Cortical Visual Insufficiency

Cortical Visual Insufficiency

Cortical visual insufficiency (CVI) is now the single greatest cause of visual impairment in young children in developed countries.138,178,197,262,264,481,485 The proportion of blindness in children attributable to this disorder has increased as a result of two major factors: (1) Advancement in neonatal medicine has saved the lives of an increasing number of premature infantsandchildrenwithseverebraindamage.(2)Advancement of ophthalmologic techniques to treat other causes of blindness in children, such as cataracts, has reduced the proportion of such children in schools for the blind. Medical advances to prevent and treat CVI have lagged behind.262

At the outset, clarification of certain aspects of related terminology may be useful.158 Cortical blindness refers to completelossofvisionresultingfromdisordersofthegeniculostriate pathway. Some investigators prefer to more accurately refer to the visual deficit as cerebral rather than cortical because injury to the optic radiations, occipital cortex, or higher cortical centers can selectively diminish vision. The term cortical continues to be the one in common use and will be used in this discussion. Furthermore, because the degree of vision loss resulting from a cortical insult is highly variable, rarely complete, and often shows a degree of recovery, many investigators prefer the term CVI to cortical blindness to avoid the dismal prognostic implications suggested by the term blindness.296,613 We have advocated the term subcortical visual loss for the focal white matter lesions confined to the optic radiations in premature infants (periventricular leukomalacia).76 Despite the apparent selective white matter involvement on neuroimaging, however, these patients often have a more global brain injury, with many symptoms referable to higher cortical dysfunction.253,254,262,264,265,463 As a corollary, children whose injury appears to be confined to the subcortical visual system on neuroimaging can still have significant cognitive visual disturbance. The general term cerebral visual loss gently but nonspecifically encompasses both entities, along with the temporary dysfunction of higher cortical centers that may define DVM (discussed below).

Perinatal injury to the developing visual system is a common cause of visual impairment and neurologic morbidity in children.35,174,210,265,271,341,599 Although multiple etiologies are recognized, hypoxic-ischemic events are most frequently implicated.35,174,210,265,271,341,599 Injuries to the retrogeniculate visual pathways are heterogenous with respect to cause, mechanism, timing, degree, location, and duration. They fall into two general groups comprising term injury predominantly to the visual cortex and adjacent white matter, and preterm injury to the subcortical white matter (periventricular leukomalacia). As discussed below, these two patterns of injury give rise to distinct constellations of neuro-ophthalmologic signs.76

The territorial distributions and neurologic consequences of perinatal retrogeniculate injury reflect the developing

brain’s level of maturity at the time of injury (Table 1.1).34,37, In the full-term infant, the brain receives its vascular supply primarily from the major cere-

bral arteries, and its watershed areas lie at the interfaces between the major cerebral arterial distributions.

Mild degrees of term hypoxic-ischemic injury produce watershed infarctions in the arterial border zones (i.e., the parieto-occipital and parasagittal cortex), injuring both gray and white matter, usually resulting in encephalomalacia. Severe hypoxic-ischemic insult in the term newborn may also involve the deep central gray matter (thalami and basal ganglia) and may be associated with spastic quadriplegia, microcephaly, and seizures.599 Perinatal cortical visual loss at term is usually attributed to hypoxic-ischemic injury, but less common etiologies include meningitis, encephalitis, nonaccidental head trauma, hydrocephalus, and metabolic derangements. Inciting events include ruptured or occluded umbilical cord, ruptured uterus, profound bradycardia, or cardiocirculatory arrest, polyhydramnios, maternal eclampsia, placental abruption, and fetal-maternal transfusion with resultant hypovolemia.38

The ability of MR imaging to readily differentiate gray matter from white matter makes it particularly useful for distinguishing cortical from subcortical retrogeniculate injury.38,86 In hypoxic-ischemic term injury to the brain, MR imaging shows variable, highor low-signal changes on both T1and T2-weighted images in the cerebral cortex, basal ganglia, or thalamus, depending on the severity of injury and the timing of the injury in relation to the study (Fig. 1.5). High-signal changes on T1-weighted images, especially in the parasagittal cortex and deep gray matter, are often present in term hypoxicischemic injury. Diffusion-weighted imaging shows areas of hyperintensity (restricted diffusion) in injured tissue earlier than findings on conventional imaging.35,174,599 Proton spectroscopy shows elevated lactate in injured areas of the brain.35

The original definition of CVI is derived almost entirely from experience with adult patients who have acquired cortical lesions.253,254 In this context, the diagnosis of CVI requires very poor vision, normal pupillary light reflexes, no nystagmus, and an otherwise normal eye examination. However,

Table 1.1  Morphology of lesion relative to timing of central nervous system injury