Handbook of Genetic Counseling/18q Deletion Syndrome

18q Deletion Syndrome

Genetic Etiology

• Caused by the deletion of the long arm of chromosome 18 (or, more often, a section of it)

Incidence and Carrier Frequency

• The estimated frequency of the deletions syndromes of chromosome 18 (18p- & 18q-) is about 1/40,000 births. This would mean that about 100 babies per year in the United States are born with 18p- and 18q-.

• 80% of cases occur due to de novo mutations; 10% of cases are due to a parental translocation; the remaining 10% are mosaic, usually resulting in a less severe phenotype

Clinical Features

This syndrome is variable in its severity. The symptoms usually correlate with the size of the deletion.

Key features:

• Dysmyelination of the central nervous system (97%)
• Hypotonia (79%)
• Decreased or absent deep tendon reflexes (76%)
• Short stature (61-77% of patients are below the fifth percentile)
• Hearing impairment (70%)
• Mental retardation IQ < 70 (68%)
• Gait abnormalities (68%)
• Ear anomalies
  • Narrow or atretic auditory canals (39-64%)
  • Prominent antihelix or antitragus (58%)
  • Low-set ears (8%)
• Foot deformities (51-74%)
  • Club foot (21%)
  • Abnormal implantation of the second toe (21%)
  • Gap between first and second toe (3%)
• Endocrine problems
  • IgA deficiency (30%)
  • Growth hormone deficiency (68%)
• Facial dysmorphism
  • Microcephaly (53-56%)
  • "Carp-like" mouth (56%)
  • Microganthia (19%)
  • Midfacial hypoplasia (68%)
  • Slanted palpebral fissures (9%)
  • Hypertelorism (26%)
  • Epicanthic folds (29%)
  • Strabismus (20%)
  • Nystagmus (30%)
  • Pale optic discs (23%)
  • Broad nasal bridge (23%)
  • Cleft lip (9%)
  • Nodules at corner of mouth (6%)
• Palate abnormality (30%)
  • Cleft palate (12%)
  • High arched (15%)
  • Bifid uvula (3%)
• Abnormal genitalia
  • Females: dysplastic labia minora (35%)
  • Males (57%): cryptorchidism, hypospadias, micropenis, inguinal hernia (13%)
• Thin hair (14%)
  • Widely spaced nipples (12%)
• Hand anomalies
  • Tapered fingers (35%)
  • Proximal thumb (33-65%) with short first metacarpal
  • Transverse palmar creases (23%)
  • High whorl patterns on fingertips (36%) and high total finger ridge count
  • Fleshy tips of fingers (11%)
  • Clinodactyly (4%)
• Skin dimples (23%)
• Seizures (12%)
• Genu varum (9%)
• Brachycephaly (8%)
• Umbilical hernia (7%)
• Coloboma (4%)
• Eczema (4%)
• Inguinal hernia (4%)
• Large protruding forehead (4%)
• Dysplastic teeth (4%)
• Delayed fontanel closure (4%)
• Chorea (3%)
• Low posterior hairline (3%)
• Scoliosis (3%)
• Strawberry birthmark on face (3%)
• Short neck (3%)
• Severe feeding problems (3%)

Age of onset, natural history, life span

• Symptoms are apparent from birth in most cases
• Natural history: Most patients have IQs between 40 and 85, normal IQ (100 or above) has also been reported. Short stature is common, as are various hearing and vision problems. Behavioral problems can occur, sometimes causing patients to exhibit autistic-like behaviors. These symptoms are variable, so a patient with a smaller deletion will have a milder phenotype than will a patient missing all of 18q.
• Life span is variable, but may be decreased from the general population, depending upon severity of deletion and various congenital diseases or defects.

Testing

Diagnosis is confirmed by chromosome studies

Surveillance, management, and treatment options

Patients should be monitored by endocrinology, audiology, ophthalmology, genetics, neurology, orthopedics, immunology, physical therapy, occupational therapy, speech therapy, and Early Intervention.

Recent research has uncovered potential treatment for short stature and dysmyelination caused by 18q-. Growth hormone treatment has been shown to potentially increase IQ in children with 18q-. A 2005 study performed by Cody et al. showed that over a treatment period of 37 months an average nIQ increase of 17 pts, and an increase in height standard deviation score of 1.7. All but one of the children who tested within measureable range (>50) to the nonverbal IQ test had, at the end of the study, an IQ within normal range (i.e., >70).

Psychosocial issues

Guilt, fear, anxiety, anger, sadness, and disbelief are common emotions associated with this diagnosis. Parents may feel overwhelmed. Financial concerns, issues affecting family dynamics, and what care may be needed for the individual with 18q- are all things that should be addressed with parents.

References

• OMIM, Smith's, Chromosomal Syndromes: Common and/or Well-Known Syndromes
• http://www.chromosome18.org/Genetics/18q-.aspx
• http://www.chromosome18.org/Genetics/Facts.aspx
• Jannine D. Cody, Margaret Semrud-Clikeman, L. Jean Hardies, Jack Lancaster, Patricia D. Ghidoni,Rebecca L. Schaub, Nora M. Thompson, Lynda Wells, John E. Cornell, Tanzy M. Love, Peter T. Fox,Robin J. Leach, Celia I. Kaye, and Daniel E. Hale (2005) Growth Hormone Benefits Children With 18q Deletions. American Journal of Medical Genetics 137A:9–15 http://www.pediatrics.uthscsa.edu/centers/chromosome18/PDF/GHbenefitschildrenwith18q-.pdf

Notes

The information in this outline was last updated in 2002.