Abstracts

Rationale:
Transcranial magnetic Stimulation (TMS) is increasingly utilized as a diagnostic tool for functional motor and language mapping prior to epilepsy surgery. However, the intensity of sound emanating from the discharge of TMS coils can be concerning and is a potential cause for acoustic trauma. Per Occupational Safety and Health Administration (OSHA) standards for safety of noise exposure, hearing protection is recommended beyond restricted levels of noise and time limits. While the acoustic properties of several TMS coils have been studied in the past, we present measurements of sound pressure levels (SPLs) in a clinical setting, accounting for the effects on all parties involved, including the operator and an accompanying parent/guardian with the goal of assessing if the acoustic artifact levels are of sufficient amplitude to warrant protection from acoustic trauma per OSHA standards.
Method:
We measured the peak mean SPLs generated by a navigated TMS Nexstim air-cooled figure-of-eight coil in acute experiments (200 pulses/condition at 100% machine output) simulating short exposures in a clinical setting. In experiment 1, a sound meter (Extech model SL400; 70-140 dB range, z-weighted) was attached to the subject’s glasses either on ipsilateral or contralateral side of the TMS coil to measure SPLs experienced by the operator (n=8, 100 pulses/side/subject). In experiment 2, SPLs were measured at varying distances from the coil at 10, 50 and 100 cm (50 pulses each), simulating the operator’s movement alongside the machine. In experiment 3, SPLs were measured at the visitor’s seat (~1.7 m from the coil in our setup), to reproduce the sound levels experienced by the accompanying parent/guardian in the TMS room.
Results:
The peak ambient SPL, produced by the TMS machine in offline mode was 87.6 dB, well under the OSHA standards for short 15-min (115 dB) and long 8-h (90 dB) exposure. In experiment 1 mimicking the operator condition, the mean SPL recorded on the ipsilateral side of TMS coil was 92.3 dB, while it was 91.4 dB on the contralateral side. In experiment 2 representing operator movement, the sound levels decayed as a function of distance with 10 cm, 50 cm, and 100 cm averaging to 97.4 dB, 94.1 dB and 86.2 dB, respectively. In experiment 3, for the parent/guardian chair condition, the mean SPL recorded over 100 stimulations was 84.6 dB.
Conclusion:
In all studied conditions of acute exposures, the navigated TMS coil’s acoustic artifact levels were lower than the OSHA permissible threshold for short exposure (< 15 min). Most epilepsy related TMS procedures including pre-surgical mapping are typically 1-h long, deeming them safe. However, the TMS operator working long shifts up to 8 h may be exposed to noise levels higher than OSHA threshold ( >90 dB), warranting the use of appropriate ear protection to prevent long-term aural damage or acoustic trauma. While safe in most conditions, ear protection is nevertheless recommended for all participants present in the TMS clinical room.
Funding:
:None