Relationship of the distal optic nerve sheath to the circle of Zinn

Background: This study was designed to determine the relationship of the va scular circle of Zinn (CZ) and its branches to the termination of the subar achnoid space surrounding the optic nerve sheath, with particular reference to optic disc size. Methods: Serial sections of 29 normal human optic nerv es were performed at 6-mu m intervals. The position of the CZ and subarachn oid space were measured with WILD Heerbrugg objective graticules. The resul ts were analysed with reference to the optic disc size. Results: The positi on of the CZ was variable but two major types were recognised. In type 1 th e circle was located anterior to the distal sheath, and in type 2 posterior to the sheath. There were more small optic discs observed in type 2. Concl usion: In this study the CZ was more posteriorly located in small optic dis cs. This location may induce risk factors contributing to the development o f ischaemic optic neuropathy. The combination of small discs, posterior pla cement of the CZ, and anatomical variations in the vascular pattern may pre dispose to ischaemic events.