Monosomy 18p

Monosomy 18p refers to a chromosomal disorder resulting from the deletion of all or part of the short arm of chromosome 18.

Epidemiology

The incidence is estimated to be about 1:50,000 live-born infants.

Clinical description

In the commonest form of the disorder, the dysmorphic syndrome is very moderate and non-specific. The main clinical features are short stature, round face with short philtrum, palpebral ptosis and large ears with detached pinnae. Intellectual deficiency is mild to moderate. A small subset of patients, about 10 to 15 percent of cases, present with severe brain/facial malformations evocative of holoprosencephaly spectrum disorders.

Etiology

In two-thirds of the cases, the 18p- syndrome is due to a mere terminal deletion occurring de novo. In the remaining cases the following are possible: a de novo translocation with loss of 18p, missegregation of a parental translocation or inversion, or a ring chr18. Parental transmission of the 18p- syndrome has been reported.

Diagnostic methods

Cytogenetic analysis is necessary to make a definite diagnosis.

Differential diagnosis

Differential diagnosis may include a wide number of syndromes with short stature and mild intellectual deficiency. In young children, deletion 18p syndrome may be vaguely evocative of either Turner syndrome or trisomy 21 (see these terms).

Antenatal diagnosis

Deletion 18p can be detected prenatally by amniocentesis or chorionic villus sampling and cytogenetic testing.

Genetic counseling

Recurrence risk for siblings is low for de novo deletions and translocations, but is significant if a parental rearrangement is present.

Management and treatment

No specific treatment exists but speech therapy and early educational programs may help to improve the performances of the children.

Prognosis

Except for the patients with severe brain malformations, the life expectancy does not seem significantly reduced.