A study of institutionalized mentally retarded patients in Manitoba. I: Classification and preventability

Genetic syndromes among individuals with mental retardation

Individuals with mental retardation more commonly have malformations and other structural anomalies than individuals without mental retardation. In many cases, the associated anomalies comprise recognizable syndromes caused by genetic or environmental insults. To co-occurrence of structural anomalies with mental retardation thus assists in the diagnostic evaluation, particularly in infants and young children. The coexistence of structural and mental abnormalities also suggests that both originated in the embryonic period of development.

X-linked mental retardation with seizures and carrier manifestations is caused by a mutation in the creatine-transporter gene (SLC6A8) located in Xq28

A family with X-linked mental retardation characterized by severe mental retardation, speech and behavioral abnormalities, and seizures in affected male patients has been found to have a G1141C transversion in the creatine-transporter gene SLC6A8. This mutation results in a glycine being replaced by an arginine (G381R) and alternative splicing, since the G-->C transversion occurs at the -1 position of the 5' splice junction of intron 7. Two female relatives who are heterozygous for the SLC6A8 mutation also exhibit mild mental retardation with behavior and learning problems. Male patients with the mutation have highly elevated creatine in their urine and have decreased creatine uptake in fibroblasts, which reflects the deficiency in creatine transport. The ability to measure elevated creatine in urine makes it possible to diagnose SLC6A8 deficiency in male patients with mental retardation of unknown etiology.

Splitting and lumping in the nosology of XLMR

Although it is assumed that genes that influence cognitive function are ubiquitous in the human genome, to date, more such genes have been found on the X chromosome than on any other comparable segment of the autosomes. This is in large measure because of the power of hemizygosity in exposing mutations of X-linked genes in males. Clinical manifestations, mapping of gene loci by linkage analysis or chromosome rearrangements, and gene identification by positional cloning or mutational analysis of candidate genes have permitted extensive lumping and splitting within the large and heterogeneous category of X-linked mental retardation (XLMR). Approximately 130 XLMR syndromes have been identified, 25 gene loci have been mapped and cloned, and 55 other loci have been mapped but not cloned. Well-recognized syndromes (e.g., Fragile X and Coffin-Lowry syndromes) and syndromes represented by only a single family (e.g., Arena and monoamine oxidase-A syndromes) are among these more or less well-defined entities. In addition, more than 75 families with nonsyndromal XLMR have been regionally mapped and 7 causative genes have been identified.

Outcome of the routine assessment of patients with mental retardation in a genetics clinic

This study reviewed hospital and genetics clinic records of 411 patients evaluated in our department from 1986 to 1997 inclusive. Major objectives were to establish how often and under what circumstances a specific genetic/syndrome diagnosis was made and to determine the value of laboratory tests in the hope of gaining a more selective approach to referral, evaluation, and use of the laboratory. A specific genetic/syndrome diagnosis was made in 19.9% of cases, and in a further 4.4% the referring diagnosis was eliminated but no new diagnosis made. There was a significant excess of affected males (277:134) and of affected male sib pairs over expectation, suggesting an additional, potentially important, contribution from nonspecific X-linked mental retardation (MR). Factors associated with making a diagnosis included referral from a pediatrician or neurologist, absence of cerebral palsy, presence of more than three minor anomalies and/or an unusual appearance, a recognizable Gestalt or key anomaly. There was a linear relationship between the likelihood of making a diagnosis and the number of minor anomalies. Factors not associated with making a diagnosis included the year when the patient was seen, degree of MR, number of prior specialists seen, presence of a major malformation, occurrence of seizures, and a head circumference either <3rd or >97th centile. Although chromosome studies were somewhat less likely to be ordered in patients with less severe MR, the positive rate was unaffected by the severity of the MR. The rate of abnormal results was positively correlated with the presence of minor anomalies and/or an unusual appearance. None of 134 studies carried out on patients with </=3 minor anomalies alone were positive. Fragile X studies were less likely to be ordered with increasing levels of MR and 10 of 14 positive test results were among those with mild delay. Thirteen of 14 diagnoses were based on Gestalt. Molecular and fluorescence in situ hybridization studies had a positive rate of >60% when ordered by a clinical geneticist compared with 0% when ordered by other physicians. Results showed that use of the laboratory was inconsistent and not clearly based on the findings in a particular child. Significant changes in patterns of referral and the evaluation process could be made that would result in significant economies of time and laboratory use and a minimum level of missed diagnoses.

X-linked mental retardation syndrome with short stature, small hands and feet, seizures, cleft palate, and glaucoma is linked to Xq28

Of the gene-rich regions of the human genome, Xq28 is the most densely mapped. Mutations of genes in this band are responsible for 10 syndromal forms of mental retardation and 5 nonsyndromal forms. Clinical and molecular studies reported here add an additional syndromic form of X-linked mental retardation (XLMR) to this region. The condition comprises short stature, small hands and feet, seizures, cleft palate, and glaucoma. One affected male died at age 19 years in status epilepticus, but others have survived to old age. Carrier females do not have somatic anomalies or mental impairment. The gene is localized to the terminal 8 Mb of Xq28 with markers distal to DXS8011 showing linkage to the disorder with a lod score of 2.11 at zero recombination.

The evaluation of the child with a global developmental delay

The child with a global developmental delay presents a diagnostic challenge to the practitioner. The spectrum of possible etiologic diagnosis and laboratory investigations that could be pursued are quite extensive. This article presents the rationale for diagnostic testing in this population and provides guidelines to those tests that should be pursued.

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.

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.

Diagnostic yield of the neurologic assessment of the developmentally delayed child

OBJECTIVE

The aim of this study was to determine the etiologic yield of the neurologic assessment of a consecutive cohort of developmentally delayed children.

STUDY DESIGN

A retrospective chart review was carried out on all patients referred to a single university-based pediatric neurologist for evaluation of global developmental delay from July 1991 to December 1993. Patients referred because of isolated speech or motor delay or autism or those who had been previously evaluated by another neurologist were excluded.

RESULTS

A total of 77 patients were identified; 47 were male, and 62 were referred by a pediatrician. Neurologic evaluation did not confirm global delay in 10, and 8 did not complete diagnostic evaluation; one child was included in both groups. Of the remaining 60, an etiologic diagnosis was suspected by the referring physician at the time of referral in 13. Although parents suspected a delay at a mean age of 0.66 (+/- 0.69) year, children were examined by the neurologist at a mean age of 3.58 (+/- 2.42) years. Twenty-five were mildly delayed, 23 were moderately delayed, and 12 were severely delayed. Diagnostic studies (history, physical examination, and selected investigations, including screens for metabolic disease, karyotype, fragile X testing, electroencephalography, and neuroimaging) yielded an etiologic diagnosis in 38 (63.3%) of the 60 patients. Etiologic categories included cerebral dysgenesis (16.7%), hypoxic-ischemic encephalopathy (10.0%), chromosomal abnormalities (10%), toxins (8.3%), metabolic disorders (5.0%), and neurocutaneous (3.3%), neuromuscular (3.3%), genetic/dysmorphic (3.3%), and epileptic (3.3%) syndromes. Etiologic yield was equivalent across categories and degree of developmental delay.

CONCLUSION

Referral to a pediatric neurologist and application of a selected battery of investigations yield etiologic findings with important implications with respect to management, prognosis, and recurrence risk estimate in a significant portion of globally delayed children.

Evaluation of the child with idiopathic mental retardation

A review of the advances in diagnostic techniques for evaluation of children with idiopathic mental retardation is presented. The current status of the use of clinical genetics, cytogenetics, molecular genetics, and neuroimaging in evaluating children with mental retardation is emphasized. Special attention is given to the evaluation of children with mental retardation and "autism" or "cerebral palsy."

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.

Causal origins of major mental handicap in the Canadian Maritime provinces

From a retrospective epidemiological survey, the prevalence of major mental handicap among seven- to 10-year-old children in the Maritime region of Canada in 1980 was estimated to be 36.5 per 10,000 children. Based on information obtained for 221 of the 307 children ascertained by the survey, prenatal origins dominated (58 per cent), followed by perinatal (10 per cent) and postnatal (4 per cent). No specific cause could be determined for 27 per cent of the children, but 41 per cent of this group had epilepsy and/or cerebral palsy in addition to major mental handicap. Significant differences were found in the causal spectrum between the Maritime region of Canada and regions studied in other developed countries. The results of this survey have implications for planning prevention programs, and for epidemiological surveillance and monitoring of adverse reproductive outcomes.

A paediatrician asks--why is it called birth injury?

It is irrational to ascribe a child's so called 'brain damage' to labour or delivery without considering other factors. 'Brain damage' occurs without difficult labour or perinatal hypoxia and caesarean section is no guarantee against it. Severe difficulties in delivery and severe hypoxia at birth are in the great majority not followed by evidence of 'brain damage'. In the maternal history there is a significantly greater incidence of relative infertility, and of pregnancies associated with low birthweight or intrauterine growth retardation, postmaturity, antepartum haemorrhage, pre-eclampsia or infections. There are often genetic factors, more congenital anomalies and pathological evidence of underlying abnormality. There is an interaction of numerous factors, prenatal, perinatal and postnatal and it is simplistic to ascribe 'brain damage' to single factors, such as breech delivery or hypoxia at birth, without considering the antecedent causes of those factors.

Dermatoglyphic and palmar-crease alterations as indicators of early intra-uterine insult in mental retardation

A comparative study of unusual dermatoglyphic and palmar patterns revealed significant differences between the frequencies of certain patterns among 200 congenitally affected mentally retarded children and 500 normal controls. A scoring method demonstrating the significance of eight unusual patterns as non-specific indicators of early intra-uterine fetal insult was devised. 10 per cent of the children previously classified as idiopathically mentally retarded were shown to have been exposed to early intra-uterine insult. Dermatoglyphic and palmar-crease analysis should be included as a routine investigation for children with mental retardation of unknown cause.