Transcranial Magnetic Stimulation in the Differential Diagnosis of Unilateral Peripheral Facial Nerve Palsy

(1) Background: This study aims to assess the diagnostic accuracy of parameters based on a combination of transcranial magnetic stimulation (TMS) and electrical stimulation (ES) in the differentiation between idiopathic and secondary facial palsy in a large cohort of patients. (2) Methods: Patients with unilateral facial palsy ≤7 days after symptom onset were included. Compound muscle action potential (CMAP) amplitudes were measured after stimulation of both facial nerves at (A) the internal acoustic meatus using TMS, CMAP-TMS, and (B) at the stylomastoid foramen using electrical stimulation, CMAP-ES. To express the degree of nerve dysfunction in the facial canal specifically, the amplitude reduction of the CMAP-TMS in relation to CMAP-ES was calculated and expressed as a percentage (amplitude reduction over the facial canal, ARFC). Receiver Operating Characteristic (ROC) curves were constructed to assess the diagnostic accuracy of ARFC as a marker to discriminate between patients with idiopathic and secondary facial palsy. (3) Results: Data from 498 patient records were analyzed. Idiopathic facial palsy was diagnosed in 424 patients, and secondary facial palsy in 74 patients. The area under the ROC curve for ARFC was 0.398. (4) Conclusions: The overall diagnostic accuracy of this method to differentiate secondary from idiopathic facial palsy is low.