Familial congenital saccade initiation failure and isolated cerebellar vermis hypoplasia

The underlying lesion in congenital saccade initiation failure (c-SIF) ('co ngenital ocular motor apraxia','Cogan's apraxia') is uncertain. Often no ab normality call be found, yet in others a midline cerebellar abnormality has often been reported. We examined this cerebellar association in a brother and sister. In addition to standard ophthalmological and neurological exami nations, both siblings underwent ocular motor testing and neuroradiological investigations including CT and MRI, Both siblings exhibited the typical s igns of c-SIF, including headthrusting, synkinetic blinking, missed-nystagm us quick phases, mild developmental delay, and speech difficulties. CT and MRI revealed cerebellar vermis hypoplasia in the brother, but appeared norm al in the sister, No other neuroradiological abnormalities were detected. T hese cases highlight the wide variability in the association of vermis abno rmalities with c-SIF, despite the inheritance and similar clinical manifest ations. They show that either: (1) the vermis is causal in saccade triggeri ng, but that c-SIF may result from very subtle damage that is beyond MRI re solution in some-cases; or (2) that a vermis abnormality per se is not caus ative but only a marker of another subtle abnormality, either structural or possibly biochemical.