USMLE Guide: Down Syndrome

Introduction

This USMLE guide provides an overview of Down Syndrome, a genetic disorder caused by the presence of an extra copy of chromosome 21. This article aims to cover key aspects of Down Syndrome, including its etiology, clinical features, diagnostic techniques, and management options. Use this guide to enhance your understanding and preparation for the USMLE examination.

Table of Contents

1. Etiology and Genetic Basis
2. Clinical Features
3. Diagnostic Techniques
4. Management

Etiology and Genetic Basis

• Down Syndrome, also known as Trisomy 21, is caused by the presence of an extra copy of chromosome 21, leading to a total of three copies instead of the usual two.
• The majority of cases (95%) result from non-disjunction during meiosis, where both copies of chromosome 21 are inherited from one parent.
• The remaining cases can be due to translocation (3-4%) or mosaicism (1-2%).

Clinical Features

• Intellectual Disability: Down Syndrome is characterized by varying degrees of intellectual disability, which can range from mild to moderate.
• Physical Features: Individuals with Down Syndrome often exhibit distinct physical characteristics, including upward slanting eyes, flat facial profile, small nose, and a single deep crease across the palm (simian crease).
• Congenital Heart Defects: Approximately 50% of individuals with Down Syndrome have congenital heart defects, such as atrioventricular septal defect (AVSD) or ventricular septal defect (VSD).
• Hypotonia: Low muscle tone (hypotonia) is commonly observed in individuals with Down Syndrome.
• Increased Susceptibility to Infections: Down Syndrome individuals are more prone to respiratory infections, otitis media, and autoimmune disorders.
• Higher Risk of Leukemia: Children with Down Syndrome have a higher risk of developing acute lymphoblastic leukemia (ALL) or acute myeloid leukemia (AML).

Diagnostic Techniques

• Prenatal Screening: Maternal serum screening, combined with ultrasound, can identify individuals at higher risk for Down Syndrome.
• Diagnostic Testing: Definitive diagnosis is made through karyotyping, which analyzes the number and structure of chromosomes. This can be performed prenatally (chorionic villus sampling or amniocentesis) or postnatally (peripheral blood sample).
• Noninvasive Prenatal Testing (NIPT): This screening test, based on cell-free fetal DNA in maternal blood, can detect an increased risk of Down Syndrome (trisomy 21) with high accuracy.

Management

• Multidisciplinary Approach: Managing Down Syndrome requires a multidisciplinary team including pediatricians, geneticists, pediatric cardiologists, speech therapists, occupational therapists, and special education teachers.
• Early Intervention: Early intervention programs focusing on speech therapy, physical therapy, and cognitive development can improve functional outcomes.
• Regular Health Check-ups: Individuals with Down Syndrome should undergo regular health check-ups to monitor for associated conditions, including thyroid dysfunction, vision/hearing problems, and gastrointestinal abnormalities.
• Surgical Interventions: Surgical correction may be required for congenital heart defects or other associated conditions.
• Genetic Counseling: Parents of a child with Down Syndrome should receive genetic counseling to understand the risk of recurrence in future pregnancies.

Remember to study additional resources and refer to the latest guidelines for a comprehensive understanding of Down Syndrome. Best of luck with your USMLE preparation!