Handbook of Genetic Counseling/Developmental Delay and Mental Retardation-2

Developmental Delay and Mental Retardation

Introduction

What is your understanding of why you were referred?
What is your main concern?
I understand that some members of your family have MR
Do you know what caused their MR?
You were referred to GC because sometimes there is an underlying genetic condition that is the cause MR
Sometimes a cause of MR remains unknown
Obtain prenatal history

Many causes of developmental delay and MR
Common causes are genetic syndromes, multifactorial conditions, environmental exposures, infections during pregnancy, drug and alcohol use during pregnancy, problems with the birth or delivery, prematurity
Cause in many individuals remains unknown
In mild MR, a specific risk factor has been identified in about 43% of the cases studied. Only 24% of these cases yield a fairly clear or convincing diagnosis. 2, 3 Of those, 14% are classified as genetic and 10% environmental.
With moderate-severe MR, a cause is determined in up to 64% of cases with 45% of those being linked to genetic causes and 19% to environmental factors. 4
The seemingly lesser role of genetics in milder retardation may have something to do with the fact that some who fall into the mild MR category may simply represent the low end of the normal range of human intelligence.
Severe mental retardation is more likely to have a genetic component (50% of cases have a major chromosomal cause).
Down's syndrome accounts for 5% of mild MR cases and 30% of severe MR cases, 6, 7
telomere deletions or duplications account for about 7.5% of MR 19
Fragile X is seen in 5% of both mild and moderate-severe MR. 8, 9
more than 500 other genetic diseases, mostly very rare, have also been associated with MR 10

2-3% of the general population is affected with some level of mental retardation
85-90% of patients with mental retardation fall into the mild range (IQ of 50-70)
Most live at home or in the community and have some level of independence.

This is largely dependent on the cause of the mental retardation.
If cause is unknown, recurrence can be quoted as 1-25%, (this takes into account causes ranging from a non-genetic environmental exposure to an autosomal recessive condition)
Studies have shown that a couple who has had one child with MR are about ten times more likely to have another
Study at Greenwood genetic center preliminary results
- 452 probands with average IQ of 41.1 and head circumference in 43%ile with idiopathic MR and a subsequent sibling
- 502 male and 468 female subsequent sibs
- 21% of brothers and 14% of sisters also had MR.
- Recurrence in blacks was 18.3% and whites 17%
- Higher recurrence risks were positively correlated with higher IQ
Question remains of whether it is genes, environment or a combination

Taking the Challenge: Finding Recurrence Risks in Idiopathic Mental Retardation. J.S. Collins, A.F. Nave, G.A. Satten, R.E. Stevenson. Greenwood Genetic Center, Greenwood, SC;
Centers for Disease Control and Prevention, Atlanta, GA. http://www.faseb.org/genetics/ashg01/f243.htm
Genetics and Mental Retardation. V. Reid Sutton, M.D. (article found at www.thedoctorwillseeyounow.com/articles/other/mr_16/)

The information in this outline was last updated in 2002.