Children with mental retardation (MR) and developmental delays experience a wide range of psychopathology.18 Problems of definition, level of retardation, measurement, and assessment make it difficult to clarify the nature of psychopathology in this population, however.
Prevalence studies have suggested that 7% to 21% of children with MR have ADHD.19 Some of the disorders with prominent ADHD symptoms and for which there is an etiology for MR include fragile X syndrome, fetal alcohol syndrome, and Williams syndrome.
Fragile X syndrome is the most common inherited cause of MR. It is the result of an excessive repetition of the nucleotide sequence CGG on the X chromosome.
Children with fragile X syndrome range from those mildly affected without MR to those with severe MR and autistic features. The degree of involvement depends on the amount of fragile X mental retardation 1 (FMR1) protein that is produced. Fully affected male individuals produce no FMR1 protein. Most male patients with fragile X syndrome have distinctive physical features, including a long face, prominent ears, and macro-orchidism, which often begins to develop at age 8.
Boys and men with fragile X syndrome have a spectrum of difficulties with social relatedness. Poor eye contact, perseveration, and sensory sensitivity are prominent symptoms. Hyperactivity and attentional problems occur in most individuals with the fragile X syndrome.20
Fetal alcohol syndrome (FAS) is a common cause of MR, first noted in the early 1970s. Children with FAS have a characteristic facial appearance, including an elongated midface, thin upper lip, flattened maxilla, and microcephaly.21 In addition, they often have growth retardation.
Children with FAS are prone to a variety of developmental delays, along with symptoms of ADHD and memory and abstraction difficulties.22,23
Williams syndrome is caused by a microdeletion on chromosome 7 that includes the elastin locus. The syndrome is characterized by congenital facial and cardiovascular abnormalities and MR.
Children with Williams syndrome often are socially disinhibited and are seen as outgoing and friendly. Inattention, impulsivity, and hyperactivity are commonly associated features. Unlike typical youngsters with ADHD, however, these children have a marked degree of persistent fears, phobias, and anticipatory anxiety.24
Generalized resistance to thyroid hormone is a rare, autosomal dominant, genetic disorder. It is characterized by reduced responsiveness of pituitary and peripheral tissues to the action of thyroid hormone. Children with this disorder appear clinically euthyroid or mildly hypothyroid. Laboratory measures indicate elevated triiodothyronine and thyroxine levels without thyroid-stimulating hormone suppression.
Affected children have a variety of cognitive difficulties. In a recent study, 70% were found to have ADHD.25 Because generalized resistance to thyroid hormone is extremely rare, however, thyroid testing of children with ADHD is not recommended as part of a routine evaluation.26,27
Lyme disease is caused by a spirochete transmitted to humans by the deer tick. Common first signs include a distinctive rash followed by flulike symptoms. Fallon et al.28 reported a case of Lyme disease in a 7-year-old girl who first presented with complaints of problems focusing in school. She met criteria for ADHD–inattentive type but demonstrated additional symptoms of lethargy, irritability, forgetfulness, and headaches, along with poor coordination, joint pain, word-finding difficulties, and sensitivity to light and sound. Her symptoms resolved with antibiotic treatment.
Brown et al.29 studied 31 children with severe closed head injuries (CHI) over the course of 2.25 years. A new psychiatric disorder developed in nearly half the subjects. Five developed symptoms resembling a “frontal lobe syndrome,” including excessive talking, carelessness, and impulsiveness. Psychiatric sequelae were limited to the group of patients whose injury resulted in a post-traumatic amnesia of at least 7 days. Psychosocial adversity was a good predictor of psychiatric disorder.
Max et al.30 studied 42 patients with CHI. New psychiatric disorders developed in 36% of their subjects over the course of 2 years, and 6 of them met criteria for ADHD.
Using structured interviews, Gerring et al.31 examined 99 children and adolescents who had experienced moderate to severe CHI. Nineteen (19%) met criteria for ADHD prior to injury, suggesting that children with ADHD are disproportionately represented in the CHI population. At reevaluation 1 year after the injury, an additional 15 children met criteria for ADHD. The number of new cases is higher than in the general population. Consistent with the findings of Brown et al.,29 this study found that subjects who developed secondary ADHD had a higher level of psychosocial adversity than those who did not.
Reviewed and revised June 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.
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