Fig. 1.19  Periventricular leukomalacia in a child with 20/20 vision in both eyes. (a) and (b) Pseudoglaucomatous atrophy. Both optic discs are normal in size, with large, round cups. (c) and (d) Automated visual field examinations show bilateral constriction with symmetrical inferior depression that

spares fixation. (e) and (f) MR imaging. (e) High signal intensity in the optic radiations and contiguous enlargement of the posterior ventricles. (f) Similar lesions involving the corticospinal pathways are shown. With permission from Brodsky et al72

Perceptual Difficulties

Even when visual acuity is normal, children with PVL may have particular difficulties with interpretation of their visual environment that are impossible to predict with standard acuity testing.464 This central visual impairment is often

incorrectly attributed to regressed ROP.101 Jacobson et al281 have emphasized the difficulties that children with PVL experience special difficulties with visual crowding, visuospatial orientation, and interpretation of complex visual patterns such as faces and words. In school, their inability to read is often puzzling to parents and teachers because their visual acuity, intelligence, and ambitions indicate higher

ability. These perceptual difficulties, together with their tendency toward reduced inferior visual fields, often necessitate educational reprogramming.280

In general, children with PVL are much more visually debilitated than expected from their optotype acuity. In new and unfamiliar surroundings, they often behave insecurely, have difficulty in finding their way around, wish to hold hands, and fail to recognize the familiar faces of others. Parents of younger children may report that, at one moment, they would spot a small crumb on a table and reach for it, while at another moment they would seem not to like looking at things. They like to listen and not to watch. Pictures and comic strips are seldom interesting to them. They prefer to sit closer to the television to eliminate crowding. They start to draw later than normal children.280,281

Perceptually, these children are confronted with a complex array of real-world difficulties. Although these children effectively see too much to be called visually impaired, they are still visually disabled.273 A number of strange perceptual consequences result. Some of these elusive visual symptoms that these children experience include the ability to recognize simple figures but not faces. Children with PVL may also display a striking inability to distinguish steps from flat ground, which seems to be a form of impaired visual guidance of the lower limbs because it can occur in the presence of intact stereopsis.510 Distortions in motion perception cause blurring or disappearance of objects when walking,273 cause cars and bicycles to appear from nowhere when they stop, and small dogs that move fast to be seen as large blurred balls that pop up out of nowhere. Informational “overload” from movement in the peripheral visual field causes these children to fall frequently. They have low tolerance to stress and sensory distraction, poor awareness of body concept and body parts (needing to touch their body frequently to make sure it still exists), difficulty seeing and talking at the same time, abnormal spatial awareness and impaired egocentric direction, impaired visual information in communication using lipreading, fingerspelling, and gestures, and difficulty with communication.273,397,398,509

In test situations, all children have difficulties in distinguishing forms but use a variety of cognitive strategies to overcome this obstacle.281 Although they may not be able to recognize faces, they know who is coming by the sound of footsteps and sound of voices. They often use colors to identify objects, symbols, and persons. They are able to guess the rest of the picture from parts and details. Guessing and use of memory serve as compensatory mechanisms. It takes a long time for them to sort out a complex visual scene. However, development of speech is often reported to start early, and the parents state that these children talk a lot and listen carefully. They have a poor visual memory and great difficulty copying, but auditory memory is often remarkably good.281

Concentration is a problem for some, and they are easily distracted by sound and visual stimulation. These difficulties are less conspicuous in children with definite cerebral palsy.

Reading is especially difficult. Although they can often read short words, they have difficulty with long words, often lose track while reading, and have difficulty in finding the place where they left off. Many have difficulty with mathematics. When trying to read, they may see a few letters at a time but not the whole word. However, they can use the intelligence they have to piece words and sentences together. Hearing is intact, so it is helpful for them to learn their lessons verbally (or in Braille). It is important to listen to children with PVL, because intelligence is relatively spared, and these children will tell you about their vision what the children with cystic PVL would tell you if they could.

For the ophthalmologist, these children are often hidden away in the strabismus group.424 However, screening with structured history taking using a question inventory can identify these children.257 According to Lena Jacobson, these children are often intellectually unhappy largely because they know themselves to be “normal,” yet when information is presented to them in a way that everyone else can handle, they cannot handle it and cannot understand why. Children with cerebral palsy may be less frustrated because these difficulties are more consonant with their self-image. These children tend to be in normal classes but lag behind despite having good verbal skills. Many have been evaluated for visual problems in the past, so parents are grateful just to know that their physician recognizes that something is wrong. Several classroom measures can be taken to accommodate the visual difficulties that these children experience. Teachers can be advised to have the child sit in front of the class, use large-print books, utilize their color vision, study in a space with reduced surrounding visual stimulation, teach lessons verbally (because the auditory system functions well), and use closed-circuit television.281

At home, parents can take additional steps to address these complex visual symptoms. Such steps include storing toys in clear compartments; using plain carpets, bedspreads and decorations; identifying caretakers through waving and speaking; using constant identifiers such as shoes; training in seeking and identifying landmarks; using a cane to walk over uneven surfaces; using enlarged double-spaced text and masking surrounding text with a piece of paper; limiting conversation while walking; limiting distractions by reducing background clutter and activity; training in tactile recognition; and visual orientation to surroundings.397

Dorsal and Ventral Stream Dysfunction

From a functional point of view, there are two pathways, the dorsal stream, which links the visual cortex with the parietal lobes, and the ventral stream, which links the visual cortex

with the temporal lobes.412,537 As a result of the seminal work of Ungerleider and Mishkin,576 visual cortex anatomy is considered to be composed of two parallel and interconnected pathways, both supplied by the primary visual cortex area V1.594 As mentioned earlier in our discussion of CVI, the dorsal pathway mainly subserves visuo-motor transformations, while the ventral pathway neurons represent information from low-level features to more and more abstract stages of identity processing, thus subserving object identification.137,412,594 Some operations in the dorsal pathway operate unconsciously, while the ventral pathway activity subserves phenomenal visual consciousness.594

More simplistically, the ventral stream is for object vision while the dorsal stream is for space vision (Fig. 1.20).309,310 Although these two parallel neural processing pathways systems are segregated, they have developmental and functional overlap.309,310

The ventral stream connects the occipital and temporal lobe territories and subserves recognition of geometric and biological form, route finding, and visual memory. The dorsal stream connects the occipital area with the posterior parietal cortex, which allows the mind to encompass the whole visual scene and to elect to pay attention to chosen components. The posterior parietal cortex is also thought to work in harmony with the motor cortex by subconsciously providing the “online” action plan for visually guided movement of the limbs and body through three-dimensional space. It also interacts with the frontal territory responsible for both choosing which elements of the visual scene to pay attention to and bringing about saccades to the object of interest. Dorsal

stream dysfunction caused by bilateral posterior parietal pathology gives rise to simultanagnosia, in which there is a profound difficulty in registering the presence and identity of any object that is not being attended to.133,142,194,509 The ventral stream thus provides a conscious analysis and understanding of the visual world, while the dorsal stream facilitates and brings about accurate movement of the body through visual space, ostensibly at a subconscious level.

The functions of the dorsal stream comprise the analysis of the complexity of a visual scene, the ability to accord selective visual attention to specific elements, the ability to suppress other elements so that they do not distract, and the handling of other incoming data such as hearing and touch. The dorsal stream also determines the visual coordinates of elements within the visual scene, informs the motor cortex to facilitate visually guided movements of the body, and informs the frontal eye fields to bring about rapid eye movements to view objects of interest.137 The ventral stream is comprised of the fusiform gyri of the inferior temporal lobes, which ostensibly act as an image store for the wealth of imagery encountered. If the incoming data from the occipital lobes match what is already known, recognition takes place.140 Following injury, affected individuals have an inability to interpret the totality of the scene despite preserved ability to apprehend individual portions of the whole. If the ventral stream is selectively affected, a child with cerebral impairment and good acuities may therefore mistake a stranger for a parent. Because the ventral stream also subserves orientation and navigation, children with ventral stream dysfunction can easily become lost in known locations.140