The second patient is a 20-year-old Warmblood gelding presenting with a 3-week history of deviation of the muzzle to the left side. Approximately 3 weeks prior to presentation, the patient exhibited abnormal mastication and hypersalivation. While these signs abated, intermittent fevers (up to 103.0 degrees Fahrenheit) were noted in the week prior to presentation. Physical examination revealed evidence of facial nerve (cranial nerve VII) deficits, with a right-sided ear droop, ptosis of the right eye, and deviation of the muzzle to the left (Image 2). Application of fluorescein stain to the affected eye yielded no uptake. Hematology (CBC, chemistry profile, fibrinogen) and oral examination are within normal limits.
Image 2 (Horse 2 - facial nerve paralysis)
Question 1: In both cases, what is a highly suspected differential diagnosis based upon their history and preliminary evaluation?
Answer 1: Temporohyoid osteoarthropathy (THO).
THO is a progressive disease of the middle ear and components of the temporohyoid joint, including the stylohyoid bone, cartilaginous tympanohyoid, and squamous portion of temporal bone. Clinical signs most commonly develop in middle-aged horses and, throughout the literature, Quarter Horse-type breeds are predisposed. Additionally, cribbing has been associated with THO, with horses with neurologic disease associated with THO being 8 times more likely to be cribbers in one study.
Two potential etiologies are proposed for THO: 1) degenerative joint disease resulting in osseous proliferation in an attempt to ankylose the joint, in which lesser degrees of aural disease is present. 2) Aural disease that spreads to the osseous structures of the temporohyoid region and results in subsequent osseous proliferation, which is suspected in patient 2.
Early clinical signs may be associated with discomfort in the affected region and include head shaking, ear rubbing, refusing to take the bit, difficulty in achieving collection when under saddle, and resistance to pressure at the base of the ear or on the basihyoid bone in the intermandibular region. Clinical signs of neurologic dysfunction become apparent when proliferation of the affected bones and/or fusion of the temporohyoid joint result in compression of the facial nerve (cranial nerve VII) and vestibulocochlear (cranial nerve VIII) as these nerves emerge from the skull adjacent to the temporohyoid region. Signs of facial nerve paresis include an ear droop, deviation of the muzzle away from the affected side, ptosis, inability to close the palpebral orifice, decreased tear production, and facial hyperesthesia. With the effects of facial nerve dysfunction on ocular structures, corneal ulceration is not uncommon due to the combination of keratoconjunctivitis sicca and exposure keratitis. Signs of vestibulocochlear neuropathy include asymmetrical ataxia, head tilt with the poll directed toward the affected side, and spontaneous horizontal nystagmus, with the slow phase directed toward the affected side.
Severe bony proliferation and inflammation can result in damage to the glossopharyngeal (cranial nerve IX) and vagus nerve (cranial nerve X) as they exit the medulla caudal to the vestibulocochlear nerve. Such cases may present with dysphagia, pharyngeal collapse, and, with the vagus nerve affected, respiratory distress due to laryngeal dysfunction.
After fracture of the petrous temporal bone, inflammation and/or infection from the middle or inner ear may result in further development or worsening of neurologic signs due to development of meningoencephalitis. In such cases, leakage of serous and/or serosanguinous fluid from the ear represents egress of cerebrospinal fluid (CSF) from the compromised calvarium into the ear canal. Severe clinical signs associated with meningoencephalitis may include seizure activity, recumbency, and acute death.
Question 2: What diagnostic imaging can be performed to confirm this diagnosis?
Answer 2: Radiography can be performed, with a dorsal-ventral projection often most useful to demonstrate unilateral or bilateral thickening of the stylohyoid bone and/or sclerosis of petrous temporal bone (Image 3).
Image 3 (Radiograph of a dorsal-ventral projection)
Endoscopy examination of the guttural pouches is a reliable diagnostic imaging modality to detect changes of the stylohyoid bone and temporohyoid articulation (Image 4, Image 5). As bilateral changes of the stylohyoid bone and temporohyoid articulation have been identified in endoscopic examination of up to 22.6% of horses with unilateral clinical signs, endoscopic examination of both guttural pouches is imperative.