Learn More (sporadic)

Sporadic disorders may have a genetic or non-genetic etiology.

  • Genetic etiology of sporadic disorders
    • Uniparental disomy - The affected individual has both chromosomes of one pair from one parent and none from the other, a chance occurrence of a genetic abnormality not likely to recur in other family members.
    • Sporadic tumors - All cancer is genetic at the cellular level; however, all cancer is NOT hereditary. A single tumor occurring in an older individual is likely to be caused by acquired (not inherited) pathogenic variants in cell regulator genes leading to uncontrolled cell proliferation.
  • Non-genetic etiology of sporadic disorders
    • Congenital rubella syndrome in an infant due to maternal rubella infection during pregnancy
    • Ataxia caused by alcoholism

'Sporadic' is sometimes incorrectly used to refer to the following:

Autosomal dominant or X-linked disorders that occur in a single individual in a family, often the result of a new pathogenic variant. Because recurrence risk is increased for sibs of the affected individual and is as high as 50% for the offspring of the affected individual, the use of the term "sporadic" is not appropriate. Use of the term "simplex case" is correct.

Some Clinical Implications

  • If a particular condition is truly sporadic, recurrence risk should be the same in families with an affected individual as compared to families with no history of the condition.
  • Care must be taken to distinguish a true sporadic case from a simplex case (single occurrence of a condition in a family) with recurrence risk implications.
  • A sporadic case is usually simplex (a single case of a disorder in a family ); a simplex case may or may not be sporadic.
  • The following are examples by mode of inheritance of simplex cases that are NOT sporadic. All have recurrence risk implications for subsequent offspring of the parents and of the affected individual.
    • Autosomal recessive. A couple with no family history of cystic fibrosis gives birth to a child with cystic fibrosis.
    • Autosomal dominant
      • A couple of normal stature has a child with achondroplasia.
      • A woman mildly affected by myotonic dystrophy type 1 is unaware of the diagnosis until she gives birth to a child with severe congenital myotonic dystrophy.
    • X-linked. A healthy woman with no family history of hemophilia A has a son with hemophilia A.
    • Multifactorial inheritance. A healthy couple has a child with a congenital heart defect.
    • Chromosome abnormality. A healthy couple has a child with Down syndrome (trisomy 21).

Updated: 4-27-16

Related terms: recurrence risk; simplex

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From: Illustrated Glossary

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