Temporohyoid Osteoarthropathy
Basic Information 
Definition
Progressive degenerative osteoarthropathy of the articulation between the stylohyoid bone and the styloid process of the petrous temporal bone (temporohyoid articulation) leading to eventual ankylosis of this joint. The condition is generally unilateral, although bilateral involvement or disease can be seen.
Epidemiology
Risk Factors
Hematogenous infection leading to otitis media or interna. The author speculates that cribbing predisposes horses to this condition.
Associated Conditions and Disorders
• The enlarging osteoarthropathy of the temporohyoid articulation leads to pain and discomfort and to a compressive neuropathy with clinical signs dependent on the affected nerve(s). Nerves at risk are VII (facial), VIII (vestibulocochlear), IX (glossopharyngeal), and X (vagus). Finally, ankylosis of the temporohyoid articulation may lead to fracture of the petrous temporal bone, leading to cerebral hemorrhage, which can also cause facial and vestibular nerve damage as well as death.
• Ear polyps are occasionally seen associated with this disorder.
Clinical Presentation
Disease Forms/Subtypes
Temporohyoid osteoarthropathy (THO) presents in one of three forms: Incidental findings, nonneurologic disease, and neurologic disease.
History, Chief Complaint
• In the nonneurologic form, horses present with one or more of the following: Difficulty chewing; head shaking; cessation of cribbing; and behavior problems, especially when being ridden.
• In the neurologic form, horses present with one or more of the following: Ataxia, head tilt, deviation of the nose, falling while eating, and difficulty eating. Depression may also be present.
Physical Exam Findings
• In the nonneurologic forms, the following signs are observed: Difficulty chewing; head shaking; and behavior problems, especially when being ridden.
• In the neurologic form, the clinical signs are related to the specific nerve involved.
Facial nerve: All branches to the head are affected, causing a deviated muzzle (away from the side of the lesion) and a dropped lower lip, ear, and eyelid on the side of the lesion. Additionally, loss of innervation to the lacrimal gland may lead to keratoconjunctivitis sicca.
Vestibulocochlear nerve: Signs vary from mild head tilt and circling to severe horizontal nystagmus (quick phase away from the side of the lesion), recumbency, and seizure-like activity.
• In both clinical presentations, pain may be induced by palpation of the base of the ear or by dorsal pressure on the ventral aspect of the basihyoid bone. In addition, ear polyps may be present on the affected side. One may also note abnormalities in both left and right temporohyoid articulations, although signs relating to these lesions occurring in both sides during the same clinical presentation are unlikely.
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