There are conditions in which obesity is part of a recognized genetic defect, clinical syndrome, or acquired pathological condition (Table 2). Together, these conditions account for only a very small
Table 2 Specific conditions associated with obesity in childhood
Inherited syndromes associated with childhood obesity
Inherited syndromes affecting mobility
Inherited disorders of growth Chromosomal abnormalities
Single gene defect affecting leptin metabolism
Autosomal dominant Autosomal recessive
Deletion or uniparental disomy for q11-q13 fragment of chromosome 15 Trisomy 21
Abnormalities of sex chromosomes
Congenital leptin deficiency Leptin receptor defect Prohormone convertase-1 defect Melanocortin-4 receptor defect Peroxisome proliferators activated receptor POMC deficiency Biemond's syndrome Alstrom's syndrome Bardet-Biedl syndrome Biemond's syndrome (some) Carpenter's syndrome Cohen's syndrome
Borjeson-Forssman-Lehmann syndrome Duchenne muscular dystrophy Spina bifida Achondroplasia
Down's syndrome Klinefelter's syndrome Turner's syndrome
Hypothyroidism Growth, hormone deficiency Cushing's syndrome Polycystic ovarian syndrome Hydrocephalus Meningoencephalitis Steroid treatment Sodium valproate proportion of obese children. With the exception of very rare single gene defects in leptin metabolism, obesity is a secondary feature in these conditions and presentation is usually for some other aspect of the condition. Single gene defects affecting leptin are associated with progressive gross obesity from early life and may respond with dramatic fat loss with leptin treatment. Where obesity is only a part of a spectrum of abnormalities, common associated features are short stature, developmental delay, and craniofacial and other bony abnormalities.
Chromosomal abnormalities are more frequent causes of a predisposition to obesity. Prader-Willi syndrome, due to deletion or uniparental disomy of part of the long arm of chromosome 15, is associated with characteristic facies, small hands and feet with tapering fingers, hypogonadism, early hypotonia, difficulty feeding, and initially failure to thrive. From the second year of life many of these children show voracious appetite, progressive obesity, and negative behavior (stealing food and refusing to follow a diet). Many also commonly have psychodevelopmental problems with moderate mental retardation that exacerbates the difficulties maintaining normal weight for height and age. Gross obesity commonly leads to early death associated with hypoventilation (Pickwickian syndrome) and/or complications of type 2 diabetes mellitus.
Down's syndrome children are also prone to develop obesity in late childhood and adolescence. This is generally unrelated to recognized pathophy-siological explanations for the obesity, although the syndrome is associated with an increased incidence of autoimmune thyroiditis and hypothyroidism (which exacerbates obesity).
Obesity may be an associated feature of other pathology in childhood. Endocrine problems, such as hypothyroidism and Cushing's syndrome, lead to obesity, but linear growth retardation does also, which often draws attention to the problem before obesity is severe. Hypothalamic damage (e.g., hydro-cephalus and meningoencephalitis) and problems leading to immobility (e.g., spina bifida and Duchenne's muscular dystrophy) may also predispose to obesity. Nonpathological childhood obesity is usually associated with normal intelligence, relatively tall stature before puberty, and no overt abnormalities, so brief assessment of growth, general health, and intelligence usually distinguishes obese children for whom investigation for possible underlying pathology is required.
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