Mosaic trisomy 20 syndrome

Mosaic trisomy 20 syndrome (also known as trisomy 20 mosaicism) is a rare chromosomal disorder in which some cells in the body contain three copies of chromosome 20 instead of the usual two, while other cells have the normal two copies. This mosaic state arises from a postzygotic nondisjunction event during early embryonic development. The condition is classified under ICD-10 code Q92.1 (trisomy of a whole chromosome, mosaicism). Full (non-mosaic) trisomy 20 is generally considered lethal and is not seen in liveborn individuals; therefore, all reported cases in living patients are mosaic. The clinical presentation of mosaic trisomy 20 is highly variable, largely depending on the proportion and tissue distribution of trisomic cells. Commonly reported features include mild to moderate intellectual disability, speech and developmental delays, hypotonia, and subtle dysmorphic facial features such as a broad nasal bridge, upslanting palpebral fissures, and micrognathia. Spinal abnormalities, including vertebral anomalies and spinal stenosis, have been described in some patients. Growth retardation, urogenital anomalies, and cardiac defects may also occur. Some individuals with a low percentage of trisomic cells may be phenotypically normal or very mildly affected, and the condition is sometimes detected incidentally during prenatal amniocentesis. There is no specific cure or targeted therapy for mosaic trisomy 20 syndrome. Management is supportive and symptom-based, involving early intervention programs, physical therapy, speech therapy, and educational support for developmental delays. Cardiac, orthopedic, or urological issues are addressed as needed by the appropriate specialists. Genetic counseling is recommended for affected families, though the condition is typically sporadic and recurrence risk is considered low.

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