Page 1070 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1045

VetBooks.ir Mental state and neck to one side, but without a head tilt (their
poll and muzzle are still in the same vertical plane).
Mental state is defined as the level of consciousness
and awareness. A decrease in level of conscious-
ments of the head, neck and limbs. Disease of the
ness could manifest as depression, lethargy, lack The cerebellum modulates and smooths move-
of response to environmental stimuli, somnolence, cerebellum results in jerky movements or an inten-
stupor or semi-comatose or comatose states. These tion tremor that may be exacerbated when looking
are usually a result of damage to the cerebral cortex for food. At rest, cerebellar disease may present as a
and ascending reticular activating system (ARAS) bobbing of the head.
in the brainstem.
PHASE 3 – PHYSICAL EXAMINATION
Head posture and coordination
A head tilt is where the poll is laterally deviated rather A complete physical examination should be per-
than being in a vertical line, aligned with the muzzle formed. Often, at this point, clinical signs asso-
and base of the neck. It is described by the direction ciated with a nervous system abnormality are
of the poll deviation (Fig. 10.1). A head tilt usually detected such as patchy sweating, muscular asym-
indicates a unilateral central or peripheral vestibu- metry, postural abnormalities or muscle fascicula-
lar lesion. Where bilateral vestibular disease is pres- tion. It may also highlight disease of other body
ent, the horse will show wide swinging movements systems that may account for the neurological signs,
of the head, rather than a tilt. Sometimes horses such as infectious respiratory disease, colitis (sus-
with a cerebral lesion will hang their whole head pect intestinal hyperammonaemia or electrolyte
derangements), paraneoplastic dermatopathies or
melanomas. It may also be apparent that the horse
10.1 is in urgent need of other medical therapy such as
reversal of hypovolaemic or haemorrhagic shock,
oxygen supplementation or even tracheostomy
tube placement when bilateral laryngeal paralysis
is present (hepatic failure).

PHASE 4 – CRANIAL NERVE
(CN) EXAMINATION

The examination of the head begins with the
observation of head posture and movement and is
completed by a detailed assessment of each of the
twelve pairs of CNs. Abnormalities tend to suggest
disease near or within the brainstem, although the
lesion could theoretically be anywhere along the
nerve, its nucleus or the part of the body inner-
vated by it.

Olfactory nerve (CN I)
It is almost impossible to assess a horse’s sense of
smell. An inability to smell (anosmia) could be due
Fig. 10.1 Three-week-old foal with a left-sided head to pathology within the nasal passages or the nerve
tilt secondary to trauma. The foal was diagnosed with itself, which is rare. Crude tests include observ-
vestibular disease secondary to haemorrhage within ing whether a horse will stretch for food when
the petrous temporal bone. blindfolded.