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Background: Communication or anatomic continuity of the fourth ventricle outlet (FVO) with the central cervical ependymal canal (CCEC) of the spinal cord in both humans and mammals is controversial.

Aim: We hypothesize that in chronic inflammatory brain conditions (CIBC) and in early stages of age this communication can be reopened. For this purpose we have conducted a study to check the potential continuity of FVO with the CCEC of the spinal cord in small domestic carnivores presenting with obstructive hydrocephalus (OH) secondary to CIBC.

Methods: Retrospective neuroradiological evaluation of a case series involving 23 domestic carnivores with CIBC presenting with both OH and cervical hydromyelia/syringomyelia. MR images checked specifically the continuity between the FVO and the CCEC.

Results: There were 18 adult and five young domestic carnivores. Anatomical continuity between the FVO and a dilated CCEC (hydromyelia) could be demonstrated on MR imaging in all young cases but in only 16 % of adult cases.

Conclusions: This study provides additional insights into understanding the relationship between the development of hydrocephalus and hydro/syringomyelia. MRI findings support that domestic carnivores have a virtual CCEC that is connected with the FVO at birth and might disappears over the years in normal, healthy animals, thus explaining hydromyelia in early stages of age rather than syringomyelia, in hydrocephalic conditions. When this anatomical continuity is present, the hydrodynamic theory have a pivotal role in the pathogenesis of hydromyelia. If not (most adult cases) other mechanisms may be activated and lead to spinal cord syringomyelia.

Key words: CNS, inflammatory diseases; CNS, animal diseases; Hydromyelia; Syringomyelia; MR imaging, hydromyelia; MR imaging, syringomyelia; MR imaging, obstructive hydrocephalus;