Prediction of Optic Chiasm Fixation Based on Cranial Metrics

Background: The suprasellar space is a common location for intracranial lesions. In this study the authors provide information predicting relationships between common intracranial distances and spaces that influence access to the suprasellar space. The major limiting factor in the transcranial surgical access to this space include the positions of the optic nerves and the optic chiasm (pre-fixed vs. post-fixed). A better understanding of these anatomic relationships and their possible relationship to cranial anatomy may help surgeons choose proper surgical approaches.

Methods: An analysis of T2-weighted magnetic resonant images (MRI) from fifty subjects (25 males, 25 females) was performed. A comparative analysis was performed regarding the interpupillary distance (IPD), optic nerve length (ONL), optic nerve distance (OND), interoptic space (IOS), optic tract length (OTL), optic tract distance (OTD), optic tract interval (OTI), cranial length (CL), bi-parietal diameter (BPD), and cephalic index (CI).

Results: The size of the IOS varies directly in relation to the fixation of the optic chiasm: for pre-fixed chiasms (n = 7) the mean IOS is 205.14 mm², for normal chiasm position (n = 33) the mean IOS is 216.94 mm² and for post-fixed chiasms (n = 10) the mean IOS is 236.20 mm² as seen in Table 1 (p = 0.002). The position of the optic chiasm did not directly correlate with CL, BPD, or CI.

Conclusion: This study provides insight into known intracranial landmarks and their relationship to transcranial access of the suprasellar space. This study shows that position of the optic chiasm has a predictable effect on the IOS, with a post-fixed chiasm being associated with a larger IOS. However, optic chiasm fixation cannot be predicted by cranial anatomy.