Page 1104 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1079

VetBooks.ir nerve (facial paralysis, exposure keratitis/corneal 10.29
ulceration, reduced tear production, facial hyperaes-
thesia). Signs are usually unilateral, although with
advanced imaging modalities such as CT, bilateral
disease is commonly recognised, with clinically silent
abnormalities in the TH apparatus contralateral to
the side with neurological signs. Cerebral sequelae
include seizures, coma and death, from incursion of
respiratory commensals to the CNS. Rarely, damage
to the glossopharangeal and vagus nerves may occur,
resulting in mastication problems and sometimes
dysphagia. Some non-neurological signs reported in
horses with THO include headshaking, ear rubbing
and resentment of bridling.

Differential diagnosis
EPM; WNV encephalitis; trauma; ear ticks; poly- Fig. 10.29 Endoscopy of the guttural pouch
neuritis equi. showing right-sided temporohyoid osteoarthropathy.
Note the osseous proliferation of the temporohyoid
Diagnosis joint (arrow).
In a clinical scenario, THO should be suspected in
cases showing any of the signs above, and outside
countries with EPM, THO may be a highly likely 10.30
differential diagnosis for vestibular or facial nerve
deficits.
A diagnosis can be made via guttural pouch
endoscopy (Fig. 10.29) or CT. Plain radiogra-
phy has been shown to generate too many false-
negative diagnoses to be useful in practice for
investigation of possible THO cases. Enlargement
of the stylohyoid bone, particularly the proximal
part, is uniformly recognised (Fig. 10.30). It can
be helpful to flush a little water into the patient’s
mouth so that the temporohyoid articulation can
be examined in motion, highlighting the anky-
losis and immobility often seen in THO cases.
Particularly in institutions where CT can be
performed as a standing procedure, negating the
risks of a general anaesthetic in horses with ves-
tibular ataxia and a strong likelihood of petrous
temporal bone fractures, it should be considered Fig. 10.30 Reconstructed transverse CT image. Left
in THO cases because it provides a comprehen- is to the right of the image. The right temporohyoid
sive assessment of the bony structures of the hyoid joint is enlarged as a result of callus formation. There
apparatus and the TH articulation. CT can also are less severe degenerative changes affecting the left
help to rule in or out middle ear disease and has temporohyoid joint. This 12-year-old mare presented
increased sensitivity for the detection of fractures with a 2-week history of facial paralysis including right
of the petrous temporal bone in comparison with ptosis, right ear-drop and mild vertical headshaking.