Introduction Compression of the optic chiasm gives rise to bitemporal hemianopia. The reason for this is unclear, but one hypothesis suggests it relates to the fact that nasal retinal fibres cross each other while temporal fibres do not. This ‘crossing’ hypothesis has been investigated using finite element modelling but this requires accurate anatomical data. The precise shape of the chiasm is not clear: nasal fibres may not decussate centrally (as if the chiasm were ‘X’-shaped) but, instead, decussate paracentrally and run parallel to each other in the central arm of an ‘H’. This study aimed to determine the population variance in chiasmal shape.

Methods 68 MRI scans of healthy individuals without visual abnormality were randomly selected. A 2D image was created and images were analysed using AutoCAD software to determine the offset between lines drawn down the centres of the optic nerves and contralateral optic tracts. A positive offset would suggest an ‘H’ shape while an ‘X’-shaped chiasm would have a offset of 0.

Results The mean width of the chiasm was 12.0 mm, and the mean offset was 4.7 mm generating a mean offset:width ratio of 0.38. No chiasm had an offset of zero. Fibre crossings occurred approximately 2.35 mm lateral to the midline, and nasal (crossing) fibres travelled an average of 4.7 mm in the mediolateral plane before entering the contralateral optic tract.

Conclusions The human optic chiasm is H-shaped, not X-shaped. This information will inform future models of chiasmal compression.