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Clinical Case Challenge: acute onset of cranial nerve deficits in a 20 -year-old Warmblood gelding and a 11- year-old Thoroughbred gelding
The first patient is an 11-year-old Thoroughbred gelding presenting for evaluation of an acute onset of head tilt to the left and circling behavior.  Physical ...
June 15, 2021

History:

The first patient is an 11-year-old Thoroughbred gelding presenting for evaluation of an acute onset of head tilt to the left and circling behavior.  Physical examination reveals evidence of vestibulocochlear nerve (cranial nerve VIII) deficits, with a left-sided head tilt (Image 1 below) and horizontal nystagmus with the slow phase of the nystagmus towards the left (Video 1 below).  Additionally, throughout the examination, the patient persistently circles to the right and appears agitated when this behavior is restrained.  Hematology (CBC, chemistry profile, fibrinogen) is within normal limits.

vestibulocochlear nerve (cranial nerve VIII) deficits, with a left-sided head tilt

Image 1 (Horse 1 - head tilt)

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)

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)

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.

Image 4 (endoscopy)
Image 4 (endoscopy)
Image 5 (endoscopy)
Image 5 (endoscopy)

Computed tomography (CT) is the superior diagnostic imaging modality for diagnosing and characterizing THO, as it allows for precise demonstration of bony changes to the stylohyoid bone, petrous temporal bone, and tympanic bullae and soft tissue changes in the middle and inner ear (Video 2, Video 3, Image 6, Image 7).  Compared to radiography and endoscopy, CT examination allows for simultaneous evaluation of both the left and right temporohyoid articulations and, in recent studies, bilateral changes are apparent in up to 92% of horses with clinical signs consistent with THO that underwent CT examination.  Additionally, with the improved detail afforded by this imaging modality, identification of fractures of the temporal bone has been identified in up to 41% of cases, with certain fracture types associated with a trend for decreased prognosis for return to performance.

Image 6 (CT collage)
Image 6 (CT collage)
Image 7 (CT 3D recon)
Image 7 (CT 3D recon)

Question 3:  What is the ideal treatment for THO?

Answer 3:  Medical treatment for THO has been described and consists of anti-inflammatory medication (NSAID, corticosteroids, +/- aural medications) to address inflammation of the affected nerves, temporohyoid articulation, and tympanic bullae, broad-spectrum antimicrobials, and any additional supportive care required for THO-associated comorbidities, such as corneal ulcers and/or dysphagia.  However, such treatment may only allow for temporary relief.

Surgical intervention with a ceratohyoidectomy under general anesthesia (Figure 1, Image 8) allows for decreased forces on the affected side of the ankylosed temporohyoid joint. This, in turn, reduces the continued enlargement of bony callus and potential for recurrent fracture of the callus that can exacerbate nerve compression.

 

Figure 1 (ceratohyoidectomy)
Figure 1 (ceratohyoidectomy)
Image 8 (ceratohyoid bone)
Image 8 (ceratohyoid bone)

As a trend toward a decrease in return to work in horses with severe neurologic deficits and/or fracture into the cranial vault is evident in recent literature, early diagnosis and treatment is absolutely critical to establishing a good prognosis, as delay in surgical intervention may increase the likelihood of persistent neurologic deficits associated with permanent nerve damage. Following ceratohyoidectomy, up to 80% of horses that were performing athletically prior to surgery returned to their previous levels of use despite some horses having persistent mild neurologic deficits.

References:

Freeman, DE.  Guttural pouch. In: Equine Surgery, 5th Edn (2018). Eds: Auer JA, Stick JA, Prange T, Kuemmerle JM. Elsevier Saunders, St. Louis. pp 770-796.

Divers TJ, Ducharme NG, de Lahunta A, Irby NL, Scrivani PV.  Clinical Techniques in Equine Practice. 2006; 5: 17-23.

Tanner J, Spriet M, Espinosa-Mur P, Estell KE, Aleman M. The prevalence of temporal bone fractures is high in horses with severe temporohyoid osteoarthropathy. Vet Radiol Ultrasound. 2019;60(2):159-166.

Walker AM, Sellon DC, Comelisse CJ, et al. Temporohyoid osteoarthropathy in 33 horses (1993-2000). Journal of Veterinary Internal Medicine. 2002;16(6):697-703.