The middle interhemispheric variant of holoprosencephaly.
OBJECTIVE
The middle interhemispheric variant of holoprosencephaly (MIH) is a rare malformation in which the cerebral hemispheres fail to divide in the posterior frontal and parietal regions. We herein describe the structural abnormalities of the brain in a large group of patients with MIH, compare these features with those of classic holoprosencephaly (HPE), and propose a developmental mechanism, based on current knowledge of developmental neurogenetics, by which MIH develops.


METHODS
Brain images obtained in 21 patients with MIH (MR images in 16 patients and high-quality X-ray CT scans in five patients) were retrospectively reviewed to classify cerebral abnormalities. The cerebral parenchyma, hypothalami, caudate nuclei, lentiform nuclei, thalami, and mesencephalon were examined for the degree of midline separation (cleavage) of the two hemispheres. The orbits, olfactory apparati, and presence or absence of a dorsal cyst were also assessed.


RESULTS
In all patients, by definition, midportions of the cerebral hemispheres were continuous across the midline, with an intervening interhemispheric fissure. The sylvian fissures were abnormally connected across the midline over the vertex in 18 (86%) of 21 patients. Two patients had relatively normal-appearing sylvian fissures; one had unilateral absence of a sylvian fissure owing to substantial subcortical heterotopia. Heterotopic gray matter or dysplastic cerebral cortex was also seen in 18 (86%) of 21 patients. MIH differed from classic HPE as follows. 1) In all subjects, the midline third ventricle separated the hypothalamus and lentiform nuclei. 2) The caudate nuclei were separated by the cerebral ventricles in 17 (89%) of the 19 [corrected] patients in whom they could be assessed. 3) The most commonly affected basal nucleus was the thalamus (non-cleavage in seven [33%] of 21 cases, abnormal alignment in 1 [5%]). 4) Three (18%) of the 17 [corrected] patients in whom the mesencephalon could be assessed showed some degree of mesencephalic non-cleavage. 5) No patients had hypotelorism (four had hypertelorism, the remainder manifested normal intraocular distances). Dorsal cysts were present in five (25%) of the 20 patients in whom they could be assessed (dorsal cysts could not be assessed after shunt surgery), and as in classic HPE, were associated with severe thalamic non-cleavage in three of these five patients.


CONCLUSIONS
MIH appears to cause non-cleavage of midline structures in a completely different pattern than does classic HPE. In MIH, impaired induction or expression of genetic factors appears to influence the embryonic roof plate, whereas in classic HPE, induction or expression of the embryonic floor plate seems to be affected.
Simon
Erin M
EM
Department of Diagnostic Radiology, University of California-San Francisco, 505 Parnassus Avenue, San Francisco, CA 94143, USA.
Hevner
Robert F
RF
Pinter
Joseph D
JD
Clegg
Nancy J
NJ
Delgado
Mauricio
M
Kinsman
Stephen L
SL
Hahn
Jin S
JS
Barkovich
A James
AJ
D001921:Brain / Q000473:pathology; D002540:Cerebral Cortex / Q000473:pathology; D002675:Child, Preschool; D004292:Dominance, Cerebral / Q000502:physiology*; D005260:Female; D016142:Holoprosencephaly / Q000145:classification / Q000175:diagnosis*; D006801:Humans; D007089:Image Enhancement; D007223:Infant; D007231:Infant, Newborn; D008279:Magnetic Resonance Imaging; D008297:Male; D011379:Prognosis; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed