CT Clues in Equine Headshaking? New Study Reveals What Really Matters

Computed Tomography of the Hyoid Apparatus in Equine Headshaking Syndrome

Ralph A. Lloyd-Edwards, Eva Mulders, Marianne M. Sloet van Oldruitenborgh-Oosterbaan, Stefanie Veraa

Background

Headshaking in horses is a common and often debilitating clinical condition, frequently attributed to trigeminal nerve–mediated hypersensitivity when no structural cause is identified. Computed tomography is widely used to detect potential structural contributors, but the clinical relevance of hyoid apparatus abnormalities remains unclear. Prior literature has identified a variety of hyoid and temporohyoid joint changes, yet their direct association with headshaking has not been well established. This study investigates whether specific CT-detected hyoid abnormalities correlate with clinical signs of headshaking in horses.

Methods

The authors conducted a retrospective analysis of CT examinations of 141 horses with documented headshaking (2015–2022) and 149 control horses who underwent head CT for other reasons. Horses were excluded if under 1 year old or if CT quality was insufficient for hyoid evaluation. Headshaking cases were further classified as having either (1) “likely relevant findings” on CT (e.g., sinus disease, dental pathology, intracranial lesions) or (2) none of these findings. CT evaluations included a standardized grading of temporohyoid osteopathy (0–3) and assessment of additional features such as joint narrowing or widening, tympanohyoid cartilage mineralization, gas, fusion, fractures, arthropathy, and ossification centers. Statistical comparisons employed Fisher's exact tests, Mann–Whitney U tests, Spearman correlations, and odds ratios.

Results

Age strongly correlated with several CT features, including temporohyoid grade, tympanohyoid cartilage mineralization, and joint widening. While some hyoid changes (e.g., increased new bone formation, joint widening) showed higher odds ratios for headshaking, none reached statistical significance. No meaningful difference in temporohyoid grade was observed between headshaking horses and controls. Horses with fractures, arthropathy, or deformation of the remaining hyoid apparatus consistently showed higher temporohyoid grades, suggesting linked degenerative processes. A separate ossified epihyoid was common but significantly more frequent in control horses. Ultimately, no CT-based hyoid feature consistently predicted headshaking.

Limitations

The retrospective design limited consistency in clinical work-up and prevented acquisition of histopathological correlation. Subjective assessments such as joint narrowing and widening may have been influenced by anatomical variability and imaging factors. Classification of “likely relevant findings” depended on literature-derived criteria, which may not fully capture true clinical relevance. The heterogeneous breeds and indications for CT in the control group may also affect comparative interpretation.

Conclusions

CT abnormalities of the hyoid apparatus—especially age-related changes of the temporohyoid joint—are common, but no specific feature demonstrated a statistically significant association with clinical headshaking. Although some hyoid changes increased odds ratios, the findings did not allow refutation of the null hypothesis. The study highlights the complexity of interpreting CT findings in headshaking horses and underscores the need for prospective, standardized, and more objective evaluation methods.

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