There are several important nerves which run through
and immediately adjacent to the middle and inner
ears. These structures are affected with progressive
otitis and can also be damaged during surgery.
The middle ear lies beyond the tympanic membrane
and consists of the mucosa lined bulla which contains
the three auditory ossicles which transmit sound
from the external ear to the inner ear. The auditory
(eustachian) tube connects the nasopharynx to the
The facial nerve and the sympathetic supply to the
eye are closely associated with the cavity of the middle
ear. Deficits may include facial paralysis, horners
syndrome or pain on opening the mouth.
The bony cochlea together with the vestibule and
semicircular canals is situated within the petroustemporal
bone and comprises the inner ear. Inflammation
of this area will result in peripheral vestibular
disease. The facial nerve runs, along with and just
above, the vestibuloc ochlear nerve (CNVIII) through
the petrosal bone and emerges from the skull through
the stylomastoid foramen.
Chronic ear infection can result in pyogranulomatous
otitis media – interna with the development of
osteomyelitis of the tympanic bulla. This will generally
result in peripheral or central vestibular disease – often
without any of the other intracranial neurological
Figure 1. Graphic depiction of the first (spinal) second (brachial plexus to cranial cevical ganglion) and third order (cervical ganglion to eye) of the sympathetic supply to the eye. The smaller graphic illustrates the close proximity the nerves have to the middle ear.
Source: BSAVA Manual Small Animal Neurology
deficits expected with space occupying lesion:head/
neck pain, lethargy, seizures.
The sympathetic supply to the eye originates in the
hypothalamus descends through the brain stem
and spinal cord to T1-T3 where it synapses and exits
through the brachial plexus nerve roots and travels
rostrally again within the vagosympathetic trunk, synapsing
on the cranial cervical ganglion. The axons
finally travel through the middle ear along the floor of
the skull and exit via the orbital fissure to innervate the
smooth muscles of the eye.
Miosis: Ipsilateral. Best seen when lights are dimmed
and failure to dilate is noted. With aniscoria it is always
important to decide which pupil is actually affected
– the small pupil or the dilated pupil – and this interpretation
will depend on levels of light and stress.
Ptosis: Ipsilateral loss of tone to the eyelid causes
them to droop – narrowing the palpebral opening.
Enopthalmosis: Ipsilateral loss of sympathetic tone
to the smooth muscles of the orbit resulting in the
retractor bulbi muscles pulling the eyes, to sink further
into the orbit. This will also cause the third eyelid to
Vasodilation of ipsilateral aural, facial or conjunctival
blood vessels may be noted.
Figure 2. Anatomy specimen of a dogs head, showing dissection of the facial nerve. Note the exit from the skull where the horizontal ear canal inserts into the bulla.
|Paresis||Never||Possible – brainstem disease may affect ascending proprioceptive and and descending motor tracts|
|Proprioceptive deficits||Never||Possible – brainstem disease may affect ascending proprioceptive and and descending motor tracts Possible|
|Consciousness||Alert. May be confused, distressed , disoriented||May be depressed, stuporous or comatose|
|Cranial nerve deficits||Facial nerve deficits – runs close to middle ear||Facial nerve deficits – exits brain stem close to vestibulocochlear nerve (CN VIII)
Additionally Cranial nerves V and VI all the way down to XII may be affected (brain stem lesion)
|Nystagmus||Horizontal or rotatory – NEVER VERTICAL
Fast phase away from side of lesion
Direction doesn’t change with head position
|Horizontal, rotatory or vertical
Fast phase in any direction
Direction may change with head position