Cortical Visual Impairment

After completing this article, readers should be able to: Vision loss caused by central nervous system damage often is referred to as CVI but also may be referred to as cerebral visual impairment or neurologic visual impairment. These terms refer to the fact that the primary defect may not necessarily be limited to the striate (primary visual) cortex and may affect other areas subserving vision, such as the visual associative cortex, optic radiations, and visual attention pathways. Although it is not yet certain which of these three terms best describes the visual deficit, it is clear that use of the label cortical blindness must be abandoned. That term not only evokes negative connotations for the child and the family but is inaccurate because in almost every instance, some degree of residual vision remains, and visual improvement can occur.CVI has become the greatest cause of pediatric visual impairment in developed countries (1)(2)(3)(4)(5) and is becoming increasingly prevalent in developing nations. CVI is a problem of increasing frequency for two primary reasons. First, the progress in neonatal care has resulted in improved survival of children who have brain damage from hypoxia. As a result, these children may suffer from CVI. (6)(7)(8) Second, the better outcomes associated with improved treatment of other causes of pediatric vision loss such as retinopathy of prematurity and congenital cataracts have diminished their roles as causes of pediatric blindness. (9)Various studies from the developed world illustrate the preeminent place of CVI among the causes of pediatric visual damage. Comparing current findings with previous prevalence studies, a population-based prospective study in five Nordic countries identified an increase from 11% to 23% in the relative frequency of CVI as a cause of pediatric blindness. (7) Along with optic atrophy, CVI accounted for 45% of cases of pediatric blindness in this study. In a recent survey of blind children in Ireland, the most common morphologic diagnoses were optic atrophy, optic nerve hypoplasia, and cortical blindness. (4) The Blind Babies Foundation of Northern California found that CVI was the leading cause of visual impairment in children younger than 5 years of age. (10)As recently emphasized by Hoyt, (5) understanding the mechanisms involved in pediatric CVI requires consideration that the pathologic processes may affect not only the primary visual cortex but also the associative areas, optic radiations, optic nerves, and even visual attention pathways.Hypoxic-ischemic brain injury is, by far, the most common cause of pediatric CVI. Matsuba and Jan (11) recently reported that hypoxia was the cause of CVI in 151 of 423 (35.7%) children diagnosed with CVI. Other authors have described similar figures in smaller series. (2)(12) The resulting pattern of injury is, to a large extent, defined by the age at which the insult occurs and differs in term and preterm children. (13)(14)(15) In term infants, the areas between circulation of the anterior and middle cerebral arteries and the medial and posterior cerebral arteries most typically are affected (watershed zones of the cerebral cortex). The loss of vascular flow autoregulation induced by hypoxia leads to hypoperfusion of the watershed territories, resulting in infarction of the frontal and parieto-occipital areas (parasagittal regions). The striate cortex is affected frequently but not exclusively. More anterior structures, such as the associative occipital visual areas and temporal and parietal cortices, commonly also are involved. (5)Unlike term infants, preterm babies rarely suffer parasagittal infarctions from hypoxia-ischemia. The periventricular deep white matter, where the germinal matrix is located, is involved when the insult occurs earlier, between 24 and 34 weeks of gestation. (16)(17) There is a transient, susceptible watershed zone in the periventricular white matter that later is replaced with the adult vascular configuration. (18) Capillaries in this region are prone to hemorrhage from hypoxia-ischemia. (16)(18) The characteristic injury is termed periventricular leukomalacia (PVL). The germinal matrix produces glial and neuronal cells that migrate eccentrically to populate the cerebrum. Immature oligodendrocytes and subplate neurons present around the ventricles at that moment are more vulnerable to ischemia than mature oligodendrocytes located elsewhere, which explains the specific location of the damage, believed to result from free radicals and decreased antioxidation. (19)(20)(21)(22)The optic radiations run precisely around the ventricles, and although this anatomic location partially explains the visual involvement, the pathogenesis appears to be much more complex. Circuits formed by these subplate neurons are necessary for the establishment and later maturation of visual connections between the thalamus and the cortex. (23)In the past, infections (Fig. 1) and hydrocephalus were the most common causes of CVI. (24) In more recent series, infections account for 11.8% to 15% of cases of CVI. (2)(5)(11)(12) The occipital cortex is more susceptible to damage produced by Haemophilus influenzae, the most common organism causing CVI. (25)(26)(27)(28)(29) Pneumococci and meningococci are other causative bacteria. (28)(30) Neonatal herpes simplex virus also causes ocular and cerebral visual problems. (31) The onset of visual impairment typically is late in the course of the infection, (1)(29) and multiple accompanying neurologic sequelae commonly occur. (32) The different mechanisms by which infection might injure the brain include thrombophlebitis, arterial occlusion, hypoxic-ischemic damage, venous sinus thrombosis, and hydrocephalus. (26)(27)(29)(33)Hydrocephalus, which has an incidence of about 1 in 1,000 during the first postnatal year, (34) can affect vision by causing optic atrophy through various mechanisms, but it also can also affect the posterior visual pathways that run close to the lateral ventricles. Furthermore, a combination of anterior and posterior visual involvement is frequent. A recent population-based study in Sweden revealed that due to an increase in the incidence of posthemorrhagic hydrocephalus in very preterm children, a previously decreasing incidence of hydrocephalus no longer is observed. Eighty percent of this population had ophthalmologic abnormalities such as strabismus, optic atrophy, and refractive errors, and 33% had visual impairment (visual acuity <0.3). (35) Houliston and associates (36) found that more than 50% of children who had CVI from hydrocephalus had higher cognitive visual deficits (“problems understanding and interpreting the visual world”). (17)Although ventricular dilatation can occlude the posterior cerebral arteries, (37) chronic distention of the posterior cortex is a more 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. (38)Head trauma is a significant cause of pediatric CVI (approximately 4% of cases in two studies). (2)(12) The damage may be transient or permanent. Shaken baby syndrome is a common cause of posttraumatic CVI. Fifty percent of the head injuries in a study published by Groenveld and colleagues (3) were the result of battering. Transient vision loss in children may occur after trivial injuries and typically is accompanied by headache, confusion, drowsiness, vomiting, and seizures. (39) A link with migraine has been mentioned by different authors, and this condition may be underdiagnosed because of the inability of young children to communicate their symptoms. (40) The pathophysiology is unknown, but an abnormal vascular response may result in vasospasm, ischemia, and edema. (39)(40)(41)Epilepsy and especially infantile spasms can cause central visual inattention (Fig. 2). Castano and associates (42) found some degree of visual improvement in only five of ten children who had severe visual inattention due to infantile spasms. Despite improvements at last follow-up, all of the patients retained significant visual impairment. Due to the severity of the visual and neurologic impairment, an objective measurement of visual acuity was possible in only one of the ten patients. Visual improvement was judged on the children's abilities to fixate and follow. The precise cause of visual impairment in patients who have infantile spasms is unknown, but it is likely the same mechanism that results in seizures and abnormal findings on electroencephalography. (39)(40)(41) Anticonvulsants are known to cause visual problems as well. (43)Congenital brain malformations (lissencephaly, schizencephaly, holoprosencephaly) also may be associated with CVI. (44) Metabolic and neurodegenerative disease, (32) hypoglycemia, (45) hemodialysis, (46) cerebrovascular accidents, and brain tumors (13) are among other reported causes of CVI.The diagnosis of CVI remains essentially a clinical one. (47) The possibility of CVI should be raised when there is greater delay in visual development than in other areas and when the degree of vision loss is unexplained by ocular findings. (48) Such a situation, paired with a characteristic clinical history such as hypoxia, should alert the clinician to the presence of CVI. In a typical case of CVI, results of the eye examination are normal, although this is not always the case. Concomitant ophthalmologic problems such as optic atrophy, nystagmus, and other conditions very frequently accompany CVI. This situation is exemplified by the preterm baby who is treated successfully for retinopathy of prematurity, but whose vision remains deficient and insufficiently explained by the ocular findings. In such a case, coexistence of some degree of CVI, often from PVL, should be considered.Habits that represent adaptations to the disease frequently are present in patients who have CVI. (1)(49)(50) Such habits are appropriately termed “neurobehavioral” signs and may aid in the diagnosis. To a large extent, many of these signs were identified and described by James Jan, a pediatric neurologist who has contributed enormously to the understanding of CVI in children. Light-gazing, a tendency to stare at bright light (eg, the sun or fluorescent lights), is frequent even in children who do not otherwise fixate. (51) Flicking the fingers in front of the eyes against the light for self-stimulation sometimes is seen. (52) Enigmatically, photophobia can be prominent and sometimes even coexist with light-gazing. (53) The explanation for this phenomenon is unknown, but thalamic or cortical damage may be responsible.A highly variable visual performance also is characteristic, with such variation even seen from hour to hour. (48) Children who have very limited vision may appear unexpectedly very responsive to color stimulation. (52) Such a response possibly is due to the bilateral cerebral representation of color, which is less likely to be eliminated unless the lesions are very extensive. Head shaking often is present, (54) as is pronounced head turns to search for objects, conceivably to make use of residual peripheral vision. (52) Visual function also appears to be better in more familiar environments. (48)Obtaining a measurement of visual acuity can be challenging in a child who suffers from accompanying neurologic deficits. If optotypes (standardized tables to test visual acuity) prove useless, preferential looking can be attempted (ie, Teller Acuity Cards, in which high-contrast gratings of different spatial frequencies are shown beside blank areas on the same card). The child usually prefers to fixate on the pattern of such cards, if seen. Even this method has limitations in testing patients who are unable to gaze directly because of eye or head movement problems, which are not uncommon in CVI. (55) It is possible to attempt to determine whether fixation or following capacity exists, but the inherent fluctuation of vision cannot be ignored in children whose vision can be influenced by fatigue, light conditions, or an unfamiliar environment. Therefore, obtaining an idea of a child's daily visual behavior is more important than assigning a numeric value to the visual acuity. Comments from the family as to how the child functions in his or her environment can be very useful.Most patients who have CVI have associated neurologic deficits. Khetpal and Donahue (12) found that 65.3% of children who had CVI had neurologic deficits. Whiting and associates (48) found that all of the patients in their study had some degree of neurologic deficit, and Wong (56) found deficits present in all congenital cases in her series. Epilepsy, cerebral palsy, hemiparesis, microcephaly, hydrocephalus, 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. (2)(11)(12)(48)(56)Associated ocular findings, such as optic atrophy, nystagmus, strabismus, gaze palsy, and retinal disease, also are common. (2)(5)(11)(12)(48)(56) Although the absence of nystagmus in a case of congenital visual inattention suggests a cerebral cause, it is now known that the presence of nystagmus should not dissuade the clinician from diagnosing CVI in the appropriate setting. Nystagmus may result from concomitant anterior pathway disease, such as optic nerve or retinal disease, or it may indicate subcortical rather than cortical damage. (57) In fact, nystagmus is common in PVL. (58) When optic atrophy coexists with CVI, clinical judgment is needed to define the relative contribution of each factor to the visual limitation.Consideration of both the clinical presentation and neuroimaging findings often is sufficient to diagnose CVI. Nonetheless, certain entities should be included in the differential diagnosis for this disorder. The lack of interest characteristic of autism, the inability to generate saccades (fast eye movements) in a child who has oculomotor apraxia, or simply a delay in visual maturation may simulate CVI, but in these cases, the diagnosis of CVI is inappropriate.In many circumstances, neuroimaging can support or confirm the diagnosis of CVI, and imaging can aid in estimating the final visual outcome. The abnormalities seen on computed tomography (CT) scan and magnetic resonance imaging (MRI) for a child who has CVI are diverse and range from normal to substantially altered anatomy of the posterior visual pathways, depending on the cause, severity, and moment of the insult. Globalized cortical atrophy, ischemic encephalopathy, PVL, and structural malformations were the most common findings in a study by Khetpal and Donahue. (12) Diffuse cerebral atrophy was the most prevalent CT scan abnormality in a study by Lambert and associates. (59)Given the importance of neonatal hypoxia as a cause of CVI, imaging findings in this condition require discussion. First, the value of ultrasonography in evaluating preterm babies in the neonatal care unit deserves mention. This technique detects intraventricular and germinal layer hemorrhage, hemorrhagic parenchymal infarctions, and cystic changes. (16) The presence of cystic leukomalacia on ultrasonographic examination was found to be highly predictive of CVI. (60)(61)In older children who were is the most test for damage to the periventricular white matter (Fig. is by dilatation of the lateral ventricles and of the periventricular white the occur to a greater degree in the posterior The severity of has a more severe with vision. Lambert and associates also found a between the degree of involvement of the optic on CT scan and and a visual outcome. found no between visual and in the striate and and associates similar results and described the pattern as for visual hypoxia in term to cause frontal and parieto-occipital later cortical and decreased white matter, dilatation of the ventricles, and location of lesions in cases of hypoxia from and may with age. In preterm infants, the and are often with white matter damage. In babies at the lateral posterior (Fig. and are often with damage to the lateral and optic Due to limited these are seen less the value of in some cases, vision not with damage seen on imaging of the optic radiations or the primary and associative visual cortex. In these cases, it may be to use imaging to the damage to areas such as the frontal cortex, parietal cortex, and which the anatomic for visual attention and may a in the pathogenesis of CVI. it is not neuroimaging (Fig. may in CVI. and colleagues found large areas of decreased cerebral flow tomography in patients who had CVI in had revealed no of in the diagnosis of CVI in children and visual has been in many studies, and the is not free of such as and are to and not in diagnosing CVI because were unable to in response between children who had cortical loss of vision and who had neurologic lesions and no visual and associates found a predictive value for because of children who had abnormal and because only of who improvement had had normal results on Other authors have that a result not necessarily a visual outcome. other have reported better of in the of pediatric CVI. and reported that abnormal results during the of vision were in a negative outcome. and reported that the response after the insult is in and found in visual acuity of children who had CVI as the visual appear to be a but have and should not to to a or on the of these findings. The the of the visual pathway but has limitations in higher of improvement of vision always occurs in children who have CVI. The mechanism by which improvement occurs to be a of In many cases, the likely explanation may be the presence of residual visual which also may result from the of some areas rather than of visual cortical function at the of the in make it to the possibility that more to damage of central vision may occur in studies support this in whose visual cortex is is of mention. The results be as anatomic and After cortical lesions are and a response that of anatomic from the through the thalamus to the lateral visual in neonatal but not in adult the young the adult a after cortical at later in the lateral cortex Although the cells in the do not the of the striate cortex, do young whose striate cortex is older a capacity to to in the the as well as the to with eye to 5 years after the children with CVI present with lesions of the striate cortex. The associative areas of the occipital cortex or temporal or parietal frequently are sometimes from abnormal visual many children who have CVI higher visual deficits visual and these deficits can occur in with visual acuity. Such deficits might result from lesions to the to or the and associates published a children who had of or some of a visual can be but not the and some degree of periventricular white matter It is important to that CVI can much more than visual acuity. has been that evaluating the damage seen on neuroimaging to involved with mechanisms of visual attention may be of A child who has vision have among multiple because of lesions visual spatial attention and or of for attention parietal cortex, This possibility has been in by Hoyt, and it may not only to cases of CVI in which the imaging findings of the striate and and periventricular appear but also to more typical cases such as CVI from PVL. that thalamic damage, for is frequent in PVL. all children with CVI some degree of visual improvement with different Matsuba and Jan (11) found improvement of visual acuity in of and and associates improvement in The of vision on many the cause, age of and severity and of Children who have subcortical damage to have a In a (5) found some of vision in of children who had striate cortex injury with of children who had periventricular white matter Lambert and colleagues found that imaging abnormality of the optic with a visual Other causes of CVI such as and have been associated in the with a outcome. other than cause (eg, of the must be for each case. and associates not a between cause and than of children who have CVI some improvement of vision. outcomes other than vision also diagnosis and are important in pediatric CVI. who have in for affected patients have specific the of visual by rather than visual is believed to vision in children who have CVI. by and are among the many for residual vision. A for each child is CVI almost is an it is important to diagnose and accompanying neurologic and can be of the best possible of associated seizures that may with visual function by their treatment also may visual to for concomitant cerebral may affect children who have visual impairment, resulting not only from of the that vision is known to have on the but also from concomitant of the of and other treatment are elsewhere, but can to the of in a pediatric in limited is a treatment at the specific cause of CVI. are hydrocephalus or shunt in which the appropriate is pediatric or is to a not only in with the diagnosis of CVI but also in associated ocular abnormalities and when a child who has CVI and significant may from involved in the care of an affected child must that the of visual development results in better must the child as as possible to of and to by the and and for their aid in this

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