The prominent and very mobile external part that we recognise as a horse’s ear comprises a funnel of skin and cartilage called the auricle, or pinna.
Within this is a tube, the acoustic meatus, which leads into the head. The external ear stops at the tympanic membrane (eardrum), a thin sheet of tissue stretched across the opening of the skull, the other side of which is a chamber called the tympanic cavity (see diagram, right).
Sound waves are transmitted across the tympanic cavity by a chain of very small bones known as the auditory ossicles. Befitting for the horse, these are called the malleus, incus and stapes, which translate from Latin as the hammer, anvil and stirrup (so-called because of their shape).
The handle of the “hammer” is embedded within the tympanic membrane, while its protruding head is connected to the “anvil”, which in turn is connected to the “stirrup”. As a result of this arrangement, any movement of the tympanic membrane caused by sound waves is converted and magnified through the chain of ossicles.
Movement of the base of the stirrup bone causes fluid within the inner ear to vibrate, which is detected by receptor cells in the inner ear and conveyed to the brain — providing the perception of sound.
Sound system
Following on from this middle-ear area is the inner ear, the function of which is to convert mechanical stimuli produced by sound or movement into signals to the brain. These inform the horse of his body position and the noises around him.
The conversion of mechanical stimuli to electrical impulses through nerves, and hence signals in the brain, takes place within a very complicated arrangement of fluid-filled ducts and cavities known as the membranous labyrinth. At the centre of this are two large chambers: the saccule and utricule.
The saccule gives rise to the long, spiral cochlear duct, which is housed within a similar spiral of bone and is responsible for hearing. A layer of fluid between the duct and the surrounding bone is compressed by movements of the stirrup bone in response to sound. The complex structure of the cochlear duct results in sounds of different frequency generating different signals.
Balance is controlled by three semicircular ducts originating in the utricule. Movement of the horse’s head results in movement of the fluid within these semicircular canals, causing disturbance of small hairs which protrude from the cells and sending signals to the brain.
In some areas of the utricule and saccule there are sensory cells and hairs covered with a layer of gel and crystals, the position of which is determined by gravity. As the horse’s head position changes, the crystals (and therefore the hairs) move, resulting in nerve impulses to the brain.
Impulses from different sensory cells are passed to the brain in the vestibulocochlear nerve. The areas of the inner ear concerned with head position and balance, along with the vestibulocochlear nerve, make up a major component of the vestibular system, which, when it fails, results in vestibular disease.
Balancing act
A horse suffering from vestibular disease will lose awareness of his head position, resulting in loss of balance.
He may fall to one side or walk in circles, or feel so disoriented that he is reluctant to move and leans on a wall or walks in a short, uncoordinated manner. The most obvious sign of the disease is tilting of the head, which often develops very suddenly. The vestibular apparatus also controls eye position, so the eyeball may be abnormally positioned within its socket and may flick rapidly and repeatedly side to side, up and down or in a circular motion.
Middle- and inner-ear disease can also affect the nerve which controls the side of the face. The ear, eyelid and muzzle may all droop on the affected side, and the eye can become damaged because tear production is reduced and the eyelids do not function normally.
Most cases of middle- and inner-ear disease are caused by fracture of the bone around them, either through severe trauma, usually when a horse goes over backwards and lands on his head, or following arthritic change and then fracture of the abnormal and inflexible bone. Bony change most often occurs around the temporohyoid joint, which sits just below the inner and middle ear. This increases with ageing and is probably associated with chronic wear.
Disease can also be caused by infection, but this is very uncommon.
Diagnosis can generally be made from clinical signs. However, identifying whether disease of the middle and inner ear is responsible, rather than disease within the brain, can be challenging.
Until recently, we would use an endoscope to look for enlargement of the temporohyoid joint within the guttural pouch, along with X-rays to identify enlargement of the middle and inner ear or fractures of the skull. Computed tomography (CT) has made diagnosis more reliable, allowing the affected area to be viewed three-dimensionally.
Fractures of the bone around the middle and inner ear cannot be repaired. Treatment is designed to reduce inflammation and minimise damage to the facial and vestibulocochlear nerves, usually using anti-inflammatories such as phenylbutazone (bute) and/or steroids.
Antibiotics may be used if there is a possibility of infection or if fracture leads near to the brain. Nursing care is very important in horses that are wobbly, have secondary eye disease or have difficulty eating.
If disease is secondary to degeneration of the temporohyoid joint, the hyoid bones can be surgically cut or one of the smaller ones removed. Reducing movement helps to relieve pressure and limit inflammation, which often helps to reduce clinical signs.
Most cases of vestibular disease improve with treatment and time, but few return completely to normal.
Ref Horse & Hound; 23 March 2017
