Down Syndrome

Down syndrome is a genetic disorder and the most common cause of learning difficulties. Individuals with Down syndrome usually present with moderate to severe learning difficulties, characteristic facial features (including a flat nasal bridge, a small mouth with a protruding tongue, and reduced muscle tone) and are at increased risk of vision and hearing difficulties, heart problems, and Alzheimer-type dementia. Down syndrome results from additional genetic material on chromosome 21, and it is often referred to as Trisomy 21. This entry provides an overview of Down syndrome, including its history; its prevalence, diagnosis, and prognosis; its relationship to hearing loss; its neurocognitive profile; and its role in language-acquisition debates.