Diagnostic utility of trigeminocervical reflex and quantitative sensory testing in horses with trigeminal-mediated headshaking.

BACKGROUND: Assessment of trigeminal nerve (TN) function in horses with trigeminal-mediated headshaking (TMHS) using trigeminocervical reflex (TCR) and quantitative sensory testing (QST) has not been investigated. HYPOTHESIS/OBJECTIVES: Horses with TMHS show lower TCR and QST thresholds than controls. A subset shows left-to-right differences. ANIMALS: Privately owned horses with TMHS (n = 12) and secondary headshaking (sHS; n = 4), from a referral hospital. Headshaking-free controls (n = 14), from the hospital's teaching herd or privately owned. METHODS: Prospective case-control-study. In unsedated horses, TCR was assessed using an automated threshold-tracking device via infraorbital nerve stimulation with concurrent splenic muscle electromyography. QST included evaluation of mechanical nociceptive, tactile sensory, and thermal nociceptive thresholds, assessed bilaterally at 2 sites. Group comparisons used t-tests or Mann-Whitney U tests. Univariate logistic regression assessed TCR and QST as predictors of TMHS. RESULTS: Horses with TMHS had significantly lower TCR thresholds than controls (0.82 ± 0.22 mA vs. 1.16 ± 0.30 mA, Hedges' g = 1.24, 95% CI, 0.41-2.06, t = -3.31, P = .003), and logistic regression predicting TMHS showed good performance (R2 of 0.73, area under the curve of 0.83). No significant left-to-right differences were identified. In sHS horses, lower thresholds were observed on the affected side. QST revealed no significant differences between groups. CONCLUSIONS AND CLINICAL IMPORTANCE: Lower TCR thresholds support altered TN function in TMHS. TCR might be a useful adjunct diagnostic tool, whereas QST did not identify affected horses, reflecting heterogeneous somatosensory phenotypes or limited sensitivity of the QST protocol.