Causal origins of major mental handicap in the Canadian Maritime provinces

The outcome of diagnostic studies on the etiology of mental retardation: Considerations on the classification of the causes

In the different studies on the etiology of mental retardation (MR), a diagnosis has been reached in a remarkably variable proportion of cases. This is due to differences in methodology (population examined, age of the patients, degree of MR, time and protocol of investigation, expertise), method of classification (according to timing of defect, type of defect or both), and, in particular the different use of the term diagnosis. The latter varies with the employed criteria (causal vs. pathomorphological vs. pathophysiological), with the degree of exclusiveness (single cause vs. multifactorial/predisposing background factors) and particularly with the degree of implied certainty (definite vs. provisional). All these factors contribute to the fact that in general, the results of different clinical studies on the etiology of MR are not strictly comparable and cannot be pooled. A dynamic method of classification is proposed which combines both time of occurrence and type of defect, and includes the degree of certainty of a diagnosis. The method allows identification of patients which need follow-up and reconsideration of their cases.

Klinefelter syndrome is a common cause for mental retardation of unknown etiology among prepubertal males

Klinefelter syndrome (KS) has not typically been associated with mental retardation (MR), however, in recent years a growing body of evidence suggested that KS boys often experience language deficits and academic difficulties. In this study, we screened DNA samples from 1205 patients originally referred for fragile X syndrome (FRAX) testing, because of MR of unknown etiology and detected 8 KS patients. A similar number of males in the same age group were found to have FRAX; 3 of them had a family history of FRAX. Based on these findings, KS might be the most common cause of MR of unknown etiology among prepubertal males. Because of the significant benefits of early recognition and treatment of KS, we emphasize the importance of cytogenetic testing of all prepubertal males with cognitive impairment even without dysmorphic features.

Evaluation of mental retardation: recommendations of a Consensus Conference: American College of Medical Genetics

A Consensus Conference utilizing available literature and expert opinion sponsored by the American College of Medical Genetics in October 1995 evaluated the rational approach to the individual with mental retardation. Although no uniform protocol replaces individual clinician judgement, the consensus recommendations were as follows: 1. The individual with mental retardation, the family, and medical care providers benefit from a focused clinical and laboratory evaluation aimed at establishing causation and in providing counseling, prognosis, recurrence risks, and guidelines for management. 2. Essential elements of the evaluation include a three-generation pedigree: pre-, peri-, and post-natal history, complete physical examination focused on the presence of minor anomalies, neurologic examination, and assessment of the behavioral phenotype. 3. Selective laboratory testing should, in most patients, include a banded karyotype. Fragile X testing should be strongly considered in both males and females with unexplained mental retardation, especially in the presence of a positive family history, a consistent physical and behavioral phenotype and absence of major structural abnormalities. Metabolic testing should be initialed in the presence of suggestive clinical and physical findings. Neuroimaging should be considered in patients without a known diagnosis especially in the presence of neurologic symptoms, cranial contour abnormalities, microcephaly, or macrocephaly. In most situations MRI is the testing modality of choice. 4. Sequential evaluation of the patient, occasionally over several years, is often necessary for diagnosis, allowing for delineation of the physical and behavioral phenotype, a logical approach to ancillary testing and appropriate prognostic and reproductive counseling.

Reported biomedical causes and associated medical conditions for mental retardation among 10-year-old children, metropolitan Atlanta, 1985 to 1987

This report describes biomedical causes of mental retardation (MR) among school-age children and associated medical conditions in children for whom no cause was reported. This study involved 715, 10-year-old children with MR (intelligence quotient [IQ] 70 or less) born between 1975 and 1977. We determined biomedical causes of MR using a hierarchical approach based on the timing of the event (i.e. prenatal, perinatal, or postneonatal). Among children with no identified biomedical cause the occurrence of associated medical conditions was examined. No reported biomedical cause could be found in 78% of children with MR (87% mild, IQ 50 to 70; 57% severe, IQ < 50). Prenatal causes were present in 12%, perinatal causes in 6%, and postneonatal causes in 4%. On the basis of these findings it was concluded that intensive use of public health prevention strategies can reduce the number of children who receive a diagnosis of MR.

Multihandicapped blind and partially sighted children in south Germany. II: Aetiology and pathogenesis

The results of a classification of the aetiology of visual impairment and mental retardation in a series of 239 multihandicapped blind and partially sighted children are reported. These children had been born between 1981 and 1987 and were examined in a population-based study in South Germany. There was marked aetiological heterogeneity. Most prominent were causes related to the perinatal period and late gestation which were found in 41% of the children. Among these, lesions of the visual pathways in preterm children were especially frequent and could be explained by periventricular leukomalacia in a large proportion. The distribution of the major aetiologic groups is different from populations with isolated mental retardation and populations with isolated visual impairment but similar to a Swedish population with spastic quadriplegia. The implications for the prevention of severe multiple impairments in children are discussed.

The aetiology of learning disability in preschool children with special reference to preventability

The aims of the study were to determine the aetiology of moderate and severe learning disability in preschool children in Cape Town, to consider the causes by race and to assess the degree of preventability. All children under the age of 6 years who presented to the Developmental Assessment Clinics at the Red Cross Children's Hospital with a developmental quotient of less than 50 over a 1-year period were included. The Development Assessment Clinics form a regional multidisciplinary service for the evaluation of preschool children with development disabilities. Over the 12-month period, 232 children were identified, of whom 65% were Coloured, 32% African and the remaining 3% white. This roughly corresponds with teh breakdown of births by race in the region. Regarding the aetiology, in 45% the disability was prenatal in origin, in 17% perinatal, in 9% postnatal and in a quarter it was idiopathic. In the Coloured and white children, the cause in over a half arose prenatally and in 8.3% postnatally. In contrast, the cause in less than a third of the African cases arose in the prenatal period, but 36% resulted from perinatal complications. More of the African children had multiple disabilities and additional disabilities were more common in the perinatal and postnatal categories. Forty per cent of all cases were considered to be preventable, 16% with existing knowledge and current practice. There was no significant difference in preventability between the races.

The detection of subtelomeric chromosomal rearrangements in idiopathic mental retardation

A major challenge for human genetics is to identify new causes of mental retardation, which, although present in about 3% of individuals, is unexplained in more than half of all cases. We have developed a strategy to screen for the abnormal inheritance of subtelomeric DNA polymorphisms in individuals with mental retardation and have detected three abnormalities in 99 patients with normal routine karyotypes. Pulsed-field gel electrophoresis and reverse chromosome painting showed that one case arose from an interstitial or terminal deletion and two from the de novo inheritance of derivative translocation chromosomes. At least 6% of unexplained mental retardation is accounted for by these relatively small chromosomal abnormalities, which will be an important resource in the characterization of the genetic basis of neurodevelopment.

Cerebral palsy--an increasing contributor to severe mental retardation?

It is estimated that the prevalence of nongenetic SMR associated with cerebral palsy has risen from 0.7 to about 0.9 per 1000 live births in the last decade. This is due to the predicted rise in total cerebral palsy prevalence to 2.5/1000 live births. This predicted prevalence of cerebral palsy is similar to that given for Western Australia in 1979-82, allowing for postnatal causes, but is higher than prevalence data from England and Sweden for that period. The estimated rise is due largely to improved survival and increased proportion of low birthweight babies since 1983 and also reflects the use of prevalence rates based on Mersey data. Improvements in prenatal diagnosis, and a parental choice of selective termination may lead to reductions in other causes of SMR, such as Down's syndrome and neural tube defects, so it seems that children both physically and mentally handicapped due to brain damage will contribute a greater proportion of the SMR population. The careload of these children is greater than that associated with many other causes of SMR and most survive into adult life. The implications for planning future services will need to be recognised.

The aetiology of intellectual disability in Western Australia: a community-based study

A register of intellectual disability is being established to assess the level and aetiology of intellectual disability in all children born and/or living in Western Australia. 1602 children aged between six and 16 years were identified who had IQs less than 70. 40 per cent had a definite genetic basis, 20 per cent an environmental cause and 40 per cent were of unknown aetiology. The insult was prenatal in 61 per cent, 10 per cent had a possible perinatal cause, 8 per cent were postnatal and for 21 per cent the timing could not be assessed. Approximately 20 per cent had concomitant cerebral palsy and 13 per cent were epileptic. A disparity was found between rural and urban areas, the prevalence being 9.9 and 6.5 per 1000 live births.

An epidemiological study on severe mental retardation among schoolchildren in Bologna, Italy

The prevalence, causal origin of and impairments associated with severe mental retardation (SMR) were investigated among all school-age children (six to 13 years) living in the city of Bologna, Italy. 90 children (57 boys, 33 girls) with IQs less than or equal to 50 were identified. The prevalence of SMR was 4.2 per 1000 for males, 2.5 per 1000 for females and 3.4 per 1000 for both sexes. Causal origin was prenatal for 33.3 per cent, perinatal for 14.4 per cent, combined pre- and perinatal for 5.6 per cent and postnatal for 13.3 per cent. Another 12.3 per cent of the children with IQs less than or equal to 50 had autism or childhood psychosis, while there was no evident cause of mental retardation for the remaining 21.1 per cent. 50 per cent had at least one associated physical or neurological impairment other than mental retardation, with epilepsy and cerebral palsy predominating.

Prevalence of mental retardation related to fragile X syndrome and other chromosomal abnormalities in the Republic of San Marino

The prevalence of mental retardation related to chromosomal abnormalities, including fragile X syndrome, was studied among 2735 males aged between five and 20 years living in the Republic of San Marino. Five cases of chromosomal abnormalities were found, including one case with fragile X syndrome, with partial epilepsy and a CT scan showing a large arachnoid cyst in the posterior fossa. The prevalence of mental retardation due to chromosomal abnormality was 1.83 per 1000, and that due to fragile X syndrome was 0.37 per 1000. Although these findings relate to only a small population, they are the first from a Mediterranean area.

Genetic causes of mental handicap and opportunities for prevention

Prior to 1970 nearly one half of severe mental handicap (IQ less than 50) was unexplained and genetic causes were recognized in one third of all patients. In recent years the genetic contribution has risen to 50%, with the decline in importance of environmental causes and the growing recognition of X-linked forms of mental retardation and chromosomal microdeletion syndromes. Recognition of genetic conditions is vital for accurate assessment of recurrence risks and in order, in some instances, to provide specific prenatal diagnosis. With full utilization of existing prenatal screening procedures and a more detailed family history, severe mental handicap due to genetic conditions should be reduced by 30-40%.