Concise Summary of Chromosomal Abnormalities

Terminology

  • Karyotype: Complete set of chromosomes in a cell.
  • Euploid: Normal balanced set of chromosomes (23 in humans).
  • Aneuploid: Unbalanced set of chromosomes (too few or too many).
  • Trisomy: Presence of an extra chromosome.

Genetic Diseases

  • Caused by:
    • Abnormal number of chromosomes.
    • Abnormal structure of chromosomes.
    • Single gene defects, which can be inherited in two ways:
    1. Autosomal Dominant: Single defective gene from one parent (50% likelihood).
    2. Autosomal Recessive: Pair of defective genes (25% likelihood).

Teratogenesis

  • Congenital malformations are structural abnormalities at birth.
  • Causes: Genetic factors, environmental factors, multifactorial inheritance.
  • Teratogens: Substances causing abnormal structures in embryos.

Types of Teratogenic Agents

  1. Physical Agents: e.g., radiation, heat.
  2. Drug & Chemical Agents: e.g., Alcohol (fetal alcohol syndrome), Coumadin.
  3. Maternal Factors: Infections, diabetes.

Neural Tube Defects (NTDs)

  • Types:
    • Anencephaly: Underdeveloped brains and incomplete skull.
    • Encephalocele: Brain tissue protrudes through skull.
    • Spina Bifida: Range from mild to severe spinal defects.

Specific Conditions

  • Myelomeningocele: Severe spina bifida where spinal cord develops outside the body.
  • Meningocele: Protrusion of meninges through spine.
  • Iniencephaly: Severe defect with cervical rachischisis and encephalocele.

Cranial Abnormalities

  • Dandy-Walker Malformation: Involves cerebellum and fourth ventricle.
  • Agenesis of Corpus Callosum: Failure of corpus callosum development.
  • Holoprosencephaly: Abnormal forebrain leading to a single midline ventricle.
  • Microcephaly: Small head due to infections or teratogens.

Abdominal Wall Defects

  • Omphalocele: Herniation of abdominal contents covered by a sac.
  • Gastroschisis: Evisceration of intestines through an abdominal wall defect.
  • Bladder Exstrophy: Bladder develops outside the fetus.

Congenital Diaphragmatic Hernia

  • Caused by a defect in the diaphragm leading to herniation of abdominal contents into the thorax.

Cardiac Anomalies

  • Common defects include Ventricular Septal Defect (VSD) and Hypoplastic Left Heart Syndrome.

Renal Anomalies

  • Renal Agenesis: Absence of one or both kidneys; can lead to oligohydramnios.
  • Obstructive Uropathies: Including UPJ and UVJ obstructions leading to hydronephrosis.

Fetal Syndromes

  • Associated with chromosomal defects such as Trisomy 21, 18, and 13; Turner syndrome.

Hydrops Fetalis

  • Abnormal fluid accumulation due to various causes; can be immune or non-immune.