Trisomy 13 Syndrome
Trisomy 13 syndrome is characterized by microcephaly, posterior scalp lesions, microphthalmia, cleft lip (Figure 157.1 [A]), coloboma of the iris, low-set dysplastic ears, deafness, cleft palate, polydactyly, typical trisomy 18 hands (Figure 157.1 [B]), prominent heels, cryptorchidism, and abnormal scrotum.

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Figure 157.1.— Trisomy 13 syndrome. [A] Typical facial features of trysomy 13 syndrome and [B] typical positions hands position of distal athrogryposis.

The most frequent central nervous system anomaly is holoprosencephaly. Holoprosencephaly can be diagnosed by brain ultrasound but MRI is the study of choice (Figure 157.2). Holoprosencephaly may be alobar, semilobar and lobar. This distinction may at times be difficult since they often overlap.

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Figure 157.2.— Brain ultrasound demonstarting Holoprosencephaly. [A] Coronal anterior view. [B] Coronal posterior view.

Alobar holoprosencephaly is characterized by fused thalami, no third ventricle, no interhemispheric fissure, no corpus callosum, an anteriorly displaced pancake-like mass of tissue, and a cresent-shaped holoventricle continuous with a large dorsal cyst.
Semilobar holoprosencephaly is characterized by partially separated thalami, small third ventricle, an interhemispheric fissure that is only present posteriorly, a corpus callosum that is only present posteriorly, normal occipital or temporal lobes, and a telencephalic ventricle that is continuous with a dorsal cyst (Figure 157.3).

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Figure 157.3.— MRI studies demostrating semilobar holoprosencephaly.

Lobar holoprosencephaly is characterized by hypoplastic anterior falx, hypoplastic frontal lobes, absence of the anterior regions of the corpus callosum, absence of the septum pellucidum but separated thalami, and a normal third ventricle and interhemispheric fissure (Figure 157.4).

Figure 157.5.— CT scan of the brain demonstrates holoprosencephaly. Partially fused frontal lobes, partially formed interhemispheric fissure.

Full trisomy 13 occurs more often in neonates born to older mothers. Translocation of chromosome 13 material produces a similar phenotype. The parents of an infant with translocation should have chromosomal studies because they may be asymptomatic carriers of a balance translocation. The chance of recurrence is higher if either parent is a carrier of a balance translocation.