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Russell-Silver Syndrome
Introduction
Russell-Silver syndrome (RSS) or Silver-Russell syndrome is one of the growth deficiency disorders that form part of the increasing group of congenital imprinting disorders. RSS is characterized by severe intrauterine and postnatal growth restriction. Other abnormalities include relative macrocephaly, hemihypertrophy (one side of the body is larger than the other), fifth finger clinodactyly, and triangular-shaped facies. RSS is genetically heterogeneous and the phenotype can vary. This disorder illustrates the importance of recognizing genetic syndromes associated with prenatal onset intrauterine growth restriction (IUGR) and significant shortening of the long bones, but is not considered a classic skeletal dysplasia. There are more than 1000 genetic syndromes associated with prenatal growth deficiency, and differentiating them by ultrasound (US) is extremely challenging.
Disorder
Definition
This syndrome was independently identified by Silver in 1953 and Russell in 1954, thus called Russell-Silver syndrome in the United States or Silver-Russell syndrome in Europe. RSS is a growth disorder characterized by growth restriction before birth that persists after birth. RSS is associated with very low birth weight and often failure to thrive. Head circumference is normal, but the head may appear unusually large when compared to the remainder of the body. RSS children are thin with poor appetites, and many develop hypoglycemia associated with poor feeding. Consistent newborn features include small triangular facies, narrow chin, smallish jaw, downturned corners of the mouth, limb asymmetry, and fifth finger clinodactyly. RSS is also associated with developmental delays, speech and language problems, and long-term learning disabilities.
Prevalence and Epidemiology
The precise incidence of RSS remains unknown, but estimates place the frequency at between 1 : 100,000 and 1.3 : 100,000 individuals.
Etiology, Pathophysiology, and Embryology
RSS is a genetically heterogeneous entity and is caused by epigenetic alterations (changes in the expression, not the sequence of the gene, by biochemical modifications). Forty to sixty percent of cases result from hypomethylation of the chromosome 11p.15.5 imprinting center region 1 (ICR1) on the paternal chromosome 11. This is the same region that is affected in the overgrowth syndrome Beckwith-Wiedemann syndrome ( Chapter 110 ). In about 5% of cases the etiology of RSS results from maternal uniparental disomy for chromosome 7 (UPD[7]mat) (inheriting two maternal chromosomes with no paternal contribution). Other abnormally imprinted chromosomal regions have been associated with RSS-like phenotypes. Autosomal dominant and recessive inheritance have been seen infrequently.
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