decreased hindbrain size hindbrain hypoplasia hypoplastic hindbrain rhombencephalon hypoplasia MPheno.ontology 2013-11-14T09:53:41Z underdevelopment or reduced size, usually due to a reduced number of cells, of the part of the brain developed from the posterior of the three primary brain vesicles of the embryonic neural tube from which the metencephalon and myelencephalon are derived hypoplastic rhombencephalon MP:0012264 csmith