Catagorising Hearing Loss
Hearing loss can be catagorised as either
- sensorineural or conductive
- hereditary or acquired
- congenital or late onset
Sensorineural deafness results from the loss of sensory hair cells which may have been destroyed by viral infection, ototoxic drugs such as aminoglycocide antibiotics, exposure to prolonged loud noise or old age. In young dogs carrying the piebald or merle genes sensorineural deafness is a result of the death of the cochlear hair cells following the degeneration of the stria vascularis in the cochlear duct. This degeneration starts at about 3 to 4 weeks of age and is ususally complete by 6 weeks of age and is thought to be due to the absence of vascular melanocytes. As in humans, sensorineural hearing loss is permanent.
Conductive deafness occurs when the sound waves cannot reach the inner ear. This is due to pathologies of the outer or middle ear such as polyps, middle ear exudate, rupture of the tympanic membrane or foreign objects. It can often be corrected or reduced by drug therapy or surgery (bulla osteotomy).
The Brainstem Auditory Evoked response (BAER) Test
The BAER test is an objective and convenient method of assessing hearing in animals. It is non-invasive and can be performed on conscious pets. With patience and gentle handling most animals do not require sedation; but valid results are obtained on sedated or even anaesthetised animals. Each ear is assessed individually.
The BAER test requires advanced electrophysiologic equipment together with a click stimulus generator and a signal averager (an EMG machine with EP facilities). To perform the test earphones delivering the click stimulus are placed against the animal's ears. Tiny recording needles are inserted just under the skin on the head and the mastoid processes. These record the brainwaves as the potential moves along the acoustic nerve and a wave form is generated on the screen. The BAER is measured on the side ipsilateral to the stimulus and consists of a wave composed of 5 peaks occuring within the first 5 milliseconds after the stimulation. Absence of the peaks indicates deafness.
Indications for performing the BAER test include early litter screening in breeds where deafness is a problem, detection of acousrtic neuromas and posterior fossa tumours and intraoperative monitoring during brain surgery.