Chapter 108: Common Chromosomal Trisomies 21, 18, and 13

INTRODUCTION

Chromosome abnormalities account for a significant portion of genetic disease and are important causes of congenital malformations and pregnancy loss. Cytogenetic disorders are found in nearly 1% of live births; thus, performing a karyotype on a newborn with multiple congenital anomalies can provide valuable information with respect to management questions and prognosis counseling. Chromosome analysis is indicated as a diagnostic procedure in a number of different general clinical situations, such as problems with early growth, development, stillbirth, and neonatal death. For infants in the neonatal period, performing a chromosome analysis may be considered if any of the following features are demonstrated:

• Abnormal growth parameters

• At least 1 major malformation

• 2 or more minor anomalies

• Abnormal neurologic findings

The most common chromosome abnormalities a care provider is likely to encounter in the newborn nursery are trisomies for chromosomes 21, 18, and 13. It is important to be able to recognize characteristic features of these conditions in order to initiate the most appropriate evaluations. For children who have had the diagnosis made prenatally, a formal copy of the chromosome report should be obtained. This report allows the clinician to confirm the diagnosis, review the results with the family, and add the formal diagnosis to the child’s medical record. If the results of prenatal testing are not available, a blood sample can be obtained for postnatal cytogenetic analysis to confirm the diagnosis and rule out a chromosome translocation.

TRISOMY 21 (DOWN SYNDROME)

Introduction

Down syndrome (DS) is the most common autosomal trisomy seen in live births. Incidence is estimated to be 1/600–1/800. The facial appearance of individuals with DS is characteristic and can be the first noticeable sign on physical examination to suggest this diagnosis. Other minor anomalies (eg, small ears, single transverse palmar crease, increased sandal gap), hypotonia, and malformations of other body systems (most importantly the cardiovascular and gastrointestinal systems) can be appreciated in the newborn period. DS is also associated with developmental delay/cognitive impairment, hearing loss, eye anomalies, thyroid dysfunction, atlantoaxial instability, and transient myeloproliferative disorder (TMD)/leukemia.

Diagnosis

Clinical Findings

History

The first step in evaluating a newborn infant suspected of having trisomy 21 is a careful review of the family history and prenatal information, including prenatal screening, chromosome studies done via amniocentesis or chorionic villi sampling (CVS), or any other genetic testing performed. Previous children born with trisomy 21, developmental differences, or pregnancies that ended in miscarriage may be significant clues that a family may carry a balanced translocation that predisposes them to having children with trisomy 21.

Physical Examination

A physical examination is the most sensitive test in the first 24 hours of life to diagnose trisomy 21 in an infant. If the clinician ...