Glossary of terms

Chiasmal
A crossing or intersection of two tracts, as of nerves or ligaments.

Cortical
Of, relating to, associated with, or depending on the cerebral cortex.

Hydrocephalus Shunt Failure
A failure of the surgically created passage which is used to divert or permit flow from an abnormal accumulation of fluid in the cerebral ventricles. This causes enlargement of the skull and compression of the brain, destroying much of the neural tissue.

Hypogylcemia
An abnormally low level of glucose in the blood.

Hypoxic ischemic encephalopathy - An acquired syndrome characterized by clinical and laboratory evidence of acute brain injury due to asphyxia.

Lesion
A localized pathological change in a bodily organ or tissue.

Nystagmus
A rapid, involuntary, oscillatory motion of the eyeball.

Occipital
Of or pertaining to the occiput, or back part of the head, or to the occipital bone.

Ocular
of or relating to the eye.

Periventricular Leukomalacia
Periventricular leukomalacia (PVL) is caused by a lack of oxygen or blood flow to the periventricular area of the brain, which results in the death or loss of brain tissue.

Parietal Lobe
The division of each hemisphere of the brain that lies beneath each parietal bone.

Photophobia
An abnormal sensitivity to or intolerance of light, especially by the eyes, as may be caused by eye inflammation, lack of pigmentation in the iris, or various diseases.

Sub-Cortical
Of, relating to, involving, or being nerve centers below the cerebral cortex.

Scotomata
An area of diminished vision within the visual field.

Strabismus
A visual defect in which one eye cannot focus with the other on an object because of imbalance of the eye muscles.

Visual Acuity Sharpness of vision, especially as tested with a Snellen chart. Normal visual acuity based on the Snellen chart is 20/20.

The following preliminary Current Perspective on Cortical Visual Impairment (CVI) is presented for review by leading medical specialists in the field of ophthalmology. The purpose is to encourage an exchange of informed opinions contributing to a dynamic up-to-date resource on the major eye disorders. Thus, these descriptions are continuously under development and are not offered as final statements.

Cortical/Cerebral Visual Impairment (CVI)

Presently, CVI is the most common cause of permanent visual impairment in children (1-3). The diagnosis of CVI is indicated for children showing abnormal visual responses that cannot be attributed to the eyes themselves. Brain dysfunction must explain the abnormal visual responses, as abnormal ocular structures, abnormal eye movements, and refractive error do not. Fixation and following, even to intense stimulation, may be poor and the child does not respond normally to people's faces. Visual regard and reaching (in the child with motor capabilities) toward objects is absent.

Causes of CVI

It is now widely accepted that "cortical blindness" is not an appropriate diagnostic term for children with early, acquired visual impairment due to non-ocular causes (4). The term "cortical" is misleading because the visual impairment is due to abnormality of bilateral post-chiasmal visual pathways, including damage to cortical (gray matter), subcortical (white matter) or both. Non-anatomical lesions, for example, seizures, metabolic derangements, also can cause CVI. Thus, "cerebral visual impairment" is preferred to "cortical blindness." Common causes CVI in infants and young children include hypoxic ischemic encephalopathy (HIE) (in the term born infant) periventricular leukomalacia (PVL) (in the preterm infant), traumatic brain injury due to shaken baby syndrome and accidental head injuries, neonatal hypoglycemia, infections (e.g. viral meningitis), hydrocephalus shunt failure, severe epilepsy, and metabolic disorders (5). Other causes include: antenatal drug use by the mother, cardiac arrest, twin pregnancy, and central nervous system developmental defects (5). Accompanying features of CVI include cerebral palsy and developmental delays.
Confusing diagnostic entities include: delayed visual maturation (6), autism spectrum disorders, severe bilateral central scotomata (with eccentric fixation), dyskinetic eye movement disorders (7) and profound mental retardation.

Eye examination findings

The eye examination may show anomaly of the optic nerves (paleness, large cups) that, however, is not severe enough to result in the visual impairment exhibited by the child. Strabismus is common; nystagmus is less common. Pupillary reactions are usually normal. High refractive error corrected by glasses may improve some visual behaviors and should be tried if present (2, 8, 9).

Symptoms

The most common CVI symptoms presenting to the ophthalmic clinician are:

• Abnormal light response - light gazing OR photophobia
• Blunted or avoidant social gaze
• Brief fixations, intermittent following
• Poor visual acuity
• Visual field loss - generalized constriction, inferior altitudinal, hemianopic defect

Behaviors

Behaviors reported by parents, teachers and low vision specialists include:

• Variable or inconsistent visual responses to the same stimuli
• Better responses to familiar than to novel stimuli
• Fatiguing from visual tasks
• Peripheral vision dominates when reaching
• Colored stimuli elicit better responses than B&W stimuli
• Visual attention for moving stimuli is better than for static stimuli
• Vision for navigation is unexpectedly good
• Difficulty seeing an object or image in a "crowded" array or a busy background
• Reduced responses to visual stimuli when music, voices, and other sounds are present, and often, when the child is touched

All or most of these behaviors are not observed in individual children with CVI. Conversely, a child showing only one or two of the behaviors above does not indicate CVI.

Parents are most disturbed by the child's lack of social gaze and direct eye contact. Active avoidance of or withdrawal from unfamiliar visual stimulation, including people's faces, is frequently reported. Tactile stimulation may be avoided by the child, while in others, touch may be utilized in preference to vision. Commonly, the child positively responds to voices and music. Therapists and teachers are rightly concerned about the child's reaching without looking at the object or the hand.

Visual development

Partial recovery of vision in many children with CVI and severe visual impairment occurs. Improvements are seen in visual acuity, orienting to peripheral stimuli, attention to and reaching for objects and for social gaze. Effective management of intractable seizures often results in improved visual behaviors (personal experience).

Future

To date, limited attempts have been made to subgroup CVI by etiological or diagnostic criteria, or by types of CVI behaviors. Likewise, limited study of association with neurological, motor and cognitive difficulties has been done. Research in these areas is much needed. Case studies showing course of visual development and other aspects of development are needed.

Clinical evaluation and monitoring

In addition to the complete eye examination, objective measures of visual abilities should be done where feasible. Visual acuity is measurable in most children with CVI using large, black and white gratings (stripes) presented using preferential looking tests (15-17), or using cortical visually evoked potentials (17-19). Acuity may be very poor in infancy and remain so. In others there is gradual improvement in acuity. In most children with CVI, acuity does not reach normal levels. And, when measurable, recognition acuity for pictures, symbols or letters may be much poorer than the acuities previously measured for gratings. Glasses should be given if warranted, as visual abilities may improve, surprisingly so.
Visual field abnormalities are much more common in children with CVI than realized probably because of the difficulties in assessing peripheral vision in children with poor fixation, poor orienting, and visually avoidant behaviors. Certainly, in individuals with diffuse, extensive lesions of the posterior visual pathways, visual field defects would be expected. Inferior field defects, often dense and complete, are seen in patients whose CVI is attributable to HIE or to PVL (20, 21).

Visually guided responses, especially reaching and environmental scanning, should be interpreted in the context of the child's visual field status. Referral to a pediatric low vision specialist for further evaluation may be helpful.

Rehabilitation and education

In all children with cerebral visual impairment, services of trained and experienced teachers are very important for the child's development and education:

See the American Printing House for the Blind CVI website http://www.aph.org/cvi/index.html and Linda Burkhart's Technology Integration website http://www.lburkhart.com/

Referral of the child with CVI to state services for visually impaired children should be done promptly after diagnosis. Specific recommendations based upon the clinical measurement of visual abilities, such as visual acuity and visual fields, should be provided to parents, therapists and teachers. Teachers of visually impaired children should assess broader, "functional" visual behaviors and, often in conjunction with other therapists, devise interventions appropriate for the specific needs of the child (11, 14, 22, 23). Appropriate additional support services for the school aged child, including for non-verbal learning disabilities, will be needed.

In children with visual field defects, visually guided mobility and spatial orientation can be expected to be impaired. Evaluation and instruction by a certified orientation and mobility instructor should be provided when the child is independently mobile.

Conceptual framework for understanding the visual difficulties in CVI

Understanding the basis of complex visual difficulties of children with CVI may be aided by description of the specific problems associated with lesions in specific areas of visual association areas of the brain in adults (21, 24, 25):

1) Visual motor disturbances, as in moving the eyes to direct visual attention to an object, fixating on an object of interest, shifting fixation and gaze to a new visual stimulus, and accomplishing fine motor tasks such as copying a drawing, are associated with posterior parietal (-occipital) lobe lesions. These are considered due to damage to the "dorsal" visual association pathway.

2) Visual spatial disturbances, as in localization of objects, judgment of direction and distance of objects, and orienting the body in relation to the physical world (the "Where is it?" aspect of vision), are associated with posterior parietal (-occipital) lobe lesions (also "dorsal" pathway).

3) Visual perceptual disturbances, as in discrimination, recognition, and integration of visual images and objects (the "What is it?" aspect of vision), are associated with inferior posterior temporal lobe lesions (due to a different visual pathway, the "ventral").

Brain damage in children with CVI is more diffuse than in adults with the specific difficulties indicated below. Thus, children with CVI may show more than one of the specific domains of visual impairment (for example, both dorsal pathway difficulties - visual motor and visual spatial). During early development, visual motor disturbances are more evident than at later ages. Aspects of abnormal reaching and visual avoidance may be affected by abnormal sensory integration and motor output difficulties. In some children with minimal visual acuity loss, specific difficulties in visual perception or in spatial orientation become more evident as they mature. Specific visual cognitive dysfunctions are common in children with traumatic brain injury (personal experiences).

Additional Websites:

http://www.tsbvi.edu/Education/anomalies/cortical_blindness.htm

http://www.tsbvi.edu/Education/corticalassess.html

http://www.ode.state.or.us/groups/supportstaff/specializedservices/deafblind/neurological.pdf

http://www.unr.edu/educ/ndsip/factsheets/cvi.pdf

http://www.nhbvi.com/Agenda/Goal1/Book1/commoncauses.html

http://home.connexus.net.au/~dba/palmer.doc

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