Somatotopic sensory trigeminal rootlets identification via unmasked trigemino-facial reflexes. Insights from trigeminal nerve surgery.

OBJECTIVES: Among the brainstem reflexes, the well-known blink reflex is commonly used to assess pathologies involving the trigeminal and/or facial nerves and to enhance surgical safety through monitoring. Taking advantage of percutaneous access to the trigeminal root during radio-frequency thermo-rhizotomies for trigeminal neuralgia, sensory rootlets corresponding to the trigger zone were identified by electrical stimulation prior to lesioning. During stimulation, we observed evoked motor responses (EMRs) not only in the orbicularis oculi but also in lower facial muscles-an observation not previously reported, to our knowledge.

METHODS: Stimulation at 5 Hz was used to evoke paresthesias in the patient's face. Concurrently, EMRs were observed in the orbicularis oculi or, independently, in the levator labii or orbicularis oris muscles, depending on electrode position within the targeted sensory rootlets.

RESULTS: EMRs were clinically observed in 44.2 % of 459 procedures when the electrode contacted closely targeted rootlets. A positive correlation was found between EMR topography (orbicularis oculi, levator labii, orbicularis oris) and trigeminal territories (V1, V2, V3) in 95 % and 96 % of procedures for evoked paresthesias and post-thermolesion hypoesthesia, respectively.

CONCLUSIONS: EMRs topography, when observable, accurately predicts trigeminal territories.

SIGNIFICANCE: Those EMRs interpreted as trigemino-facial reflexes (TFRs) may serve as a useful tool for identifying retrogasserian sensory fibers.