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

Overview

• This study investigated the utility of the trigeminocervical reflex (TCR) and quantitative sensory testing (QST) for assessing trigeminal nerve function in horses suffering from trigeminal-mediated headshaking (TMHS).
• Results showed that horses with TMHS had lower TCR thresholds compared to controls, suggesting altered trigeminal nerve function, while QST was not effective in distinguishing affected horses.

Background and Objective

• Trigeminal-mediated headshaking (TMHS) is a condition in horses characterized by involuntary head movements believed to be linked to trigeminal nerve dysfunction.
• Current diagnostic approaches for evaluating trigeminal nerve (TN) function in these horses are limited or underexplored.
• The trigeminocervical reflex (TCR) is a neurophysiological reflex that can assess TN function by stimulating the infraorbital nerve and recording muscle responses.
• Quantitative sensory testing (QST) measures sensory thresholds such as mechanical, tactile, and thermal sensitivity and is used in various species to assess nerve function.
• The study aimed to investigate whether TCR and QST can differentiate horses with TMHS from healthy controls and those with secondary headshaking (sHS).

Methods

• Study Design: Prospective case-control study involving three groups:
  • TMHS horses (n=12)
  • Secondary headshaking horses (sHS, n=4)
  • Headshaking-free control horses (n=14)
• Setting: Horses were selected from a referral hospital and a teaching herd.
• In unsedated horses, TCR assessment involved:
  • Electrical stimulation of the infraorbital nerve using an automated threshold-tracking device.
  • Recording electromyographic responses from the splenius muscle.
• QST included:
  • Evaluation of mechanical nociceptive (pain-related), tactile sensory, and thermal nociceptive thresholds.
  • Testing done bilaterally at two sites for comparison within and between groups.
• Statistical analyses:
  • Group comparisons used t-tests or Mann-Whitney U tests depending on data distribution.
  • Univariate logistic regression assessed predictive value of TCR and QST for TMHS diagnosis.

Results

• TCR thresholds:
  • Horses with TMHS showed significantly lower TCR thresholds (mean 0.82 ± 0.22 mA) compared to controls (mean 1.16 ± 0.30 mA).
  • Statistical evidence showed a strong effect size (Hedges’ g=1.24) and high significance (P=0.003).
  • This suggests increased excitability or altered function of the trigeminal nerve reflex pathway in TMHS horses.
  • No significant differences between left and right sides were observed in TMHS horses, indicating bilateral involvement.
• In the sHS group:
  • Lower thresholds were observed on the affected side, consistent with localized nerve involvement.
• QST results:
  • No significant differences were found between TMHS horses and controls for mechanical, tactile, or thermal thresholds.
  • This suggests QST, as applied here, might not be sensitive enough to detect trigeminal nerve abnormalities in TMHS.
• Logistic regression:
  • TCR thresholds predicted TMHS status well, with a model R2 of 0.73 indicating good explanatory power.
  • The area under the curve (AUC) of 0.83 showed good diagnostic accuracy of TCR testing for TMHS.

Conclusions and Implications

• Lower TCR thresholds in TMHS horses support the hypothesis that altered trigeminal nerve function underlies the headshaking condition.
• TCR testing could serve as a valuable adjunct diagnostic tool in clinical assessment of horses with suspected trigeminal-mediated headshaking.
• The lack of QST differences might be due to:
  • Heterogeneous somatosensory phenotypes in TMHS horses, meaning variability in sensory symptoms.
  • Limited sensitivity of the QST protocol used; alternative or more refined methods might be necessary.
• Future research should explore:
  • Refinements in QST protocols or additional sensory testing methods.
  • Longitudinal studies to validate TCR and QST as monitoring tools for disease progression or treatment response.