Monosomia

Monosomia is a chromosomal condition characterized by the presence of only one copy of a chromosome in a cell rather than the typical two copies. In humans, monosomy of autosomes is almost always lethal during embryonic development; viable cases are extremely rare. The most common viable monosomy is monosomy X, historically called Turner syndrome, in which individuals have only one X chromosome (45,X) and no second sex chromosome.

Causes include nondisjunction during meiosis I or II or mitotic errors after fertilization, leading to monosomic cell lines. Mosaic forms, in which some cells are normal diploid and others monosomic, can occur and influence the phenotype.

Diagnosis is typically made by karyotyping or chromosomal microarray after birth or prenatally; noninvasive prenatal testing (NIPT) may indicate monosomy X or other monosomies but requires confirmation by diagnostic testing such as amniocentesis with karyotyping or CMA.

Management depends on the specific monosomy. For Turner syndrome, interventions include growth hormone therapy to improve height and estrogen replacement therapy to induce secondary sexual development, along with monitoring for cardiovascular, renal, and endocrine issues; education and psychosocial support are important. For other monosomies, prognosis is highly variable and often limited; many cases are detected only in pregnancy and result in miscarriage.

Epidemiology: Monosomy X occurs in about 1 in 2,000 to 2,500 female births. Autosomal monosomies are rare in live births; most are terminated early.

Note: The term monosomia appears in some texts as an alternate form of monosomy.