A Plausible Mechanism for Optic Nerve Damage: A Comparative Study of Heat Transfer during Optic Canal Decompression

Introduction: Multiple articles have described vision loss after neurosurgical intervention from heat-induced injury to the optic nerve during exposure to the optic canal. This project's objective was to determine factors associated with heat transfer during drilling of the optic canal that could lead to possible optic nerve injury.

Methods: On a non-embalmed cadaver, the optic canal was approached through a standard pterional craniotomy. A thermocouple was placed within the optic canal, and the temperature was recorded during 30-second continuous drill sessions using both CUSA NXT and Anspach eMax 2 with a 3-mm diamond burr. The drill power and irrigation rate were varied for comparison of heat transfer.

Results: There was an incremental increase in optic canal temperature with both Diamond burr and ultrasonic aspirator over time (10-, 20-, and 30-second time points). Stepwise elevations in temperature occurred with increasing sonication amplitudes. Decreases in thermal conduction were noted in both diamond burr with continuous irrigation (mean, 3.3°C reduction) and sonication with increased irrigation (from 2 to 4 mL/min) at 50% amplitude (mean, 1.1°C reduction), but was insufficient to combat the heat generation at 80% amplitude, and may result in a slight increase in heat conduction (5.5°C increase from baseline at 2 mL/min vs. 6.8°C at 80% with 4 mL/min). Thermal transfer was comparable between diamond burr with continuous irrigation and sonication at 50% amplitude with 4 mL/min irrigation (avg. increase from baseline 1.3°C drill vs. 1.1°C ultrasonic aspirator).

Conclusions: Increased drilling times correlated with increase in temperature from baseline using both bone removal devices. Elevations in sonication amplitude resulted in incremental increases in optic canal temperature. These temperature changes can be mediated by irrigation, although the effect of increasing irrigation rate to suppress the rise in temperature decreases with increasing sonication amplitude. Thermal transfer was similar between the diamond burr with continuous irrigation and sonication at low power and high irrigation. This pilot study has revealed critical information for the development of further projects with the aim of determining if thermal transfer from optic canal decompression could contribute to optic nerve damage, and what techniques could be applied to prevent it.