Middle interhemispheric fusion: an unusual variant of holoprosencephaly.

PURPOSE To describe the imaging features of a brain anomaly found on studies of three patients, and to speculate on the embryologic basis leading to the development of this abnormality.

PATIENTS AND METHODS Clinical records (three patients), MR scans (two patients), and CT scans (two patients) of three patients with fusion of the middle portions of the cerebral hemispheres in the presence of nearly normal anterior interhemispheric fissures were retrospectively reviewed. The results were correlated with the present theories of brain development in an attempt to classify the anomaly and define the underlying embryologic abnormalities.

RESULTS All three patients with middle interhemispheric fusion were severely developmentally delayed. Associated anomalies were identified in all three and included neuronal migration anomalies, callosal dysgenesis, and hypoplasia of the anterior falx cerebri. Correlation of the imaging findings with theories of brain development lead to the suggestion that this anomaly is the result of deficient or dysplastic mesenchyme, which leads to disordered brain development.

CONCLUSION Middle interhemispheric fusion may be considered as a variant of holoprosencephaly. It is suggested that the mesenchyme formed by the prechordal plate, notochord, and neural crest play an important part in the early development of the brain and that anomalies of the mesenchyme underlie this disorder as well as other forms of holoprosencephaly.