Mapping the VIIIth cranial nerve by electrical stimulation: methods for differentiating auditory from vestibular...

Area of Science:

• Neuroscience
• Otolaryngology
• Surgical Neurology

Background:

• Intraoperative monitoring of auditory brainstem responses is crucial for cerebellopontine angle surgery.
• Accurate identification of the vestibular portion of the VIIIth cranial nerve is challenging due to indistinct subdivisions in approximately 25% of humans.
• Current intraoperative techniques rely solely on acoustic stimuli, highlighting the need for improved methods.

Purpose of the Study:

• To map the VIIIth cranial nerve using direct electrical stimulation.
• To characterize auditory and vestibular evoked responses from direct electrical stimuli on the VIIIth cranial nerve.
• To compare electrically evoked responses in brainstem nuclei with extracranially recorded far-field potentials.

Main Methods:

• Experiments were conducted on 12 anesthetized cats.
• Electrical pulse stimuli were applied directly to the exposed VIIIth cranial nerve using a bipolar electrode.
• Evoked responses were recorded from scalp electrodes (far-field) and from electrodes in the inferior colliculus and abducens nucleus (near-field).

Main Results:

• Direct electrical stimulation of the cochlear nerve yielded a four-wave extracranial response and strong inferior colliculus activity.
• Stimulation of the vestibular nerve produced a two-wave extracranial response and strong abducens nucleus activation.
• Stimulation at the nerve border generated intermediate responses with both cochlear and vestibular characteristics.

Conclusions:

• Direct electrical stimulation of cochlear and vestibular subdivisions elicits distinct evoked response morphologies.
• Intraoperative electrically evoked responses are feasible and can aid in unambiguous identification of VIIIth cranial nerve subdivisions.