A brain showing holoprosencephaly can be further classified based on the neuroanatomic features.


All major types of holoprosencephaly show:

(1) a single ventricle (holosphere)

(2) cerebral cortex crossing the midline

(3) some degree of microcephaly




no evidence of the longitudinal cerebral fissure; absence of Sylvan fissures with no distinction between the lobes of the cerebral hemispheres; disorganized gyral pattern


partial longitudinal cerebral fissure separating the posterior extent of the cerebral hemispheres; absence of fissure at the rostral end; disorganized gyral pattern


longitudinal cerebral fissure extending most of the distance between the two hemispheres except for the most rostral and ventral regions of the frontal lobes; in mild cases the posterior lobes may show complete separation of the hemispheres


Additional types:

(1) septopreoptic: a mild subtype of lobar type with midline fusion restricted to the septal region or preoptic region of the telencephalon

(2) middle interhemispheric variant (MIHV): in which the posterior and parietal lobes fail to separate

(3) isolated arrhinencephaly: isolated absence of olfactory bulbs and tracts


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