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  • Updated 04.12.2022
  • Released 10.29.1995
  • Expires For CME 04.12.2025

Down syndrome

Introduction

Overview

Down syndrome, the most common genetic cause of intellectual disability, is caused by the presence of all or part of an extra chromosome 21 (175; 117). Down syndrome occurs in an estimated 8 in 10,000 live births (163). It is characterized by varying levels of intellectual disability, ranging from mild to severe cognitive impairment (118), as well as variable neurodevelopmental profiles, with significant heterogeneity and individual differences in cognition, language, and social and behavioral difficulties. In addition to congenital birth defects, persons with Down syndrome are at risk for acquired medical conditions during childhood, adolescence, and throughout adulthood, as well as premature onset of aging-related conditions beginning in the third to fourth decades of life. Some of the co-occurring medical conditions associated with Down syndrome have atypical presentations that require a high index of suspicion for diagnosis. Healthcare guidelines for persons with Down syndrome are designed to detect occult conditions in asymptomatic individuals and to guide physicians evaluating medically complex or difficult-to-diagnose patients. Successful recognition and treatment of medical conditions is paramount to managing patients with a decline in functional skills.

Key points

• Down syndrome is caused by the presence of all or part of an extra chromosome 21 and occurs in an estimated 8 per 10,000 live births.

• Down syndrome is the most common genetic cause of varying levels of intellectual disability, ranging from mild to severe cognitive impairment, as well as variable neurodevelopmental profiles.

• There is significant heterogeneity and individual differences in cognition, language, and social abilities as well as a range of mental health disorders and behavioral difficulties in individuals with Down syndrome.

• Healthcare guidelines for children and adults with Down syndrome exist to guide appropriate screening for co-occurring medical conditions.

Historical note and terminology

The first comprehensive description of this condition was provided by Dr. John Langdon Down in 1866. In his report of 1866, Down highlighted some of the salient aspects of the physical appearance and behavioral attributes of individuals with this syndrome.

Dr. John Langdon Down
In 1866, Dr. John Langdon Down provided the first comprehensive description of what is now known as Down syndrome. (Contributed by Dr. George Capone.)

The first illustrations in the medical literature of an individual with Down syndrome appeared in 1876 in a classic paper by Fraser and Mitchell, in which they provided marvelous illustrations of the face, foot, and skull (72). They were also the first to publish a pathologic description of the brain at autopsy.

The term "Mongolian idiocy," or "Mongolism," was used widely throughout the latter part of the 19th and first half of the 20th century. The chromosomal etiology (trisomy 21) for Down syndrome has been known since 1959 (115). In 1961, sparked by the discovery 2 years earlier of trisomy 21 in persons with Down syndrome as well as by complaints from Chinese and Japanese scientific investigators, it was proposed that the term "Down syndrome" or "trisomy 21" be adopted to replace the inaccurate and anachronistic designation "Mongolism" (03). The complete DNA sequence of human chromosome 21 (long-arm) was determined in 2000 and is revolutionizing our genetic understanding of this condition (77; 94).

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