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Neuroradiology Case of the Month

December 2001

Ahmed N. Abdelhalim MD, Jose Echeverri MD, Henry Z. Wang MD, PhD

Clinical Presentation: A 36-year-old female who is 18 weeks pregnant with suspected fetal CNS abnormality on a routine ultrasound.

Imaging Findings: Single shot fast spin echo Axial T2 weighted images through the fetal head reveal a large single monoventricle filling most of the intracranial cavity with fused frontal lobes and basal ganglia. The falx cerebri and interhemispheric fissure are absent.

Diagnosis: Alobar holoprosencephaly.

Figure 1. Axial T2W FSE through the fetal head shows a large single monoventricle (arrow) and fusion of the frontal lobes.	Figure 2. Axial T2W FSE Shows fused basal ganglia (arrows).

Discussion: Complete or partial failure in division of the developing cerebrum (prosencephalon) into hemispheres and lobes results in the holoprosencephalies. In holoprosencephaly, there is failure of lateral cleavage into distinct cerebral hemispheres and failure of transverse cleavage into diencephalon and telencephalon [1]. The differentiation of the telencephalon from the diencephalon and the separation of the telencephalon into two cerebral hemispheres are in progress by the end of the fifth week of gestation [2].

DeMyer has divided holoprosencephaly into three subcategories: alobar, semilobar and lobar. These categories are useful for classifying holoprosencephalies of different severities with alobar type as the most severe and lobar type as the mildest form [2].

Holoprosencephaly is caused by both teratogens and genetic factors. The most common teratogen in humans is maternal diabetes [2].

Although alobar holoprosencephaly is the most common form of holoprosencephaly diagnosed by prenatal ultrasound, affected patients are rarely imaged postnatally by CT or MRI. This rarity stems from the fact that most affected infants are still born or have a very short life span [2].

Imaging studies in alobar holoprosencephaly shows a completely unsegmented rim of brain that surrounds a largely undifferentiated central CSF-filled cavity. There is no interhemispheric fissure, falx cerebri or corpus callosum. A large posterior midline cyst is usually present. Usually there are associated severe craniofacial anomalies [3].

References:

1. Byrd SE, Osborn RE, Rodkowski MA et al: Disorders of midline structures: holoprosencephaly, absence of corpus callosum, and Chiari malformations, Sem US, CT, MR 99:201-215, 1988.
2. Barkovich AJ. Pediatric neuroimaging, 3rd edition. Lippincott Williams & Wilkins, 2000; 318-320.
3. Osborn AG. Dignostic Radiology. Mosby, 1994; 37-38.