Preyalence and causes
This syndrome, also known as HHHO (hypotonia, hypogonadism, hypomentia, obesity) syndrome, was first described by Prader, Labhart, and Willi in 1956. (26) The syndrome is characterized by neonatal hypotonia, mental retardation, short stature, obesity, and cryptorchidism. This is a rare condition with an estimated population prevalence of approximately 1.2 to 1.3 per 10 000 with a boy-to-girl ratio of 1.6:1.2.(27) In approximately 50 to 70 per cent of cases cytogenetic tests can detect deletion of chromosome 15 (q11q13) which is of paternal origin in subjects with Prader-Willi syndrome. This genetic abnormality could be detected either by high-resolution chromosome analysis or fluorescence in situ hybridization using specific probes. However, this condition appears to be genetically heterogeneous, in that it is caused by inheritance of two chromosome 15 from mothers (maternal disomy) in some cases, but in others by a paternal gene mutation. The patients with chromosome deletion is clinically identical to the group without such chromosomal abnormality apart from a possible difference in pigmentation. According to Greenswag, fathers of children with Prader-Willi syndrome who have normal chromosomes were significantly older when their child was born than fathers of those children who have abnormal chromosomes.(28) He also described similar genetic heterogeneity in 'retinoblastoma' which may be caused by an autosomal dominant mutation and deletion of long arm of chromosome 13.
The majority of the affected children have mild to moderate degree of mental retardation and some have severe mental retardation or a borderline intelligence. In the initial preobesity phase, affected babies are usually limp, sleepy, unresponsive, and present typically with a narrowed bifrontal diameter, a triangle-shaped mouth, strabismus, and acromicria. Feeding difficulties in the form of absence of swallowing and sucking reflexes are common at this stage. Usually between age 1 and 4 years these children gradually develop hyperphagia caused either by persistent hunger or a decreased perception of satiety. This eventually leads to gross obesity. These subjects also manifest developmental delay and limited sexual function due to hypogenitalism. Precocious puberty or premature sexual maturation may affect some subjects. Abnormal secondary sex characteristics such as micropenis and cryptorchidism in males and amenorrhoea in females are common. Other associated features in Prader-Willi syndrome include small hands and feet, cleft palate, almond-shaped eyes, incurved foot, club foot, congenital hip dislocation, scoliosis, abnormalities in the knee and ankle joints, and squint. Systemic abnormalities include diabetes, bowel obstruction, asthma, duodenal ulcer, rectal prolapse, heart disease, cor pulmonale, boils, deafness, renal stones, gall stones, and hypothermia. (29>
A behavioural phenotype associated with Prader-Willi syndrome has been described consisting of abnormalities in speech, sleep, and behaviour along with a specific pattern of cognitive impairment in which subjects show relatively intact visuospatial skills associated with reduced short-term memory and a greater loss of information from memory over time.(30) Although poorly defined, speech abnormalities in the form of articulation disorder (23 per cent, echolalia) or abnormal syntax (17 per cent) and other speech abnormalities (18 per cent) have been described in the literature. A relatively higher proportion of males compared with females show these speech abnormalities. Various types of sleep abnormalities such as excessive daytime sleepiness, initial insomnia, sleep-onset rapid eye movement ( REM), and hyperventilation during Rem sleep have also been reported in individuals with Prader-Willi syndrome.
Although described as pleasant, good-tempered, and cheerful, subjects with Prader-Willi syndrome can manifest severe behavioural problems associated with hyperphagia and self-injurious behaviour particularly in the form of incessant spot picking and temper tantrums. A behavioural syndrome consisting of motor slowness, skin picking, sleepiness, ritualistic behaviour, impulsive talk, and stubbornness has been described.
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