Trisomy 18 Syndrome
18 syndrome is characterized by having a narrow bifrontal head diameter
with a prominent occiput, epicanthal folds, small mouth, a short upper
lip, micrognathia, low-set dysplastic ears, shield chest, short sternum,
limited hip abduction, and hand and feet arthrogryposis. The position
of the hands are typical in most cases. The feet anomalies are characterized
by talipes calcaneovalgus, short dorsiflexed big hallux, and prominent
heels. The central nervous system is affected in neonates with trisomy
18 syndrome. The most frequent anomalies are abnormal myelinization,
microgyria, cerebellar hypoplasia, agenesis of the corpus callosum, hydrocephalus,
and meningomyelocele. Neonates with trisomy 18 syndrome often die as a
result of apnea. If they survive the neonatal period, they may require
respiratory support and nasogastric feedings. The overall survival rate
is very low and those that survive are severely mentally retarded. Limitation
of extraordinary medical means for prolonging life should be considered.
Most neonates with trisomy
18 syndrome phenotype have trisomy. Full trisomy 18 occurs more frequently
in neonates born to older mothers. The risk of recurrence of trisomy 18
in the same family is probably less than one percent. Translocation of
chromosome 18 material produces a similar phenotype. The parents of an
infant with translocation may be asymptomatic carriers of a balance translocation.
The chance of recurrence translocation of chromosome 18 material is higher
if either parent is a carrier of a balance translocation.
I syndrome is characterized by hypertelorism, micrognathia, depressed
tip of the nose, low-set deformed ears, hypoplastic dermal ridges, hypoplastic
lungs, cryptorchidism, and club feet (Figure 158.1). They are often stillborn
or die within the first year of life.
Figure 158.1.— Pena-Shokeir I syndrome. Distal arthrogryposis
and lung hypoplasia.