Brain stem auditory evoked potentials and deafness in dogs
Dondi M., Bianchi E. Istituto di Clinica Medica Veterinaria

Italian please

The acoustic path
Hearing loss is a very frequent pathology among pets. However, very often, the potential forms of deafness are not recognized by the animals owners or mistaken for behavioral problems.  Moreover, the partial deafness is not so easily recognized by the veterinary for the lackness of an active cooperation by the patients. The conscious perception of a sound is a complex phenomenon which activates all the anatomic and neurologic structures constituting the auditory canals.
Hearing is defined as the trasduction of the sound, ondulatory mechanic energy in neural impulses, transmitted and interpreted by the nervous system.
The sound, before reaching the inner ear, passes through the outer ear (outer auricle and auditory canal) and makes the eardrum membrane vibrate; the membrane divides the external ear from the middle one.
The eardrum transfer these vibrations to the oval window, through the ossicular chain (hammer, anvil, stirrup); in this way the middle ear transforms and amplifies the air’s vibration in fluid of the cochlea of the inner ear.
The acoustic cell the hair cells of the organ of  Corti, through a piezoelectric mechanism, turn the mechanic energy of the sound ways in to electric potentials which spread like nervous impulses along the numerous fibers of the cochlea nerve towards the spiral ganglion.
The cochlea nerve forming together with the vestibular the 8th cranial nerve, crosses the inner acoustic meatus and ends in dorsal and ventral cochlear nucleuses set at the junction between the bridge and the medulla, were there is the sinapsi with the 2nd neuron (see table 1). .
The sound information trasmission, from the  receptors of the inner ear to the auditory cortex, occurs for propagation of potentials of action of membrane along the fibers and nervous nucleuses compounding the acoustic path. 
These dynamic electric potentials, influencing the surrounding tissues, which behave as conductory may be highlighted at distance on the outer surface of the body, maintainig their own original features.
By means of the right equipment and technics it is possible to register this electric activity and represent it as a wave function. 

Different forms of deafness

From the clinic point of view we can classify the hearing alterations of cats and dogs in two main categories:

• Peripheric deafness (the most common in pets are due to the damage of the peripheric portion of the acoustic path).
• Central deafness (provoked by damages at the back of the cochlea, very rare in the pure form).

Further subdivision of peripheric deafness: Hereditary or acquired, Connative or delayed arise
    Neurosensorial    or Conductive  When nerves and Receptors don't work ( The structures work correctly but create an obstacle and the impulse doesn't arrive at).

Theorically these three couples of characters should give origin to 8 different forms of peripheric deafness, but pratically in dogs and cats are frequent only three of these forms:

•    Hereditary connative neurosensorial deafness;
•    Lately acquired neurosensorial;
•    Lately acquired conductive.

The 1° one is often linked to the gene responsible of the white pigmentation of hear; the 2nd one occurs in presence of ototoxicity, oldness or inner otite; the 3rd is due to otites of middle and outer tract or to an excess of earwax.
In Dals we observe the hereditary deafness, connative not congenital, neurosensorial.

Congenital neurosensorial dealness

This form of deafness cannot be diagnosed until 3 - 4 weeks of age, both for anatomic reasons, the auditory canal opens at about 14 weeks of age, and for behavioral reasons, in this period puppies are at their mother's cares.
Several breeds of dogs are hit, mostly Dals, Bull Terriers, British Setters, Cocker Spaniels. The istopatologic description of the most part of cases is known as saccular cochleatic degeneration, it is permanent, non-reversible.
You cannot say it hears little, or does not hear. The only cases that deceived us have
been 5 week aged puppies with the birth of degeneration; it will be complete towards the 8 week. It still can hear little.
In older subject we can observe also ipotrophy of the cortical auditory areas and of the structures of the acoustic path. It is thought this condition due not much to a degenerative condition as to a non development because of the lackness of stimuli at
the brain cortex.
It may not be prevented because the pathology is not yet known.


Conductive deafness

This form of deafness hits all species of animals without a particular genetic proneness;     it may be the result of different pathologic conditions; among the most frequent are chronic otite, middle and outer, due to stenosi and occlusion of the outer auditory canal, or to the ammass of wax.
More rarely instead, animal development of ossicles (melting), absence of auditory canal, otosclerosi may be observed.
The transfer of sound waves is stopped at the outer middle auditory ducts.
This condition is reversible so you don't have to discard dogs, deaf for inflammatory pathologies.


Ototoxicity

Ototoxicus agents may cause hearing loss for effects direct on the hair cells both cochlear and vestibular, for prior damages at the vascular streaks with a secondary loss of the hair cells.
Antibiotics (gentemicina and streptomicin) may cause serious damages, when used frequently, to the cochlear and vestibular systems.
Clinically in some circumstances the hit dogs show behavioral alterations similar to those of man continually searching the source of a no-existent sound (tinnitus, perception of inexistent high-frequency sounds).

Presbycusis

It is a progressive loss of hearing, connected with various kinds of disorders of the aging, not to be confused with the former conditions.
The alterations are of meurosensorial kind, even though joined with a decreased fexiblity of the tympami and of a bones articulations.
It is progressive, but very often the patient becomes aware of it at the complete deafness, when the dog can no longer compensate the auditory function with the other senses.
The Symnptomatology is modest, firstly dogs shake their heads, as if they had a foreign body in the ear duct, later on they adjust to the sensation.

Deafness in Dalmata

The deafness which hits the Dals is neurosensorial, hereditary and acquired at birth. This type of deafness is connected to genius responsible for pigmentation of coat, so it is polygemetic at incomplete penetration.

• Genes are: P-Ball
                          P-Ball (utmost)
                          Merle (dominant)

The patology does not dipend upon the precence or not of genes but upon when they appear. The spots are holes.
Dals are black or liver with holes where white appears The utmost P-Ball codifies the white which is homozigote.
Merle is the gene responsable of the spotting, produces holes and not the black, already present beneath.The effects are highlighted according to the shape they have.
If the P-Ball utmost shows itself little, with little activation, there will be not enough covering of the beneath color; we will have a dark dog with colored patches.
If it shows much, it suppresses the pigmantation of the iris (blue,red eyes) or produces deafuess by means of degeneration of Stria Vascularis (vases irrorring the receptorial structure).
Inside these structures there are melanociti (inside these cells there are Promatina granules).
We are not able at the moment to explain the reasons. Sure is the lack of these produces the progressive degeneration.

Effects on progeny

Coupling subjects with patches produces very few deaf dogs:

7% monaural
3% binaural.

They are suggested to be used for reproduction.
Exams of behavorial type
1)    ultra sound whistle dishes, out of the view
Dogs adjust themself very soon, bring into play other resources, vibrations, olfaction.
2)    Elextrody Methods
They are objective, but there are few machines because of their high costs.

Electrodiagnostic methods

Acoustic stimulation.
Extremely precise headsets are used, however they can cause problems.
The patient is given a click, repeated with a certain frequency; changes, evoked by the stimuli in electronic activity of brain are registred; they are temporarly linked to them.
The result is transferred on a layout, made up by a series of 5-7 waves describing the variations of potential of eletronic field in function of time.
These waves are called short latency auditory potemtials, given that they are registered in the 10 ms following the stimulation.
The stimuli used is a sharp sound, produced by the sending of a 10 second rectangular wave to an audiometric headset.
This operation is done in 2 times, one ear a time, in order to avoid that the good ear can influence the ill ear, masking the alteration.
In spite of this the stimulating clicks reach the other ear, even though with a reduced intensity, trasmitted by the air and the bony tissue.
In order to avoid that this cross stimulation alte4the layout, it is advisable to mask the other ear with a back white noise, with an intensity 30-40 dB lower than the stimulus.
To have a significant layout it needs to effectuate 2 series of at least 500 stimulations at a 10 clicks per second speed.
The intensity of stimulation is measured in "decibel normal hearing level" (dB HL) .
The 0 dB HL represent in men the average hearing threshold for that stimulus people having no known history of ear desease.
Otherwise the auditory threshold of dog is defined as the minimum intensity of stimulation capable of evoking the V wave, you can see it on the BAEP layout.
The dog's middle hearing threshold is probably around 10 dB in non eneesthetised subjects while it can reach up to 50 dB HL in emaesthetised dogs.
Generally the test starts with a 90-95 dB HL intensity stimulation, lowering a time, as far as you reach the subject's hearing threshold.

Registration electrodes

The registration is activated through monopolar electrodes with needles under skin.
These electrodes are indicated as " reference and exploring" and are respectively put at the top of the cranium and at the caudoldorsal of the zigomatic arch ipsilateralis as to the stimulated ear, just before the euricular pinna. If you have other means of registration you can use other configuration.  As regards to the ground elechtrode with similar needle are used and are put subcutis. It may be placed either at cefalic level or extrecef and is practiced to reduce the artfacts due to the muscle activity.
Whenever the bodly electric noise is too much for the presence of unwill muscle contractions, a system of rejection of artefacts may be activated.  This test is good for patients treated with sedative, anesthetizid or awake, unless the
muscle contractions are reduced at minimum.  The position of the electrodies varies according to the points of view of the reaserchers.  With puppies are used " reference " monopolar : the positive on the head, the negative on the neck.
Other surgeons make bipolar bearings with more difficult interpretation; moreover it needs to shift the needle from one ear to another causing useless movements of puppies.
It gives no more information than the monopolar. Usually ungovernable dogs reveal deaf, do not learn from their mother, they are bitten by them without a prior acoustic signal, assuming anomalous behaviour.
"Doubt" puppies (with a single test they cannot said deaf) are tested again with sedative treatment (after some months it is used for reducing the muscle trempling).

What we gain with BA E B

With the help of these techniques and equipment above described it is possible to register the electric activity and represent it as a wave function on a cartesian plane where in ordinate are visable differences of electric potentials in MilliWalt, in abscissa the times in Milliseconds.
The tracced paths so obtined are compounded by a series of consecutive 5-7 waves, conventionally indicated in Roman numbers, they give information about anatomic structures that have generated those waves.
Valuable differences in shape can be seen in drawins obtained by different animals,   probably due to  the anatomic peculiarities of each species.
In dogs only wave I would be produced by the VIII cranial nerve. It has been hypotized that la and lb waves correspond respectively to the receptorial component of the 8th cranial nerve and to the presimaptic ending at the cochleer nucleus of the same nerve.
Wave lie, you can distinguish it even when the same configuration is used, represents, instead, the postsinaptic potential originated from the cochlear nucleus; the latency between lb and 2nd peaks has a duration close to that of the simaptic retard (0,5 ms).
The wave III seems to be produced in the caudel section of the bridge from the trapezoidal body. The IV and V waves would originate respectively from the middle and cranial section of the bridge at nucleus level and of the lateral lemnisco and inferior collicolo. The anatomic closeness of these two last structures is probably the cause of the frequent between the waves IV end V.
The VI and VII waves are not used in the clinic interpretation, it is however that they are produced respectively by the medial genicolatic body and by acoustic radiations.

Conclusion

Auditory evoked potentials technique is of extreme utility for the veterinary: it allows to get objective information of nerve function not easily investigable through the normal clinic procedures.
Even if it is a recently introduced technique which needs other perfecting in order to reach an operative completeness, has at its disposel a semiologic paradigma able to face the most frequent patological situations which hit the auditory system in pets. Moreover other studies are in progress on the possible applications in many deseases of extreme actuality in the biomedical research as the chronic degenerative patologies of the central nervous system (prionic disease) and neurological alterations present in the coma, in pain, in cerebral death.
The main limits of this exams are in the high costs of equipments and courses, training very qualified personnel who should use the equipment. For this reason medical services are available only in specialized centres of Clinical Audiology and
Neurophisiology function both of research and advise for veterinaries.
Breeders and clubs have to do much work in order to produce high quality dogs and above all state a strict regulation.

Translator:B. LAUDANTE

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