Familial congenital saccade initiation failure and isolated cerebellar vermis hypoplasia.

The underlying lesion in congenital saccade initiation failure (c-SIF) ('congenital ocular motor apraxia', 'Cogan's apraxia') is uncertain. Often no abnormality can 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 examinations, both siblings underwent ocular motor testing and neuroradiological investigations including CT and MRI. Both siblings exhibited the typical signs of c-SIF, including headthrusting, synkinetic blinking, missed-nystagmus quick phases, mild developmental delay, and speech difficulties. CT and MRI revealed cerebellar vermis hypoplasia in the brother, but appeared normal in the sister. No other neuroradiological abnormalities were detected. These cases highlight the wide variability in the association of vermis abnormalities with c-SIF, despite the inheritance and similar clinical manifestations. They show that either: (1) the vermis is causal in saccade triggering, but that c-SIF may result from very subtle damage that is beyond MRI resolution in some cases; or (2) that a vermis abnormality per se is not causative but only a marker of another subtle abnormality, either structural or possibly biochemical.