Time course of optic nerve sheath dilation: In vitro response characteristics to controlled pressure elevations.

BACKGROUND

Optic nerve sheath (ONS) dilation indicates intracranial pressure elevation under clinical conditions but limited data exist with regard to the dynamics of sheath expansion.

OBJECTIVE

To assess the time course of ONS widening and its stability under controlled pressure conditions in vitro.

METHODS

Pre-defined pressure steps up to 65 mmHg were applied to the perineural space of ex-vivo human optic nerves (n = 16). Using ultrasound, the optic nerve sheath diameter (ONSD) was monitored over 500 s. Re-tests at low-pressure levels concluded each experimental series.

RESULTS

In most cases, 50% of the total diameter-change were achieved within 50 s after pressure onset and 95% of the maximal diameter after 200 s. The diametric gains in each experiment were strongly correlated with the applied pressure levels (coefficient of variance 0,96) within preparations with variability of the transfer function across preparations. The time course of the dilation was found to follow an approximate natural logarithmic function (R2 = 0.93-0.99). The re-test condition revealed unchanged time course properties (5% significance level) despite starting regularly from a higher baseline-diameter after preceding exposures.

CONCLUSIONS

ONS dilation commences rapidly after pressure exposure with a predictable time course over 3-4 min. Elasticity changes presumably affecting trabecular structures cause upward shifts of the optic nerve sheath pressure response. Clinically, ONSD shifts should be considered in serial measurements for increasing intracranial pressure monitoring, but relevant response delays are absent for lower or higher changes of intracranial pressure.