The incidence of congenital ocular malformations is approximately 6 per 10,000, of which almost 8% are associated with abnormal karyotypes.1 This chapter is limited to the description of ocular and systemic anomalies that result from some isolated aberrations of chromosome number or structure involving a single chromosome as detected by traditional cytogenetic technologies.
Many genetic diseases are caused by mutations in a single gene that are not cytogenetically visible. Molecular genetic sequencing technologies enable the recognition of single base pair changes, single base pair deletions/duplications, whole exon deletions, and multi-exon deletions. Molecular strategies create a more sensitive means of detecting copy number variation throughout the genome, which may or may not be pathogenic. This has allowed for another dimension of genetic diagnosis.
The most common chromosomal abnormalities associated with congenital ocular malformations identified at birth are trisomy 13 and trisomy 21.1 In the presence of an ocular malformation, certain specific systemic malformations appear to be better predictors of an associated chromosomal abnormality: talipes-equinovarus (club foot), microcephaly, hydrocephalus, facial dysmorphism, and hypertelorism. Children with congenital ocular malformations tend to be smaller for gestational age and developmentally delayed. They have a higher incidence of consanguineous parents. Chromosomal aberrations can involve the whole genome, resulting in triploidy, tetraploidy, and polyploidy or they may be numerical (monosomies and trisomies) or structural (translocations, insertions, deletions, or duplications). Due to the size of a cytogenetically detectable chromosomal change, more than one gene is disrupted.
The material presented in this chapter will be confined to “pure” chromosomal aberrations. It is hoped that the reader will therefore be familiar with the phenotypes associated with abnormalities of individual chromosomes so that when more than one chromosome is altered, features of each separate aberration can be considered clinically.
Chapter. 39692 words. Illustrated.
Subjects: Clinical Genetics ; Ophthalmology
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