LEARNING DISABILITIES

Health Evidence Bulletins - Wales

Literature searches completed on: 19th April 1999

1:Epidemiology, Risk Factors and Prevention  

This document is a supplement to, not a substitute for, professional skills and experience. Users are advised to consult the supporting evidence for a consideration of all the implications of a recommendation

"The field of intellectual disability continues to be plagued with inconsistent, uncritical and unscientific terminology, apparently hiding lax conceptualisation. Issues relating to deinstitutionalisation and community and home care services also continue to be addressed but the right questions are not always asked and, again, the complexities of methodology, the requirement of very large populations for epidemiological work in this field and the limitation of resources for research in all countries argue for better co-ordinated, collectively planned research programmes."
Fryers T. Epidemiology in relation to community and residential services.
Current Opinion in Psychiatry. 1997; 10: 340-53
The Statements The Evidence
1.1 Epidemiology
1.1a. Classification of intellectual disability is a complex and confused area. There is still little agreement on terms and classes. There is a need for a general title for the field, a common terminology and taxonomy, and more innovative exploration of the potential of the multi-dimensional framework of the International Classification of Impairments, Disabilities and Handicaps (ICIDH)i. i. Fryers T. Epidemiology in relation to community and residential services. Current Opinion in Psychiatry 1997; 10: 340-53
(Type V evidence – expert opinion with review of papers published during previous 12 months)
1.1b. ICIDH is under revision and is currently available in Beta-2 draft versioni. i. World Health Organisation. ICIDH-2: International Classification of Functioning and Disability. Beta-2 draft, Full Version. Geneva: World Health Organisation, 1999
http://www.who.int/whosis/icidh/index.html
[accessed 8.12.00
]
(Type V evidence – expert opinion)
1.1c. The prevalence rate in Western countries for Severe Mental Retardation (IQ <50) is 3-4 per 1000 population*. The prevalence of Moderate Mental Retardation (IQ 50-70) is 30 per 1000 populationi,ii.
A typical District of 250,000 population would have 750 -1000 people with Severe Mental Retardation*; 25-40 of these would have severe behavioural problemsii.
*These minor differences in prevalence rates reflect the confusion in terminology pointed out in 1.1a.
i. Roeleveld N, Zielhuis GA, Gabreels F. The prevalence of mental retardation: a critical review of recent literature. Developmental Medicine & Child Neurology 1997; 39(2): 125-32
(Type IV evidence - systematic review of 43 prevalence studies)
ii. Felce D, Taylor D, Wright K. People with Learning Difficulties. pp.412-450 In Stevens A, Raftery J (eds). Health Care Needs Assessment: The Epidemiologically Based Needs Assessment Review. Oxford: Radcliffe Medical Press, 1994
(Type V evidence – expert opinion based on a survey of the field)
1.1d. The expected birth prevalence* of Down syndrome has increased from 1.67 (1996) to 1.84 per 1000 live births (1998). The observed birth prevalence has increased from 0.91 (1995) to 1.04 per 1000 live births (1998). Antenatal diagnosis has remained constant at 45-46%. This reflects a tendency for women to have babies at older ages and the greater use of antenatal screeningi. (UK)
*(assumes absence of antenatal screening and selective termination: to account for spontaneous foetal loss these figures also include 23% reduction in terminations after amniocentesis and 43% reduction in terminations after chorionic villus sampling
i. Huang T, Watt HC, Wald NJ, et al. Birth prevalence of Down’s syndrome in England and Wales 1990 to 1997. Journal of Medical Screening 1998; 5; 213-214
(Type IV evidence - observational study)

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1.1e. Down syndrome is the commonest known cause of mild and severe intellectual disability. Fetal Alcohol Syndrome is the second commonest known cause in many countriesi. i. Minns RA. Pre and perinatal conditions contributing to mental retardation. Current Opinion in Psychiatry 1997; 10(5): 354-59
(Type V evidence – expert opinion with review of 42 papers published during previous 12 months)
1.1f. Endemic cretinism caused by iodine deficiency is a common global cause of severe intellectual disability. Prevention with iodine is technically simple and cheapi.
(Health gain notation – 1 "beneficial")
i. Fryers T. Epidemiological concerns in current literature. Current Opinion in Psychiatry 1995; 8(5): 272-275
(Type V evidence – expert opinion with review of 21 papers published during previous 12 months)
1.1g. Extremely low birth weight children, even with optimal socio-economic environments, have a 50% chance of requiring special educational services and 20% are significantly disabled* i. (USA)
*(presence of one or more severe functional disabilities including autism, cerebral palsy, mental retardation, borderline intelligence with global intellectual problems)
See also statement 1.2e.
i. Halsey CL, Collin MF, Anderson CL. Extremely low-birth-weight children and their peers. A comparison of school-age outcomes. Archives of Pediatrics & Adolescent Medicine 1996; 150(8): 790-794
(Type IV evidence – an observational comparison, in the US, of 54 ELBW children matched for race, gender and socioeconomic status)

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1.1h. Between 5 and 34 years of age, the prevalence of severe intellectual disability is three times higher among the Asian community compared to the non-Asian communityi. i. Emerson E, Azmi S, Hatton C, et al. Is there an increased prevalence of severe learning disabilities among British Asians? Ethnicity & Health 1997; 2(4): 317-321
(Type IV evidence - observational studies with age specific prevalence rates for three Metropolitan Boroughs in the North East of England)
1.1i. For most children with intellectual disability, between 30-60%, the cause is unknowni. i. Minns RA. Pre and perinatal conditions contributing to mental retardation. Current Opinion in Psychiatry 1997; 10(5): 354-59
(Type V evidence – expert opinion with review of 42 papers published during previous 12 months)
1.1j. 80% of individuals with autism have significant intellectual disability. Conversely, autistic ‘traits’ are very common amongst people with intellectual disability: the full syndrome occurs in 17% overall and 27% of those with an IQ < 50i,ii. i. Berney TP. Autism – an evolving concept. British Journal of Psychiatry 2000; 176: 20-6
(Type IV evidence – wide ranging review of 93 recent papers)
ii. Deb S, Prasad KBG. The prevalence of DSM3-R autistic disorder among the children with a learning disability in the north-east of Scotland. British Journal of Psychiatry 1994; 165: 395-399
(Type IV evidence – cross sectional study)
1.1k. Fragile X syndrome is not as common as was once thought with a revised prevalence of 1 in 5700 children of school agei. i. Morton JE, Bundey S, Webb TP, MacDonald F, Rindl PM, Bullock S. Fragile X syndrome is less common than previously estimated. Journal of Medical Genetics 1997; 34(1):1-5
(Type IV evidence – population study of school children in Coventry)
1.2 Risk Factors.

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1.2a. Bacterial meningitis in childhood may result in a lower IQ score but rarely gives rise to significant intellectual disability (IQ£70)i,ii.
There is significant variation in likelihood of sequelae dependent upon the causative organism. Lower IQ score occurs with Neisseria meningitidis (2.1%), Haemophilus influenzae (6.1%) and Streptococcus pneumoniae (17%)ii.
i. Anderson V, Bond L, Catroppa C, Grimwood K, Keir E, Nolan T. Childhood bacterial meningitis: Impact of age at illness and acute medical complications on long term outcome. Journal of the International Neuropsychological Society 1997; 3(2): 147-158
(Type IV evidence - case-control study of 130 post meningitic children and 130 sex matched controls)
ii. Baraff LJ, Lee SI, Schriger DL. Outcomes of bacterial meningitis in children: a meta-analysis. Pediatric Infectious Disease Journal 1993; 12(5):389-394
(Type IV evidence - meta analysis of 45 observational studies since 1955, with a total of 4920 children with acute bacterial meningitis)
1.2b. Maternal smoking in pregnancy may be a preventable cause of intellectual disability (IQ£70). Smoking one cigarette per day on five or more days per week in pregnancy may increase the risk of intellectual disability by 50%. There is evidence of a dose – response relationshipi.
(Health gain notation – 6 "likely to be harmful")
i. Drews CD, Murphy CC, Yeargin-Allsopp M, Decouflé P. The relationship between idiopathic mental retardation and maternal smoking during pregnancy. Paediatrics 1996; 97(4). 547-553
(Type IV evidence - case control study, 221 children with idiopathic intellectual disability and 400 controls from local schools)
1.2c. Risk factors for intellectual disability need to be re-evaluated in the context of other important maternal, perinatal and neonatal characteristicsi.
Characteristics associated with intellectual disability include low socio-economic status of the family (44-50% of intellectual disability) and low level of maternal formal education (20%). Other significant associations include maternal IQ£70, multiple births, low weight gain in pregnancy (<10 pounds), maternal anaemia and maternal urinary tract infection.
i. Camp BW, Broman SH, Nichols PL, Leff M. Maternal and neonatal risk factors for mental retardation: defining the 'at-risk' child. Early Human Development 1998; 50(2):159-73
(Type IV evidence - prospective cohort study, 35704 North American children followed up to 7 years of age with data on 12 neonatal factors)

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1.2d. Low Birth Weight (LBW) (birth weight £2500g) affects intellectual, educational and health outcomesi.
Children with a birth weight £2500g have a higher rate of health and educational problems at seven years of age.
Children with a birth weight £1500g are more likely to have visited a GP, physiotherapist, speech therapist and eye specialist, and to require additional help in school.
i. Middle C, Johnson A, Alderdice F, Petty T, Macfarlane A. Birthweight and health and development at the age of 7 years. Child: Care, Health & Development 1996; 22(1): 55-71
(Type IV evidence - retrospective cohort study of 1169 UK children (aged 7) grouped by weight at birth)
1.2e. Extremely Low Birth Weight (ELBW) (birth weight £800g) affects intellectual, educational and health outcomes. ELBW survivors are three times more likely to demonstrate intellectual disability at school age compared to full term babies with a birth weight ³2500gi.
The most likely outcome for ELBW survivors at school age is a learning disorder, often multiple, or borderline intellectual functioning, combined with behavioural and motor risk factors rather than severe / multiple disability.
See also statement 1.1g.
i. Whitfield MF, Ekstein Grunau RV, Holsti L. Extremely premature (< or = 800 g) schoolchildren: multiple areas of hidden disability. Archives of Disease in Childhood Fetal & Neonatal Edition 1997; 77(2): F85-90
(Type IV evidence - prospective cohort study of 115 extremely low birth weight North American children compared with 50 age and socio-demographic controls)
1.2f. Cerebral ventricular dilatation may be the strongest predictor of intellectual disability and cerebral palsy in ELBW babiesi. i. Waugh J, O'Callaghan MJ, Tudehope DI, et al. Prevalence and aetiology of neurological impairment in extremely low birthweight infants. Journal of Paediatrics & Child Health 1996; 32(2): 120-124
(Type IV evidence - cross sectional study of 194 extremely low birthweight Australian children at 2 years of age)
1.3 Prevention
1.3a. Surgical correction of single suture craniosynostosis in the first year of life may not prevent intellectual disability* or improve global cognitive functioning at a later stagei.
(* as defined by Bayley Scales of Infant Development and the McCarthy Scales of Children’s Abilities)
(Health gain notation – 5 "unlikely to be beneficial")
i. Kapp-Simon KA. Mental development and learning disorders in children with single suture craniosynostosis. Cleft Palate –Craniofacial Journal 1998; 35(3): 197-203
(Type IV evidence - longitudinal evaluation of 84 consecutive children up to 50 months after initial assessment for surgery)
1.3b. The reduction or cessation of smoking in pregnancy may reduce the likelihood of learning disability in offspringi. See statement 1.2b.
(Health gain notation – 2 "likely to be beneficial")
i. Drew CD, Murphy CC, Yeargin-Allsopp M, Decouflé P. The relationship between idiopathic mental retardation and maternal smoking during pregnancy. Paediatrics 1996; 97(4): 547-553
(Type IV evidence - case control study, 221 children with idiopathic intellectual disability and 400 controls from local schools)

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Health Evidence Bulletins: Wales, Duthie Library, UWCM, Cardiff CF14 4XN. e-mail: weightmanal@cardiff.ac.uk