Hearing disorders in cats

Abstract

Practical relevance: Auditory function is a sense that is central to life for cats - being important in situational awareness of potential predators, pursuit of prey, and for communication with conspecifics, humans and other species. Deafness in cats is most frequently the result of a genetic disorder, strongly associated with white fur and blue eyes, but may also result from acquired causes such as advancing age, ototoxic drugs, infection, environmental noise and physical trauma. Deafness can be sensorineural, where there is loss of cochlear hair cells, or conductive, where sound is muffled on its way to the inner ear. Clinical challenges: Establishing whether a cat is deaf can be difficult as behavioral testing of hearing is subjective and does not reliably detect unilateral deafness. Brainstem auditory evoked response testing is an objective measure but is limited in its availability. Currently, sensorineural deafness is irreversible because no treatments are available to restore lost hair cells. Conductive hearing loss can usually be treated, although full hearing recovery following otitis media may take weeks as the body clears the middle ear of debris. Evidence base: The author draws on the published literature and his extensive research on clinical aspects and molecular genetics of deafness, principally in companion animals, to review types and forms of deafness in cats. He also discusses current diagnostic approaches and provides brief advice for managing cats with hearing loss.

Figure 1

Structures of the feline ear. 1 = pinna; 2 = external ear canal, consisting of a vertical canal and a horizontal canal; 3 = tympanic membrane; 4 = bulla/middle ear space; 5 = auditory tube; 6 = middle ear ossicles (malleus, incus and stapes); 7 = cochlea; 8 = vestibular structures; 9 = cranial nerve VIII. Reproduced from Ryugo and Menotti-Raymond, with permission

Figure 2

Feline audiograms compiled from published sources.^– Hearing range is usually based on the frequency range for responses below 60 dB SPL, and is typically cited as 45 Hz to 65 kHz

Figure 3

Structures of the cochlea in cross-section. Inner hair cells in the organ of Corti are the primary sensory transduction cells, while the outer hair cells utilize active processes to increase or decrease sensitivity. Pigment cells in the stria vascularis play a major role in maintaining high potassium levels in the cochlear duct to support hair cell viability. Hair cells synapse on neurons of the spiral ganglion, which in turn become components of cranial nerve VIII. Reproduced from Bloom and Fawcett,8 with permission

Figure 4

Scanning electron micrographs of cochlear hair cells. (a) Hair cells and their stereocilia. (b) Higher magnification view of the stereocilia on an outer hair cell. The stereocilia are approximately 5 μm in length. Reproduced from Bear et al, with permission

Figure 5

Deafness in blue-eyed white cats is perhaps the most recognized form of deafness in animals. ©iStock.com/Pley

Figure 6

Cat gently restrained, with electrodes in place, and BAER testing of the right ear being performed

Figure 7

(top) BAER recording from a 2-year-old normal cat, with peaks I, II, III and V labeled on the tracing. Peak I is generated by cranial nerve VIII entering the brain, while the later peaks are generated in the brainstem. (bottom) By comparison, this essentially flat line is a recording from a bilaterally deaf cat. 0.5 μv/div and 1 ms/div