Idiopathic versus Secondary Equine Headshaking Syndrome: Diagnostic Insights from CT imaging

Abstract

Background: Equine headshaking Syndrom (EHS) is a rare condition characterized by involuntary headshaking in horses without external stimuli. It is not a specific disease, but a clinical sign with various potential underlying causes. Earlier studies indicated that the majority of cases are idiopathic. However, with advancements in diagnsotic technology, particularly CT scans, there has been an increase in identifying causative pathologies. This has led to a classification of EHS into 'idiopathic' and secondary headshaking (HS). However, recent studies show varying discrepancies in the classification of CT findings as incidental or causative. Objectives: The aims of this study were to describe the computed tomographic (CT) findings in horses that were presented for investigation of headshaking. A secondary aim was to establish the distinction between incidental findings and causative pathologies and a third aim was to investigate the potential distinction between different types of HS. Study Design: Retrospective case series of horses undergoing CT examination of the head as part of the investigation of headshaking at the Diagnostic Imaging division, University of Utrecht. Methods: Patient records and CT images of horses with a history of headshaking, which underwent CT head examinations at the University of Utrecht between 2015 and 2022, were reviewed. Results: A total of 101 horses had a complete diagnostic work-up and CT examination, whereof 73.3% had also a follow-up. The mean duration of headshaking signs before admission to the university clinic for CT examination was 47.2 weeks (SD ± 64.4). Additionally, a wide variaty of clinical manifestations were noted. A definitive diagnosis leading to complete resolution of headshaking symptoms were achieved in only 12.5% (3 out of 24) of treated horses. Common abnormalities identified in CT scans included dental issues (76.2%), temporomandibular disoders (33.8%), sinus abnormalities (21.2%), and skull bone issues (26.5%). The most common treatment performed based on CT findings were dental extractions. Main limitations: Retrospective study; Wide Variation in Clinical Signs and Symptom Duration; Potential Misdiagnosis: misclassification of idiopathic headshaking versus secondary headshaking; Inconsistent follow-up duration and high loss to follow-up affecting long-term outcome interpretation; Limited amound of follow-up diagnostic imaging. Conclusion: The treatment of pathologies observed during CT scans led to the complete resolution of clinical signs in three horses, highlighting the complexity of diagnosing causal pathologies in EHS. The wide variaty of clinical manifestations in this group underscores the need for a clear subdivision of horses admitted for the diagnostic work-up of headshaking based on the involvement of the trigeminal nerve. Old perspectives that EHS is mainly allergy-induced or untreatable delay proper diagnostic work-ups, worsening horses welfare and reducing the likelihood of symptom resolution. Over time, structural abnormalities in the trigeminal nerve can lead to functional issues, and the risk of the development of chronic pain increases with prolonged disease duration, complicating the identification of causal pathologies and effective treatments. Idiopathic HS is overdiagnosed leading to a significant decreased welfare in horses as a result of symptomatic treatment instead of treating the underlying pathology, risking chronic disease. Trigeminal mediated headshaking might be a multifactorial problem based on genetic, anatomical, neurophysiological, immunological, and environmental factors; whereof currently inflammatory processes in the innervation areas of the nerve might be the most underscored cause. The seasonal or intermitted pattern of EHS might be associated with flare-ups of (chronic) inflammatory processes, rather than allergies.