Chapter 166 Down Syndrome
INTRODUCTION
Description: Down syndrome is characterized by physical and mental symptoms that have their origin in the presence of extra genetic material from chromosome 21. This may be due to errors of duplication or the translocation of genetic material that results in effective duplication. Patients with Down syndrome exhibit a spectrum of changes that range from mild to profound.
Prevalence: Based on maternal age, from about 1 in 1250 at age 25 to about 1 in 100 at age 40. Overall incidence 1 in 733 live births (United States, 2006)
Predominant Age: Most patients with Down syndrome are identified at birth; their life span is generally shorter than average (usually 50 to 60 years).
ETIOLOGY AND PATHOGENESIS
Causes: Nondisjunction of chromosome 21, resulting in two copies from one parent and one from the other, with a net of three. Balanced translocation of chromosome 21q material onto another chromosome (most often 12, 13, or 15) in 90% of patients. During cell division, this is inherited independently from the normal chromosome 21s, resulting in extra genetic material. Roughly one half of these duplications are new occurrences; one half have a parental carrier. Mosaicism of two cell lines: one normal and one with trisomy 21. This is generally associated with milder clinical manifestations.
