Background The axons of ganglion cells in the nasal retina decussate at the optic chiasm. It is unclear why tumours cause more injury to crossing nasal fibres, thereby giving rise to temporal visual field loss in each eye. To address this issue, the course of fibres through the optic chiasm was examined following injection of a different fluorescent tracer into each eye of a monkey.
Methods Under general anaesthesia, cholera toxin subunit B—Alexa Fluor 488 was injected into the right eye and cholera toxin subunit B—Alexa Fluor 594 was injected into the left eye of a single normal adult male rhesus monkey. After a week’s survival for anterograde transport, serial coronal sections through the primary optic pathway were examined.
Results A zone within the core of the anterior and mid portions of the optic chiasm was comprised entirely of crossing fibres. This zone of decussation was delineated by segregated, interwoven sheets of green (right eye) and red (left eye) fibres. It expanded steadily to fill more of the optic chiasm as fibres coursed posteriorly towards the optic tracts. Eventually, crossed fibres became completely intermingled with uncrossed fibres, so that ocular separation was lost.
Conclusions A distinct, central compartment located within the anterior two-thirds of the optic chiasm contains only crossing fibres. Sellar tumours focus their compressive force on this portion of the structure, explaining why they so often produce visual field loss in the temporal fields.
- Optic Nerve
- Visual pathway
Data availability statement
Data are available upon reasonable request. All data relevant to the study are included in the article or uploaded as supplementary information. See above responses.
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Collaborators Not applicable.
Funding This work was supported by grants EY029703 (JCH) and EY02162 (Vision Core Grant) from the National Eye Institute and by an unrestricted grant from Research to Prevent Blindness. The California National Primate Research Center is supported by a Base Grant from the NIH Office of the Director, OD011107.
Competing interests None declared.
Provenance and peer review Not commissioned; internally peer reviewed.
Author note This article is published to mark the Festschrift held at the Royal Society of Medicine in London, UK, on 25th and 26th March 2021 to celebrate the retirement of Dr Gordon T Plant . A complete recording of the event is available on the website of the United Kingdom Neuro-Ophthalmology Society: https://uknos.com