Deafness and BAER
Deaf animal is disabled and this disadvantage can range from trivial to extreme. The dog or cat with unilateral deafness experiences difficulties in localizing the source of sound but usually learns quickly to compensate. Coordinated movements of both pinnae still occur in animals with unilateral deafness when alerting to auditory stimuli. Bilaterally deaf animal is unable hear or locate the source of sound and to anticipate dangers such as motor vehicles or predators and is at increased risk.
The ear canals of dogs and cats open within the second week of life; however, the complete maturation of the auditory system does not occur until the 8 weeks of age. The entire auditory pathway, including the cerebral portion, is probably not completely matured until 12 weeks of age. The clinical assessment of a suspected deaf animal can be difficult. Animal can react to vibratory clues (shutting a door), or visual clues (clapping hands). Therefore, when examiner is producing noise, he should stay out of animal’s visual field. Complete hearing loss can be a challenge to assess clinically and partial or unilateral hearing loss is even more difficult to detect.
Three forms of deafness are known in small animals: inherited, acquired and conductive
1.Inherited deafness. Inherited early-onset sensorineural deafness is usually, but not always, associated with pigmentation genes responsible for white in the coat. In most dog and cat breeds inherited sensorineural deafness results from perinatal degeneration of the stria vascularis, the vascular bed of the outer wall of the cochlear duct, which leads to hair cell degeneration. The strial degeneration appears to result from the absence of melanocytes, but their function in this structure is unknown.
2.Acquired deafness. Acquired later-onset sensorineural deafness is most often associated with ototoxicity or ageing-related hearing loss, but can also result from otitis interna, noise, trauma and other causes. Ototoxicity may result from any of a large number of drugs and chemicals that directly or indirectly destroy cochlear hair cells (indirectly via destruction of the stria vascularis). The effects are dose dependent and in rare cases reversible. The most commonly recognized ototoxic drugs are the aminoglycoside antibiotics. Presbycusis is the decline in hearing associated with various types of auditory system dysfunction that accompany ageing. It can be sensory, neural, strial and cochlear conductive. The pathological change in most dogs and cats appears to be sensorineural, although decreased tympanum and ossicle joint articulation flexibility can potentially contribute. It is common in geriatric dogs. Although it is a progressive disorder, owner usually report an acute onset because of the ability of the animal to compensate for hearing loss until nearly complete deafness occurs.
Hearing aids have been successfully utilized in some dogs with residual auditory function, but not all dogs will tolerate the presence of the plug. There is no known way of retarding the progression of deafness. Noise-induced hearing loss can be temporary or permanent. Temporary increase in hearing threshold occurs after brief exposure to intense sounds (over 100 dB), with gradual recovery of function occurring over periods ranging from minutes to two weeks. Noise-induced hearing loss is thought to result from either disarrangement or breakage of hair cell cilia, but can also result from damage to the tympanum and ossicles. Continous or repeated exposure to noise results in a progressive loss of hair cells and a corresponding deafness.
3.Conductive deafness. Conductive deafness is due to a problem with the actual conduction or transmission of the sound waves from the external environment to the cochlea. Blockage of sound wave transmission may be due to occlusion of the external ear canal by debris, tissue, or foreign bodies. These blockages may be congenital, as in the case of birth defects, or acquired. Conduction blockage in the middle ear may be due to rupture of the tympanic membrane, fluid or exudate, excessive tissue growth, foreign body, malformation or damage to the ossicles, and stiffening of the ossicles with the age.
Hearing test or BAER
Hearing test is the most important examination in the diagnosis of deafness in dogs and cats.
Indirect evaluation of the auditory system is possible utilizing brain stem auditory evoked response (BAER). BAER plays the central role in the deafness evaluation and is performed in deep sedation or anaesthesia. To perform this test, dicks or tones are generated in the ear of the animal and the response of the brain stem and auditory pathways is recorded via needle electrodes placed on the scalp and near the ear.
The test generates an early latency waveform within 1-10ms, indicating various activities within the brain. If the sound waves get to the organ of Corti, the evoked response is visible as a series of waves that represent brain stem activity. Wave I represent vestibulocochlear nerve. The other waves represent various nuclei and pathways for auditory function in the brain. The generator sites of the remaining waves are not definitely known, and probably represent superimposition of the action potentials from multiple brain stem structures. Wave II is thought to arise from the cochlear nuclei in the medulla. Wave III is suspected to be generated by the rostral olivary nuclei and the dorsal nuclei of the trapezoid body, both located in the medulla. Wave IV likely represents action potentials from the lateral lemniscus and lemniscus nuclei of the pons. The caudal colliculi of the midbrain and medial geniculate nuclei of the diencephalon contribute to the generation of wave V. The latencies (time necessary for the generation of the wave following a stimulus), amplitude, and morphology of the waves are evaluated.
The appearance of 4/5 recognizable waves on a BAER recording confirms hearing ability on that side. A flat line is evidence of deafness. Animal with congenital sensorineural deafness typically has flat BAER at both 80 and 100 dB either uni- or bilaterally. For evaluation of brain stem integrity, inter wave intervals can be used (latencies between I and III, III and V, I and V, reference ranges, should not differ from right to left by more than 0,1ms). Brain stem lesions will cause a conduction delay, which will be measured as a prolonged latency that corresponds to the anatomic location of the lesion. Although wave amplitudes are not often calculated for diagnostic purposes, a V/I amplitude of less than 0,5 is indicative of a brain stem lesion. A patient that has experienced brain death will typically have a flat BAER, or only wave I.
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