الفهرس 
Down syndromeOnly 14 pages are availabe for public view
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Abstract
D
own syndrome (DS) is a complex genetic disorder caused by the presence of three copies of the whole or part of the chromosome 21 in humans. The resulting gene-dosage imbalance is responsible for a variety of phenotypes affecting various organs and notably leads to intellectual disability (ID) in the whole DS population. Some characteristics of the syndrome are near universal, including a characteristic facial appearance, learning disability, muscle hypotonia, motor dysfunction, and Alzheimer’s disease neuropathology after 35 years of age. However, there are large individual differences in the DS phenotype and severity of associated symptoms.
Besides the mild to severe ID, language is often reported to be a domain of particular difficulties. Children and adolescents with DS reach developmental language milestones only with a considerable delay, often lagging even behind expectations based on their nonverbal mental development.
Phonological short term memory (PSTM) is another domain where individuals with DS are significantly impaired in comparison to control participants matched in nonverbal mental age. PSTM is a component of working memory. Most prominent in research on developmental language disorders is Baddeley’s model of working memory. The model consists of two modality-specific short-term storage systems where restricted amounts of information can be maintained for only a short period of time: the visuo-spatial sketchpad concerned with visual and spatial information and the phonological loop responsible for the temporary storage of phonological representations. The phonological loop has two components; a storage component (the phonological store) and an articulatory subvocal rehearsal process. Speech-based information has automatic access to the phonological store, but suffers from forgetting from this passive store over time. However, forgetting can be countered by active subvocal rehearsal of this information which is thought to refresh items within the store.
Several researchers have investigated the underlying mechanisms for PSTM difficulty in individuals with DS, but no clear picture has emerged. For example, the impairment does not seem related to failure to rehearse, as group differences are apparent even when neither group is rehearsing. That leaves the phonological loop storage component as its likely source.
In research studies, PSTM has most often been measured using digit or word span tasks in which participants listen to lists of digits or words and repeat them back immediately in the same order. It has also been measured using non word repetition (NWR) tasks in which participants listen to pronounceable non words and repeat them back immediately. Often the number of digits/words in the list or the number of phonemes/syllables in the non word increases until performance declines.
The current study assessed language and PSTM in 20 DS children with mild degree of ID (aged 6-12 years) and compares their scores in subtest items against two control groups: 1- 20 Non syndromic children with mild degree of ID (they were selected to match the case group as regard chronological age and mental age). 2- 20 Normally developing children (they were selected to match the case group as regard chronological age) using the Modified Preschool Language Scale, Fourth Edition (PLS-4) Arabic version, and Phonological Short Term Memory test (PSTM) Arabic version which are comprehensive valid diagnostic tools. Arabic PSTM test is characterized by its comprehensive evaluation of PSTM through variable parameters which include; digit span as a function of the phonological loop, syllable repetition as a convenient measure for children with articulation disorders, NWR as a neat measure of PSTM, similar and dissimilar word set recall to measure the influence of similarity on PSTM. Words of variable length have been used in the later items to investigate the effect of word length on PSTM.
After the application of Arabic PLS-4 and Arabic PSTM test on the three groups, it is obvious that:
According to the application of the PLS-4, children with DS with mild degree of ID and non syndromic children with mild degree of ID scored significantly lower than the normally developing children matched for chronological age regarding receptive language age, expressive language age and total language age. DS group also scored significantly lower than non syndromic children with mild degree of ID matched for mental age regarding expressive and total language ages but were equal in receptive language age. DS children were also found to have better receptive than expressive language abilities.
According to the application of the PSTM test, children with DS with mild degree of ID and non syndromic children with mild degree of ID scored significantly lower than the normally developing children matched for chronological age regarding the scores of subtests of the PSTM test (digit span, syllable repetition, non sense word repetition, dissimilar word set recall, similar word set recall and total score). DS group also scored significantly lower than non syndromic children with mild degree of ID matched for mental age regarding the scores of subtests of the PSTM test. All DS children of the current study (100%) were found to have impairment in NWR task which has been proven a sensitive tool to test for PSTM capacities.
In the Down Syndrome with mild degree of ID group, mental age had statistically significant positive correlation with total language age, IQ and total raw score of the PSTM test. Statistically significant strong positive correlation were also found between total language age and total row score of the PSTM test and between IQ with both total language age and total row score of the PSTM test.
In non syndromic children with mild degree of ID group, statistically significant strong positive correlation was observed between mental age and total Language age and between total language age and total row score of the PSTM test.
In the normally developing children group, a perfect positive correlation was observed between mental age and total language age.