Computed Tomography of the Hyoid Apparatus in Equine Headshaking Syndrome.

Headshaking is a common condition in horses, most cases are presumed idiopathic/trigeminal-nerve mediated. Diagnostic work-up of a headshaking horse may involve computed tomography (CT) of the head to exclude causative structural pathology. The relevance of the presence and severity of hyoid apparatus findings at CT to headshaking is unknown. A retrospective analysis of CT changes in the hyoid apparatus in horses was carried out. Comparisons were performed between horses with signs of headshaking and a control population and a subgroup of horses with signs of headshaking and no other 'likely relevant findings' to headshaking and the control population. The grade of temporohyoid joint sheath ossification, mineralisation of the tympanohyoid cartilage, and widening and narrowing of the temporohyoid joint all showed significant correlation with age. Findings of the remaining hyoid apparatus (fracture, deformation, or arthropathy) showed significant correlation with temporohyoid joint grade. Centres of ossification of the epihyoid, thyrohyoid, and lingual processes were described. No consistent association of headshaking to hyoid changes was found. Odds ratios were increased in many cases, particularly in comparisons of the subgroup with no 'likely relevant findings'; however, statistical significance was not reached. CT findings of the temporohyoid joint are not consistently associated with clinical signs of headshaking.