Neuroradiology

Holoprosencephaly

complex congenital anomaly characterized by a failure of cleavage and differentiation of the prosencephalon. The abnormalities occur around the end of the fifth week of gestation, when the cerebrum should cleave laterally into distinct cerebral hemispheres and transversely into a diencephalon and a telencephalon.

Although holoprosencephaly has been divided into three subcategories; alobar, semilobar and lobar, no clear distinction between the categories exists and the disease is more a continuum of forebrain malformation which affects the anterior parts of the brain more severely.

No sex preference is found and the brain abnormalities may be seen in several syndromes such as trisomy 13 and 18; hypotelorism and midline facial clefts are frequenty associated.

In alobar holoprosencephaly, the most severe form, the thalami are fused, no interhemispheric fissure can be identified, a single ventricle comprising third and lateral ventricles is identified and no corpus callosum is formed. The anterior cerebral arteries are usually azygous, with a single trunk. The children have usually a very short life span, if not stillborn.

In the semilobar form (Fig.1) the brain is less dysmorphic; the thalami are partially separated, a small third ventricle may be recognized, part of the corpus callosum (splenium) is formed, as well as hippocampi and temporal horns. The frontal lobes, however, are fused anteriorly and the interhemispheric fissure and falx cerebri are only partially formed. Facial anomalies are less severe than in alobar holoprosencephaly.

Lobar holoprosencephaly (Fig.2) has a more defined third ventricle, corpus callosum, some frontal horn formation and the hippocampal are normal.

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Fig.1

a, b, c, d. Semilobar holoprosencephaly, MR images. T2-weighted axial (a), T1-weighted sagittal (b) and T1-weighted coronal (c, d). The fusion involves a wide portion of the brain contained between the rolandic and parietal area. In (a) the interruption of the interhemispheric fissure, the absent falx and the azygos anterior cerebral artery are well seen. In (b) only the genu and the most anterior part of the corpus callosum are formed. In (c, d) the fusion of the two hemispheres is clearly seen on top of the single ventricular cavity. The thalami are not fused, a third ventricular cavity is partially formed, the fornices are not present. The temporal horns are formed but appear dysplastic as do the hippocampi.
Holoprosencephaly, Fig.1 (a)
Holoprosencephaly, Fig.1 (b)
Holoprosencephaly, Fig.1 (c)
Holoprosencephaly, Fig.1 (d)
Holoprosencephaly, Fig.2 (a)
Holoprosencephaly, Fig.2 (b)