Case reveal

Findings:

• The left temporohyoid joint is surrounded by a moderate to severe amount of T1 and T2 heterogeneously hypointense (to muscle) material/tissue (yellow arrows), and the stylohyoid is moderately to severely thickened and heterogenously T1 and T2 hypointense (yellow highlight). This thickening extends approximately 3 cm distally from the proximal aspect of the stylohyoid bone (to the level of dotted line). Associated with these findings, the external ear canal is moderately to severely narrowed.
• The right stylohyoid bone is normal (blue highlight).

• The left masseter, temporal, and digastricus muscles are mildly decrease in volume in comparison to the right.
• The left ethmoid turbinates are moderately, homogeneously thickened in comparison to the right.

Conclusions:

• Moderate to severe left temporohyoid osteoarthropathy (at least Grade 3) with mild facial disuse muscle atrophy. Possible facial neuropathy associated with these findings may be compatible with the clinical signs of the patient.
• Moderate left ethmoid turbinate thickening, most compatible with normal nasal cycling.

Case follow-up:

• Endoscopy confirmed thickening of the left temporohyoid joint.
• The patient was also diagnosed with chronic laminitis, equine metabolic syndrome and an intra-abdominal mass. Palliative care at home was elected.

A little bit more…

• Stylohyoid bones articulate with petrous part of temporal bone. Connects with the skull via the temporohyoid joint (Koch et al.).
• The stylohoid bones extend through the guttural pouches, dividing the pouches into lateral and medial compartments (Platt et al.).
  • Medial compartment: Internal carotid, CN IX, X and XII nerves
  • Lateral compartment: External carotid, CN IV nerve

• Characterized by extensive bony proliferation surrounding the temporohyoid joint, tympanic bulla, stylohyoid bone, and adjacent structures of the skull (Koch et al., Walker et al., Hilton et al.).
• In chronic cases, can lead to fusion of the joint with subsequent fracture(s) (Koch et al., Walker et al.).
• Usually seen in older horses but can occasionally be diagnosed in young horses (Koch et al.).
• The underlying cause is not fully understanding. Proposed etiologies include age-related degenerative changes, upper respiratory infection, inflammation, and otitis media/interna (Koch et al., Walker et al.).
• May be unilateral or bilateral. Usually bilateral with one side less severely affected than the other (Walker et al.).
• Clinical signs (Koch et al., Walker et al., Hilton et al.)
  • Head shaking
  • Quiding
  • Cribbing
  • Vestibular ataxia and/or head tilt
  • Facial nerve paralysis, decreased tear production, corneal ulceration, keratitis and facial hyperaesthesia
• Radiography may be considered to screen for THO. Computed tomography is the preferred imaging modality (Koch et al.).
• Radiography:
  • Should include orthogonal projections
  • Osseous proliferation in the region of the tympanic bullae, temporohyoid joints and/or stylohyoid bones may be indicative of THO.
  • Radiographs have a low sensitivity to establish a diagnosis, and the absence of radiographic changes does not rule out THO
• Computed tomography (Hilton et al, Whitty et al.):
  • Allow a more complete evaluation of the osseous structures of the head, including the temporohyoid joints.
  • Other structures, such as ceratohyoid bone, tympanic bullae, and external ears also implicated in THO
  • Increase in stylohyoid bone width associated with presence of neurologic disease (>48mm width may be associated with increased risk)
  • CT is more sensitive for the identification of fractures.
• MRI (Walker et al., Inui et al.):
  • Not the first imaging modality used but may help better identify regional structures involved
  • Better evaluation of the soft tissues surrounding the temporohyoid joint, including the middle and inner ear, peripheral nerves and brain
• Treatments (Oliver et al.):
  • Medical management:
    • Decrease inflammation and pain
    • NSAIDs, steroids
    • Broad-spectrum antibiotics
    • Gabapentin?
    • Corneal ulcers and KCS managed with conventional ulcer treatments
  • Surgical options:
    • Partial stylohyoidectomy (PSHO)
    • Ceratohyoidectomy (CHO)

References:

• Koch, C. and Witte, T. (2014), Temporohyoid osteoarthropathy in the horse. Equine Veterinary Education, 26: 121-125.
• Walker, A.M., Sellon, D.C., Comelisse, C.J., Hines, M.T., Ragle, C.A., Cohen, N. and II, H.C.S. (2002), Temporohyoid Osteoarthropathy in 33 Horses (1993–2000). Journal of Veterinary Internal Medicine, 16: 697-703. 
• Hilton, H., Puchalski, S.M. and Aleman, M. (2009), The computed tomographic appearance of equine temporohyoid osteoarthropathy. Veterinary Radiology & Ultrasound, 50: 151-156.
• Whitty, JA, Miller, AV, Miller, AD, Carney, PC, Scrivani, PV. Associations between styloid process sheath CT sizes and age, body weight, and breed in horses. Vet Radiol Ultrasound. 2022; 63: 436– 449.
• Readford, P., Lester, G. and Secombe, C. (2013), Temporohyoid osteoarthropathy in two young horses. Aust Vet J, 91: 209-212.
• Oliver ST, Hardy J. Ceratohyoidectomy for treatment of equine temporohyoid osteoarthopathy (15 cases). Can Vet J. 2015 Apr;56(4):382-6.1
• Piat, Perrine, and Jean-Luc Cadoré. “Endoscopic Anatomy of the Equine Guttural Pouch: An Anatomic Observational Study.” Veterinary Sciences 10.9 (2023): 542.
• Inui T, Yamada K, Itoh M, Yanagawa M, Higuchi T, Watanabe A, Imamura Y, Urabe M, Sasaki N. Computed tomography and magnetic resonance imaging findings for the initial stage of equine temporohyoid osteoarthropathy in a Thoroughbred foal. Journal of equine science. 2017;28(3):117-21.