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Genetics Home Reference: your guide to understanding genetic conditions     A service of the U.S. National Library of Medicine®

Tetrasomy 18p

Reviewed March 2011

What is tetrasomy 18p?

Tetrasomy 18p is a developmental disorder that affects many parts of the body. This condition usually causes feeding difficulties in infancy, delayed development, intellectual disability, changes in muscle tone, distinctive facial features, and other birth defects. However, the signs and symptoms vary among affected individuals.

Babies with tetrasomy 18p often have trouble feeding and may vomit frequently, which makes it difficult for them to gain weight. Some affected infants also have breathing problems and jaundice, which is a yellowing of the skin and the whites of the eyes.

Changes in muscle tone are commonly seen with tetrasomy 18p. Some affected children have weak muscle tone (hypotonia), while others have increased muscle tone (hypertonia) and stiffness (spasticity). These changes contribute to delayed development of motor skills, including sitting, crawling, and walking.

Tetrasomy 18p is associated with a distinctive facial appearance that can include unusually shaped and low-set ears, a small mouth, a flat area between the upper lip and the nose (philtrum), and a thin upper lip. Many affected individuals also have a high, arched roof of the mouth (palate).

Additional features of tetrasomy 18p can include seizures, vision problems, recurrent ear infections, mild to moderate hearing loss, constipation and other gastrointestinal problems, abnormal curvature of the spine (scoliosis or kyphosis), and heart defects. Males with tetrasomy 18p may be born with undescended testes (cryptorchidism) or the opening of the urethra on the underside of the penis (hypospadias). Psychiatric conditions, such as attention deficit hyperactivity disorder (ADHD) and anxiety, have also been reported in some people with tetrasomy 18p.

How common is tetrasomy 18p?

Tetrasomy 18p is a rare chromosomal disorder. It is known to affect about 250 families worldwide.

What are the genetic changes related to tetrasomy 18p?

Tetrasomy 18p results from the presence of an abnormal extra chromosome, called an isochromosome 18p, in each cell. An isochromosome is a chromosome with two identical arms. Normal chromosomes have one long (q) arm and one short (p) arm, but isochromosomes have either two q arms or two p arms. Isochromosome 18p is a version of chromosome 18 made up of two p arms.

Cells normally have two copies of each chromosome, one inherited from each parent. In people with tetrasomy 18p, cells have the usual two copies of chromosome 18 plus an isochromosome 18p. As a result, each cell has four copies of the short arm of chromosome 18. (The word "tetrasomy" is derived from "tetra," the Greek word for "four.") The extra genetic material from the isochromosome disrupts the normal course of development, causing the characteristic features of this disorder.

Related Chromosome(s)

Changes involving this chromosome are associated with tetrasomy 18p.

  • chromosome 18

Can tetrasomy 18p be inherited?

Tetrasomy 18p is usually not inherited. The chromosomal change responsible for the disorder typically occurs as a random event during the formation of reproductive cells (eggs or sperm) in a parent of the affected individual, usually the mother. Most affected individuals have no history of the disorder in their family. However, rare inherited cases of tetrasomy 18p have been reported.

Where can I find information about diagnosis or management of tetrasomy 18p?

These resources address the diagnosis or management of tetrasomy 18p and may include treatment providers.

  • Chromosome 18 Clinical Research Center, University of Texas Health Science Center at San Antonio (
  • Genetic Testing Registry: Chromosome 18, tetrasomy 18p (

You might also find information on the diagnosis or management of tetrasomy 18p in Educational resources and Patient support.

General information about the diagnosis ( and management ( of genetic conditions is available in the Handbook. Read more about genetic testing (, particularly the difference between clinical tests and research tests (

To locate a healthcare provider, see How can I find a genetics professional in my area? ( in the Handbook.

Where can I find additional information about tetrasomy 18p?

You may find the following resources about tetrasomy 18p helpful. These materials are written for the general public.

You may also be interested in these resources, which are designed for healthcare professionals and researchers.

What other names do people use for tetrasomy 18p?

  • 18p isochromosome
  • 18p tetrasomy

For more information about naming genetic conditions, see the Genetics Home Reference Condition Naming Guidelines ( and How are genetic conditions and genes named? ( in the Handbook.

What if I still have specific questions about tetrasomy 18p?

Ask the Genetic and Rare Diseases Information Center (

What glossary definitions help with understanding tetrasomy 18p?

ADHD ; anxiety ; attention deficit hyperactivity disorder ; cell ; chromosome ; constipation ; cryptorchidism ; disability ; gastrointestinal ; hyperactivity ; hypospadias ; hypotonia ; inherited ; isochromosome ; jaundice ; maternal ; meiosis ; motor ; muscle tone ; nondisjunction ; palate ; philtrum ; reproductive cells ; scoliosis ; spasticity ; sperm ; testes ; tetrasomy

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.


  • Bugge M, Blennow E, Friedrich U, Petersen MB, Pedeutour F, Tsezou A, Orum A, Hermann S, Lyngbye T, Sarri C, Avramopoulos D, Kitsiou S, Lambert JC, Guzda M, Tommerup N, Brøndum-Nielsen K. Tetrasomy 18p de novo: parental origin and different mechanisms of formation. Eur J Hum Genet. 1996;4(3):160-7. Erratum in: Eur J Hum Genet 1996;4(5):291. (
  • Nucaro A, Chillotti I, Pisano T, Pruna D, Cianchetti C. Progressive spastic paraplegia as a feature of tetrasomy 18p. Am J Med Genet A. 2010 Sep;152A(9):2173-5. doi: 10.1002/ajmg.a.33576. (
  • Sebold C, Roeder E, Zimmerman M, Soileau B, Heard P, Carter E, Schatz M, White WA, Perry B, Reinker K, O'Donnell L, Lancaster J, Li J, Hasi M, Hill A, Pankratz L, Hale DE, Cody JD. Tetrasomy 18p: report of the molecular and clinical findings of 43 individuals. Am J Med Genet A. 2010 Sep;152A(9):2164-72. doi: 10.1002/ajmg.a.33597. (
  • Zavala J, Ramirez M, Medina R, Heard P, Carter E, Crandall A, Hale D, Cody J, Escamilla M. Psychiatric syndromes in individuals with chromosome 18 abnormalities. Am J Med Genet B Neuropsychiatr Genet. 2010 Apr 5;153B(3):837-45. doi: 10.1002/ajmg.b.31047. (


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? ( in the Handbook.

Reviewed: March 2011
Published: February 1, 2016