rising over the past few decades from a reported incidence of 36 per 100,000 in the late 1980s to 161 in 100,000 in 2003.
The term, cerebral visual impairment is preferred to cortical blindness as the central nervous system is still plastic and presence of extra-geniculostriate visual pathways precludes total loss of sight, even when there is complete destruction of the striate cortex.
Cerebral, is preferred over cortical owing to the subcortical involvement of optic radiations in premature infants. The sequelae of perinatal injury are also related to the timing, degree, duration, and mechanism of damage to a child’s brain. Management of children with CVI requires a combined and coordinated effort of ophthalmologist, neurologist and rehabilitation services.
CVI is commonly defined as a loss in visual function in the absence of damage to anterior afferent visual pathways or ocular structures. CVI has multiple causes but the most common is perinatal hypoxia. CVI can result from the insults at level of:
- Cortex, which includes geniculo striate lesions.
- Subcortex, which includes focal white matter lesions resulting to periventricular leukomalacia.
- Delayed visual maturation can also result from a temporary dysfunction of higher cortical centers.
The various etiologies for cortical visual impairment have been summarized in Table 1, detailed pathogenesis of each of them has been described below.
Hypoxic-ischemic brain injury is, by far, the most common cause of pediatric CVI. The resulting pattern of injury is due to hypoxia, is mainly, defined by age at which the insult occurs and differs in term and preterm children. In term infants, the areas between the circulation of anterior and middle cerebral arteries and medial and posterior cerebral arteries are the most typically affected as they are watershed areas of the cerebral cortex. Loss of vascular flow autoregulation induced by hypoxia leads to hypoperfusion of these watershed territories, resulting in infarction of the frontal and parieto-occipital areas. Striate cortex is affected frequently, associative occipital visual areas and temporal and parietal cortices, are also involved commonly. Preterm neonates, on the contrary rarely suffer parasagittal infarctions from hypoxia-ischemia. Periventricular deep white matter is involved when the insult occurs earlier, between 24 and 34 weeks of gestation. There is a transient, susceptible watershed zone in the periventricular white matter that later is replaced with the adult vasculature. Periventricular arterial end zone of long penetrating vessels derived which are mainly from the middle cerebral arteries, run from the pial surface and terminate in the deep periventricular white matter. Active development of this periventricular vasculature occurs during the last 16 weeks of gestation. The number of short penetrators and anastomoses between the long and short penetrators increases in the third trimester with a consequent decrease in vulnerable end zones and border zones. Capillaries in this region are prone to hemorrhage from hypoxia-ischemia.
As a result, of hypoxic insult germinal matrix produces glial and neuronal cells which migrate eccentrically to populate cerebrum. Immature oligodendrocytes and subplate neurons present around the ventricles are more vulnerable to ischemia than mature oligodendrocytes located elsewhere, and this explains the specific location of damage.
Thus resulting in the characteristic feature of periventricular leukomalacia (PVL).
Meningitis and hydrocephalus were considered to be the most common causes of CVI in past. In more recent series, infections account for 11.8% to 15% of cases of CVI. The occipital cortex is more susceptible to damage produced by Haemophilus influenzae, and hence, it is the most common organism causing CVI. Pneumococci, meningococci and herpes simplex virus are other causative agents, which have been implicated to cause ocular and cerebral visual problems. Onset of visual impairment typically is late in the course of the infection, and multiple accompanying neurologic sequelae commonly occur. Different mechanisms by which infection might injure the brain include thrombophlebitis, arterial occlusion, hypoxic-ischemic damage, venous sinus thrombosis and hydrocephalus.
Hydrocephalus affects vision by causing optic atrophy through various mechanisms, and can also affect posterior visual pathways which run close to lateral ventricles. A combination of anterior and posterior visual involvement is frequent. Although ventricular dilatation can occlude the posterior cerebral arteries, chronic distention of posterior cortex is a frequent mechanism by which hydrocephalus causes CVI. It is well known that shunt malfunction can cause CVI, but paradoxically, rapid correction by shunting also occasionally can produce CVI.
Head trauma is another cause of pediatric CVI (approximately 4% of cases as reported in two studies). Damage may be transient or permanent. Shaken baby syndrome is a common cause of posttraumatic CVI. Transient vision loss in children can also occur after trivial injuries and is accompanied by headache, confusion, drowsiness, vomiting, and seizures.
Epilepsy chiefly, infantile spasms can result in central visual inattention. Pathogenesis of visual impairment in patients who have infantile spasms is unknown. Anticonvulsants also are known to cause visual problems.
Congenital brain malformations (lissencephaly, schizencephaly, holoprosencephaly), metabolic and neurodegenerative disease, hypoglycemia, hemodialysis, cerebrovascular accidents, and brain tumors are among other reported causes of CVI.
Visual impairment in CVI can be as severe as no light perception to normal visual acuity. Cognitive visual function may be impaired in most of these children leading to misinterpretation in relation to what objects are there and where they are. Vernier acuity has been found to be more affected than grating acuity. There is some evidence that dorsal stream magnocellular pathway deficits may be more common in children with CVI.
Children with PVL frequently have hypermetropia in combination with astigmatism. Hypermetropia is a common refractive error in children with cerebral palsy and can be due to accommodation dysfunction
Strabismus and Ocular Motility
Infantile exotropia is more common as compared to esodeviation in patients with cortical visual loss. On the contrary children with PVL present with esotropia commonly and needs to be differentiated from infantile esotropia.
A specific trait of strabismus in these patients is dyskinetic strabismus, where esotropia changes to exotropia on a momentary basis. Repeated evaluation and constant deviation measurements are thus crucial before planning any surgical intervention for strabismus. Nystagmus is a common feature and indicates subcortical rather than cortical damage and may also be associated with concomitant anterior pathway diseases, such as optic nerve or retinal disease.
Optic disc features
Optic atrophy is common and can be secondary either to hypoxia involving the optic nerves. In cases of hydrocephalus, resulting in papilledema may lead to secondary optic atrophy. In a study done by V. Khetpal et al, optic atrophy was found to be a feature in 40% patients of cortical visual loss.
CVI patients have bilateral inferior field defects most commonly, homonymous hemianopia can also be present in association with hemiplegia and can be explained by interruption of axons of optic radiations and partly by the problems of simultaneous attention in them.
CVI can typically be associated with neurological deficits including cerebral palsy, mental retardation and hemiparesis, microcephaly, hearing problems, abnormal mental development, behavioral problems, myelomeningocele, progressive degenerative disorders, and hypotonia are among the reported anomalies, indicating that the damage may not be limited to the visual pathways. Some patients may also have superimposed anterior afferent pathology from optic nerve–related disease.
Visually impaired children have different blind mannerisms like rocking, thumb sucking, head banging, head flopping, eye pressing, light gazing and flicking their fingers.
Diagnosis of this entity is of utmost importance in a child with the normal ocular examination. However, suspicion begins in the immediate neonatal period.
Visual Evoked Response (VER)
In past, investigators heavily relied on findings of VER and EEG for a diagnosis of cortical visual loss, however, a normal flash VER recording can be obtained even in patients with cortical visual loss and can be mediated by the extra geniculostriate visual system.
EEG was once considered to be a valuable diagnostic tool and shows multiple eleptiform waves originating from occipital lobe.
CT and MRI helps to understand the ocular lesions as well as the underlying pathogenesis of CVI, also provide clues about the prognosis and visual recovery. Extent and location of brain damage is important. MRI is always recommended in children with low APGAR score.
Neuroimaging evidence of acute brain injury seen on brain MRI with Hypoxia-Ischemia is also considered as a significant feature as per the recent task force on neonatal encephalopathy. Severity of visual impairment could be predicted by the clinical severity of HIE shown at birth. Pattern of lesions on MRI can be broadly classified into 3 categories: periventricular leukomalacia, diffuse cerebral atrophy and multicystic encephalopathy.
Children with periventricular leukamalacia, encephalic cyst and diffuse cerebral atrophy are very less likely to improve, and children with mild damage on MRI have a better prognosis.
Some degree of visual recovery is seen in majority of children with cortical visual impairment, the improvement tends to be gradual over months, although exact mechanism is unclear. Lambert et al have summarized various theories proposed for the visual improvement and suggest that insult producing cortical visual impairment may not cause cellular death but interrupts the normal protein synthesis of neurons thus causing delay in myelination, dendrite formation and synaptogenesis.
It has now been postulated that improvement of sight in patients of CVI is actually a form of delayed visual maturation.
Amblyopia management with patching therapy remains the mainstay of management though compliance has been a major concern. Strabismus and motility evaluation is difficult owing to the behavioral aspects, yet has to be performed in multiple visits to know about the stability of deviation which helps decide optimal timing of surgical intervention. Esotropia being more detrimental is an indication for early surgery. Undercorrection by 15-20% is done usually to avoid consecutive exotropia over long term follow up, as overcorrection is more common.
The problems in patients of cortical visual loss are not just limited to vision, but are complex. A multidisciplinary approach is thus, necessary not just for diagnosis but also for management.
Rehabilitation services are much more needed, despite of progress in diagnostic methods, interventional treatments for CVI are still very limited. Rehabilitation support must include an approach that addresses psychosocial impact on patients and their families. Various active visual stimulation therapies have been tried for the rehabilitation of patients of cortical visual loss. Light reflex stimulation is performed in a completely dark and quiet room done by using a flashlight shined briefly in each eye, pausing 5 s between eyes. performed for 1 min and repeated 30 times per day. The ability for perceiving object outline can be developed by shining a penlight onto a target in a totally darkened room for 1 minute. Simple distinct shapes like circles, triangles, or stars either be on white cardboard with a black image or black cardboard with a white image shown 10 times a day, help the child to identify shapes, similarly black and white or colorful outline images added to the checkerboard environment can be used to develop outline perception. Further details can be added in the form of colors or expression in a circle which helps in developing the ability of the child to see the details within a configuration. Each child with CVI, hence is likely to have its own unique visual and motor deficit, necessitating an individualized approach.
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