Introduction to Trisomy 13

Trisomy 13, also known as Patau syndrome, is a chromosomal disorder characterized by the presence of an extra copy of chromosome 13. It is the third most common autosomal trisomy among live births, occurring in approximately 1 in 8,000 to 12,000 live births.

Key points:

• First described by Klaus Patau in 1960
• Associated with severe intellectual disability and multiple congenital anomalies
• Most affected individuals do not survive beyond infancy
• Slightly more common in female infants
• Risk increases with maternal age

Genetics of Trisomy 13

Trisomy 13 is caused by the presence of an extra copy of chromosome 13. There are three types of Trisomy 13:

1. Full Trisomy 13: Accounts for about 95% of cases. All cells have three copies of chromosome 13.
2. Mosaic Trisomy 13: Accounts for about 5% of cases. Only some cells have the extra chromosome 13.
3. Partial Trisomy 13: Rare. Only a portion of the extra chromosome 13 is present.

Patau syndrome or trisomy 13 is indicated by the karyotype showing 47, XY, +13. (source)

Risk factors for Trisomy 13 include:

• Advanced maternal age (risk increases after age 35)
• Previous pregnancy with a chromosomal abnormality
• Parental balanced translocation involving chromosome 13

Clinical Features of Trisomy 13

Trisomy 13 is characterized by a wide range of severe congenital anomalies affecting multiple organ systems. Common features include:

Newborn infant with trisomy 13, showing microphthalmia, bilateral cleft lip and palate, and postaxial polydactyly of the hands. (source)

Physical Characteristics:

• Microcephaly
• Holoprosencephaly (failure of forebrain to divide properly)
• Severe scalp defects
• Microphthalmia or anophthalmia
• Cleft lip and/or palate
• Low-set ears
• Polydactyly (extra fingers or toes)
• Rocker-bottom feet
• Omphalocele or umbilical hernia
• Single palmar crease

Developmental Characteristics:

• Profound intellectual disability
• Severe developmental delays
• Seizures
• Feeding difficulties
• Limited responsiveness to environment

Diagnosis of Trisomy 13

Trisomy 13 can be diagnosed prenatally or after birth. Diagnostic methods include:

Prenatal Screening and Diagnosis:

• Non-invasive prenatal testing (NIPT): Cell-free fetal DNA analysis
• First-trimester combined test: Nuchal translucency ultrasound and maternal blood tests
• Quad screen: Second-trimester maternal blood test
• Ultrasound findings: May show characteristic abnormalities
• Diagnostic tests:
  • Chorionic villus sampling (CVS): 10-13 weeks gestation
  • Amniocentesis: After 15 weeks gestation

Postnatal Diagnosis:

• Physical examination: Characteristic features may be apparent at birth
• Chromosomal karyotype: Confirms diagnosis and determines type of Trisomy 13

Associated Conditions in Trisomy 13

Individuals with Trisomy 13 often have multiple congenital anomalies affecting various organ systems:

• Central Nervous System:
  • Holoprosencephaly
  • Neural tube defects
  • Cerebellar anomalies
• Cardiovascular defects (80% of cases):
  • Ventricular septal defect
  • Atrial septal defect
  • Patent ductus arteriosus
  • Dextrocardia
• Ocular abnormalities:
  • Microphthalmia or anophthalmia
  • Coloboma
  • Retinal dysplasia
• Genitourinary abnormalities:
  • Polycystic kidneys
  • Hydronephrosis
  • Cryptorchidism in males
  • Bicornuate uterus in females
• Gastrointestinal issues:
  • Omphalocele
  • Malrotation of intestines
  • Pancreatic abnormalities
• Respiratory problems:
  • Pulmonary hypoplasia
  • Congenital diaphragmatic hernia

Management of Trisomy 13

Management of Trisomy 13 is primarily supportive and palliative, focusing on the individual's quality of life. The approach includes:

Medical Management:

• Neonatal intensive care for severe cases
• Management of respiratory difficulties
• Feeding support (nasogastric tube or gastrostomy)
• Seizure control
• Treatment of infections
• Pain management

Surgical Interventions:

• Decisions regarding surgery are complex and individualized
• May include repair of life-threatening defects in selected cases
• Ethical considerations play a significant role in decision-making

Supportive Care:

• Physical therapy
• Occupational therapy
• Nutritional support

Family Support:

• Genetic counseling
• Psychological support
• Palliative care consultation
• Support groups and resources

Prognosis in Trisomy 13

The prognosis for individuals with Trisomy 13 is generally poor:

• Approximately 50% of infants with Trisomy 13 survive beyond the first week of life
• About 5-10% of infants survive beyond the first year
• Median survival for live births is 7-10 days
• Survival rates are slightly higher for females compared to males
• Individuals with mosaic Trisomy 13 may have a better prognosis

Factors affecting prognosis include:

• Severity of congenital anomalies, particularly brain and heart defects
• Presence of holoprosencephaly
• Degree of medical intervention provided
• Type of Trisomy 13 (full, mosaic, or partial)

Long-term survivors face significant medical challenges and developmental delays. However, some individuals with Trisomy 13, particularly those with mosaic forms, have survived into adulthood with appropriate medical care and support.