Nonuniform pressure generation in the optic chiasm may explain bitemporal hemianopsia.

PURPOSE: To measure the pressure in the temporal and central aspects of the chiasm simultaneously during compression of the optic chiasm from below with an expanding simulated tumor.

DESIGN: Experimental study.

METHODS: Craniotomies were performed on 5 unfixed cadaveric specimens ranging in age from 49 to 89 years, and the optic chiasm was exposed. After a pediatric gauge Foley catheter was inserted into the sellar region, the temporal side of the chiasm (temporal aspect uncrossed fibers) was impaled with a 30-gauge needle connected to a pressure transducer. In like fashion, the central portion of the chiasm (crossed fibers) was impaled with another 30-gauge needle connected to a pressure transducer on a separate channel. The Foley catheter was inflated for a period of 30 seconds to 1 minute and the pressure in each channel continuously monitored. Multiple trials were performed on each of the 5 specimens.

MAIN OUTCOME MEASURE: Pressure change between the temporal and nasal aspects of the optic chiasm.

RESULTS: In all cases in which intrachiasmal pressure could be measured, higher pressures were always generated in the central aspect of the chiasm than in the temporal aspect of the chiasm. In cadaver 1, the mean central pressure was 19.75 mmHg, whereas the mean temporal pressure was 6 mmHg. In cadaver 5, the mean central pressure was 4 mmHg, whereas the mean temporal pressure was zero. Donors 70 and older demonstrated abnormally thin and frail chiasms that were unable to support a sustained pressure increase during simulated tumor expansion.

CONCLUSIONS: During deformation of the optic chiasm from below by a radially expanding mass analogous to a pituitary tumor, the central aspect of the optic chiasm consistently manifests a higher pressure than the temporal aspect. It is hypothesized that the peculiar geometry of the optic chiasm renders the crossing nasal fibers more prone to a deformation stress exerted from below. Nonuniform pressure generation between the central and temporal aspects of the chiasm results in a greater effective stress on the crossing fibers of the chiasm and may be responsible for the clinical phenomenon of bitemporal hemianopsia.