Work up the potential causes of mental retardation (MR) to assist families in identifying potentially modifiable conditions or conditions that are genetic in origin so that they can receive appropriate counseling for their next child or other family relatives
Work up the potential causes of mental retardation (MR) to assist families in identifying potentially modifiable conditions or conditions that are genetic in origin so that they can receive appropriate counseling for their next child or other family relatives: Excerpt from Avoiding Common Pediatric Errors
Author:
Sonya Burroughs, MD
What to Do - Gather Appropriate Data
MRiscommon,affectingapproximately1%to 3% ofthegeneral population
and defined as an intelligence quotient (IQ) of <70, coupled with limitations
in adaptive abilities.
The differentiation between mild and severe MR is clinically important because this helps to determine which educational programs are most
beneficial for patients.
MR results from genetic and environmental factors. Genetics provide
the cognitive potential, which is molded by environmental factors. Fragile
X syndrome (FXS), fetal alcohol syndrome (FAS), and Down's syndrome
have been identified as the three most common identifiable causes of MR.
FXSis the leadingcauseofinheritedMR. The prevalenceof FXSis1 in
4,000 males; 1:6,000 females. Approximately, 4% to 8% of the cases of MR
in males aredue to this syndrome. The primary mutation is an increasein the
number of CGG trinucleotide repeats in the promoter region of the fragile
X mental retardation gene (FMR1) on the X chromosome. Individuals with
>230 CGG repeats have a "full mutation." Those with a modest increase
in repeats (55–230) are considered to have a "premutation." Based on the
mutation leading to this syndrome, it makes sense that the diagnosis of FXS
is via moleculartesting of the FMR1 gene to detecta CGGrepeatexpansion.
Chromosome analysis to detect the fragile site of the X-chromosome is no
longer used as a stand alone test because of its low sensitivity.
FXS is inherited in an X-linked dominant fashion. Remember, pre-
mutations may be carried by males. These males are usually phenotypically
normal until late adulthood. The premutation is passed to all of their daughters, but none of their sons. Women with the premutation or full mutation
have a 50% chance of passing the mutation to each offspring. Sons, who
inherit the full mutation, develop FXS. Half of daughters who inherit a
full mutation manifest mild-to-moderate symptoms, and the other half are
asymptomatic carriers.
Physical features associated with FXS usually become apparent after
puberty. Large ears and macro-orchidism are the two most commonly described physical features. Developmental delay (especially speech) and MR
are the most common prepubertal findings.
FAS is the most common preventable cause of MR. IQ scores range
from 20 to 120 in affected children. Those with normal IQ scores may have
significant neurobehavioral deficits. Children with FAS have a higher likelihood of psychiatric and behavioral disorders than those with other identifiable causes of MR. Diagnosis of FAS is easier between ages 2 and 11
years when the physical features (short palpebral fissures, smooth philtrum,
thin upper lip) are still present and central nervous system dysfunction
emerges (memory deficits, slow information processing, hearing deficits,
etc).
Down's syndrome is the most common genetic disorder causing mild-
to-moderate MR. It is usually identified based on physical features, including hypotonia, flat facial profile, hyperflexibility of joints, poor Moro reflex,
excess skin on back of neck, single palmar crease, and abnormal ears. Chromosome analysis confirms the diagnosis.
These threesyndromes arethe mostcommon identifiable causes of MR,
but other causes such as Prader-Willi, Angelman, Sotos, Autism, Rett, and
cri-du-chat (5p-) exist and should be considered in the differential diagnosis
when appropriate.
Identifying a cause of MR is extremely important because it relates to
treatment, prognosis, and genetic counseling. The initial evaluation should
focusonadetailedhistoryandphysicalexam.Thiswillhelpmakesomediagnoses more likely than others. Inquire about family members with learning
disabilities, stillbirths, miscarriages, and consanguinity. Routine chromosome studies may be the next step in the evaluation, or high-resolution
chromosome analysis may be needed for more subtle chromosomal abnormalities. Fluorescent in situ hybridization may be helpful when considering
a particular syndrome.Newborn screening testsare also effectivein identifying metabolicdisorders associated with MR. Becausemost of these disorders
are amenable to early treatment, newborn screening can also be seen as preventative.
Suggested Readings
Walker WO Jr, Johnson CP. Mental retardation: overview and diagnosis. Pediatr Rev. 2006;
27:204–212.
Weisner GL, Cassidy SB, Grimes SJ, et al. Clinical consult: developmental delay/fragile X
syndrome. Prim Care. 2004;31:621–625.
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Book Source Details
- Book Title: Avoiding Common Pediatric Errors
- Author(s): Anthony D Slonim MD, DrPH; Lisa Marcucci MD
- Year of Publication: 2008
- Copyright Details: Avoiding Common Pediatric Errors, Copyright © 2008 Lippincott Williams & Wilkins.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Avoiding Common Pediatric Errors
Authors: Anthony D Slonim MD, DrPH; Lisa Marcucci MD
Publisher: Lippincott Williams & Wilkins
Copyright: 2008
ISBN: 0-7817-7489-6
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