Terratogenic agents with examples
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Teratogenic Agents
A teratogen is any agent that interferes with normal embryonic or fetal development and can cause a birth defect. Teratogens account for approximately 2% of all congenital abnormalities. Their effects depend on:
- Dose of exposure
- Timing during pregnancy (organogenetic period, weeks 4-8, is most critical)
- Susceptibility of mother and fetus
(Sources: The Developing Human - Clinically Oriented Embryology; Emery's Elements of Medical Genetics and Genomics)
Classification of Teratogenic Agents
1. Drugs and Chemicals
| Drug/Chemical | Teratogenic Effects |
|---|---|
| Thalidomide | Phocomelia (flipper/seal-like limb deformity due to absence of long bones), cardiac defects, ear abnormalities, cleft lip/palate |
| Alcohol | Fetal Alcohol Syndrome - microcephaly, short palpebral fissures, smooth philtrum, thin upper lip, cognitive deficiency, growth retardation, cardiac defects |
| Warfarin | Nasal cartilage hypoplasia, stippled epiphyses, CNS defects (most sensitive: first trimester) |
| Retinoids (isotretinoin, acitretin) | Ear and eye defects, hydrocephalus, CNS malformations |
| Phenytoin | Fetal hydantoin syndrome - cardiac defects, cleft palate, digital hypoplasia |
| Valproic acid | Neural tube defects, clefting, limb defects, characteristic facies, neurodevelopmental delay |
| ACE inhibitors | Renal dysplasia |
| Lithium | Cardiac defects (notably Ebstein anomaly) |
| Tetracyclines | Dental enamel hypoplasia, staining of deciduous teeth |
| Streptomycin | Sensorineural deafness |
| Chloroquine | Chorioretinitis, deafness |
| Diethylstilbestrol (DES) | Uterine malformations, vaginal adenocarcinoma in female offspring |
| Methotrexate | Methotrexate embryopathy - microcephaly, growth deficiency, craniofacial/limb anomalies, possible tetralogy of Fallot |
| Thalidomide | Critical period: day 20-35 post-conception; risk of congenital heart defect ~10% if exposed days 20-36 |
2. Physical/Radiation Agents
| Agent | Effects |
|---|---|
| Ionizing radiation | Microcephaly, growth retardation, CNS abnormalities, increased risk of childhood malignancy |
| Hyperthermia | Neural tube defects, CNS malformations (febrile illness in first trimester) |
3. Infectious Agents (TORCH+)
| Organism | Effects |
|---|---|
| Rubella virus | "Congenital rubella syndrome" - cataracts, deafness, congenital heart disease (patent ductus arteriosus, pulmonary artery stenosis), microcephaly |
| Cytomegalovirus (CMV) | Most common congenital viral infection; periventricular calcifications, sensorineural deafness, microcephaly, chorioretinitis |
| Toxoplasma gondii | Chorioretinitis (macular scarring), hydrocephalus, intracerebral calcifications |
| Treponema pallidum (Syphilis) | Congenital syphilis - stillbirth, hepatosplenomegaly, bone lesions, CNS involvement; crosses placenta from 6-8 weeks |
| Herpes simplex virus | Microcephaly, microphthalmia, retinal dysplasia, CNS damage |
| Varicella-zoster virus | Limb hypoplasia, skin scarring, eye defects, CNS damage |
| Zika virus | Microcephaly, cortical malformations |
4. Maternal Metabolic Conditions
| Condition | Fetal Risk |
|---|---|
| Diabetes mellitus | Cardiac defects (3-5%), caudal regression syndrome, macrosomia |
| Phenylketonuria (PKU) | Microcephaly, intellectual disability, cardiac defects (15% risk if untreated) |
| Systemic lupus erythematosus | Congenital heart block, neonatal lupus |
Key Concepts
Critical Periods:
- Thalidomide: day 20-35 post-conception (limb defects)
- Organogenesis (weeks 4-8): period of highest teratogenic susceptibility for most agents
- CNS remains susceptible throughout gestation and even after birth
The Thalidomide Tragedy:
Used in Europe from 1958-1962 as a sedative, thalidomide caused phocomelia in >10,000 infants. The drug was identified by recognizing an increase in a previously rare defect. It has since been reintroduced for leprosy treatment in some countries, with ongoing monitoring given its teratogenic risk. (Emery's Elements of Medical Genetics and Genomics, p. 248-249)
Fetal Alcohol Syndrome (FAS):
Prevalence ~1-2/1000 live births. There is no safe amount of alcohol in pregnancy. Features: microcephaly, short palpebral fissures, maxillary hypoplasia, thin upper lip, smooth philtrum, abnormal palmar creases, cardiac defects, cognitive disability. (The Developing Human, p. 1271-1272)
Proving Teratogenicity:
- Prospective: increased defect frequency in exposed pregnancies
- Retrospective: malformed infants show greater maternal exposure history than controls
- If teratogenic in ≥2 animal species, high probability of human hazard
Sources: The Developing Human: Clinically Oriented Embryology (Moore et al.); Emery's Elements of Medical Genetics and Genomics; Thompson & Thompson Genetics and Genomics in Medicine, 9th ed.
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