Cavum veli interpositi: Why this anatomical variant exists?

The cavum veli interpositi (CVI) represents a cerebrospinal fluid (CSF) filled space formed by the corpus callosum and fornix above, the roof of the third ventricle and thalamus below and the crus of the fornix on each side laterally. It is frequent in infants but infrequent in children above 2 years of age and adults. We describe the case of a 20-year-old woman, which revealed a helmet-shaped CSF space at the anatomical location of the CVI on brain MRI performed because of recurrent headaches. We present this case in order to review the literature and to discuss the possible cause of the existence of such an anatomical variation.