Tetrasomy 18p
Tetrasomy 18p is a rare chromosomal disorder that causes developmental delays, intellectual disability, changes in muscle tone, and distinctive physical features.
Overview
Tetrasomy 18p is a chromosomal condition that impacts multiple systems of the body. It commonly causes feeding difficulties in infancy, developmental delays, mild to severe intellectual disability, changes in muscle tone, and distinctive facial features. The specific signs and symptoms, as well as their severity, vary among affected individuals.
During infancy, babies with tetrasomy 18p often experience difficulty feeding and frequent vomiting, which can make it hard to gain weight. Some infants also experience breathing difficulties and jaundice (yellowing of the skin and eyes). Changes in muscle tone are common; some children exhibit weak muscle tone (hypotonia), while others have increased muscle tone (hypertonia) and stiffness (spasticity). These muscle tone abnormalities contribute to delays in motor skills such as sitting, crawling, and walking.
Symptoms & Features
The clinical features of tetrasomy 18p can include:
- Feeding difficulties, frequent vomiting, and poor weight gain in infancy
- Delayed motor development (sitting, crawling, walking)
- Mild to moderate (and occasionally severe) intellectual disability
- Muscle tone abnormalities, including hypotonia, hypertonia, and spasticity
- Distinctive facial features, including unusually shaped and low-set ears, a flat area between the upper lip and nose (philtrum), and a small mouth
- A high-arched palate or a split in the roof of the mouth (cleft palate)
- Seizures and vision problems
- Recurrent ear infections and mild to moderate hearing loss
- Gastrointestinal issues, such as chronic constipation
- Abnormal curvature of the spine (scoliosis)
- Growth hormone deficiency
- Congenital heart defects and other organ abnormalities
- Genital abnormalities in males, such as undescended testes (cryptorchidism) or the urethra opening on the underside of the penis (hypospadias)
- Psychiatric and behavioral challenges, including attention-deficit/hyperactivity disorder (ADHD), anxiety, and social difficulties
Frequency
Tetrasomy 18p is a rare chromosomal disorder. It is documented to affect approximately 250 families worldwide.
Causes
Tetrasomy 18p is caused by the presence of an abnormal extra chromosome called an isochromosome 18p in each cell. An isochromosome is a chromosome with two identical arms; while normal chromosomes have one long (q) arm and one short (p) arm, an isochromosome has either two q arms or two p arms. Isochromosome 18p consists of two p arms.
Typically, human cells contain two copies of chromosome 18. Individuals with tetrasomy 18p have the standard two copies of chromosome 18 plus the extra isochromosome 18p. This results in a total of four copies of the short (p) arm of chromosome 18 in each cell. This extra genetic material disrupts normal embryonic and childhood development, leading to the characteristic features of the condition.
Inheritance
Tetrasomy 18p is typically not inherited. The chromosomal abnormality usually occurs as a random, non-inherited (de novo) event during the formation of reproductive cells (eggs or sperm) in a parent, most frequently the mother. Most affected individuals have no family history of the disorder, though rare cases of inherited tetrasomy 18p have been reported.
Other Names for This Condition
- 18p isochromosome
- 18p tetrasomy