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The brain and cerebral visual impairment
Introduction
Many congenital and acquired disorders of the central nervous system can affect a child’s vision. Several of these directly affect vital visual structures and pathways resulting in varying disability. In many parts of the world, the prevalence of visual impairment in children from damage and disorders of the brain is equal or greater than that caused by ocular disease (see Chapter 1 ). This condition, called cerebral (or cortical) visual impairment (CVI) is now the leading single cause of pediatric visual impairment in high-income countries. The broad spectrum of clinical features of CVI probably relates to the varied etiology, extent, and timing of the underlying brain insult. Nevertheless, it is essential to carefully assess a child’s visual deficits and potential in order to provide appropriate medical, rehabilitation and educational services. New directions of investigation using novel assessment tools and advanced neuroimaging methodologies are likely to help in uncovering the neurophysiological basis of CVI.
Developmental Defects
Embryological errors
Cephalocele (encephalocele)
During the first month of embryogenesis, a neural plate is formed which invaginates into the neural groove and fuses into a neural tube. Cephaloceles (defects in the skull and dura mater with extracranial extension of intracranial structures) are probably the result of a disturbance in the closure of the neural groove. Fortification of staple foods with folic acid and the use of folic acid supplements by women of childbearing age have greatly reduced the incidence of this type of cephalocele. Alternatively, they may occur as a postneurulation event with brain tissue herniating through the mesenchyme that will give rise to the cranium and dura. Three of the four major types of cephaloceles are of interest to ophthalmologists.
1. Occipital cephaloceles
In these, portions of the occipital cortex and the occipital horn of the lateral ventricle herniate into the defect. Severe visual defects are associated both with the anomaly and the results of any surgical correction. Knobloch syndrome is a rare autosomal recessive disorder characterized by high myopia, vitreoretinal degeneration with retinal detachment, cone–rod dystrophy, macular abnormalities, glaucoma, and occipital encephalocele, but the phenotype can be quite variable. Two novel mutations in COL18A1 have been described (see Chapter 51 ).
2. Frontal ethmoidal cephaloceles
These have a geographical predilection for Southeast Asia. They do not usually impinge on any visual structures, but they may be associated with ipsilateral optic nerve dysplasia.
3. Nasal pharyngeal cephaloceles
These are uncommon, but visual function is almost always affected. The optic nerves and chiasm may be compromised as they are stretched when they extend into the sac of the defect. Optic nerve hypoplasia and retinal dysplasia and coloboma are frequent associations.
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