Teaching NeuroImages: Syntelencephaly: Middle interhemispheric fusion.

An 8-month-old infant presented to our hospital with global developmental delay. MRI brain findings revealed the diagnosis of syntelencephaly (figure, A–C). Syntelencephaly, a distinct subset of holoprosencephaly,1 is a rare brain malformation in which the hemispheric fusion does not occur at rostral forebrain but rather across posterior frontal region; hence the alternative term “middle interhemispheric fusion” or “dorsal lobar holoprosencephaly.” The clinical presentations can vary, with mental defect or global developmental delay being common.2 The septum pellucidum is absent in all cases. Unlike classic holoprosencephaly, typically the midbody of corpus callosum is deficient in this disorder.