It is often difficult to pin down a cause for headshaking in horses despite a variety of diagnostic procedures. Head and neck radiography and endoscopy of the upper respiratory tract and guttural pouches are common diagnostic procedures. Currently, veterinarians pursue MRI and CT instead of or in addition to these other diagnostic measures.
One study revealed a higher sensitivity (100%) and specificity (96.7%) of CT compared to radiographs for head conditions that might provoke headshaking. Potential conditions include dental diseases, osseous and articular disease, and sinusitis.
Retrospective Case Study: CT Findings of Headshaking Horses
Researchers at the Royal Veterinary College in London performed a retrospective study of 103 cases of headshaking. They evaluated CT findings and targeted treatment of those pathologic findings, as well as success in resolution with treatment [Perrier, M.; Manso-Diaz, G.; Dunkel, B. Computed tomography findings in horses presented with signs of head shaking. Equine Veterinary Journal Oct 2022; doi.org/10.1111/evj.13889].
Horses (and ponies) from 52 veterinary practices ranged in age from 4-26 years. In 23.4% of the group, horses demonstrated signs during exercise. In 14.3% of the group, they showed signs at rest. Horses showed signs during both exercise and at rest in 45.2% of the group.
A questionnaire or phone conversation with the referring veterinarian ensued for 81.6% (84 horses) of cases evaluated with CT. Most of the responding veterinarians felt that the CT was helpful to rule out concurrent pathology that might contribute to headshaking.
CT Findings in Headshaking Horses
The CT findings were as follows:
- 9 horses (8.7%) had no abnormalities;
- 71 horses (69%) had fewer than five findings;
- 22 horses (21.3%) had 5-10 findings;
- One horse (1%) had more than 10 findings with four of possible clinical significance and six incidental findings.
Gas within the infundibula of cheek teeth–identified in nearly 50% of cases—is an example of an incidental finding. Another common incidental finding was insertional enthesopathy of the nuchal ligament in 23.3%. Sinusitis occurred in 11.7%, temporohyoid arthropathy in 7.8%, and otitis media and externa in 5%.
Of clinical significance on CT scans, 34% had tooth abnormalities such as fractured maxillary teeth (5%), fractured mandibular teeth (2%), and diastemata (4%). Remaining tooth abnormalities included dysplastic teeth/cementomas, asymmetric tooth root and residual teeth.
Soft Tissue Abnormalities
Soft tissue abnormalities occurred in 31% of subjects, such as thickened epiglottis, hyperattenuating foci (clots and thrombi) of the tongue, mucosal thickening of the dorsal nasal conchal bulla, carotid artery enlargement, and muscle enlargement of sternohyoideus, omohyoideus and occipitomandibular muscles.
Trigeminal Nerve-Mediated Headshaking
Most notably, 60.2% of the cases were diagnosed with trigeminal nerve-mediated headshaking (TNMH). Of these, 28.2% were confirmed with this diagnosis based on lack of relevant CT abnormalities along with diagnostic efforts reported by the referring vet or owner in the follow-up questionnaire. Diagnostics included a positive response to a maxillary nerve block and/or treatment with percutaneous electrical nerve stimulation, electroacupuncture, nose nets and tack changes.
Euthanasia was the outcome for 12.6% of the horses in the study despite attempts at treatment with tooth extraction, treatment of a guttural pouch infection, or steroid injection of the atlantooccipital joint. The other euthanized horses had no relevant findings on CT.
Treatment of Primary Disease in Headshaking Horses
The study identified a primary disease in 21.4% (22 horses) of cases as a cause of headshaking. All had at least one relevant finding on CT exam. Treatment of the primary disease ended with successful resolution:
- Tooth fracture (8%) resolved with tooth removal. (Of 11 horses treated with tooth extraction based on CT findings, two horses did not improve in clinical signs.)
- Sinusitis (4%) resolved with sinus lavage.
- Temporomandibular joint arthritis (3%) resolved with intra-articular injection of hyaluronic acid and/or corticosteroids. (One TMJ case did not resolve with treatment. Nine cases were not included in follow-up information.)
- Impingement of dorsal spinous processes (1%) and cervical arthropathy (1%). Both resolved with treatment.
- Basisphenoid fracture (1%) managed with NSAIDs.
- Otitis externa (1%) responded to treatment.
- One horse with a neoplastic mass within the infraorbital nerve was euthanized.
The study did not identify any abnormal findings in 9% of subjects, thus leading to a likely diagnosis of TNMH by exclusion of other disease issues. No follow-up information was available for 10 of the 103 horses.
In summary, this study evaluated 103 horses with a primary complaint of headshaking that underwent computed tomography (CT) evaluation. Ninety-four horses had relevant pathologic findings. The most common diagnosis was trigeminal nerve-mediated headshaking in 62 (60.2%) of horses. At least 22 (22.4%) of the 103 cases of headshaking had a treatable condition that resolved with targeted treatment.