50.: Visualisation of nerve fibre orientation in the human optic chiasm using photomicrographic image analysis

Compression of the optic chiasm typically gives rise to bitemporal hemianopia due to selective damage to the decussating nasal fibres. It is unclear why nasal fibres are particularly vulnerable to the extent of generating a sharp vertical cut-off in the visual fields. One theory suggests that this is due to the geometry of individual fibres within the chiasm. Unfortunately, detailed anatomical information about the precise arrangement and crossing of nerve fibres in the chiasm is limited. This study aimed to clarify the microscopic anatomy of the chiasm, looking particularly at nerve fibre distribution and the location of fibre crossings. A human optic chiasm obtained at autopsy was stained en bloc with silver stain and sectioned in the axial plane at 5 μm intervals. Photomicrographs were digitised and subdivided into smaller regions of interest (ROI). Fibre orientation distributions for each ROI were obtained and processed using ImageJ software and custom-written MATLAB code. The orientation data and crossing angles were then represented graphically. The central portion of the chiasm was found to contain fibres travelling in parallel in a predominantly medio-lateral direction. Crossings were located in the antero-inferior and supero-posterior portions of the lateral parts of the chiasm. This study contradicts the traditional understanding that fibre crossings are located centrally in the chiasm. Data from the study will be used to inform models of the optic chiasm which, in turn, will generate further insight into the pathophysiology of bitemporal hemianopia.