Ear, Nose, and Throat – Hearing Loss and Deafness: By Carol Bauer M.D.

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Ear, Nose, and Throat – Hearing Loss and Deafness
Whiteboard Animation Transcript
with Carol Bauer, MD
https://medskl.com/Module/Index/heari...


Let’s begin with a brief overview of hearing anatomy and physiology. Sound travels through the external auditory canal to reach the tympanic membrane, transmitting vibration to the middle ear ossicles (malleus, incus and stapes). The ossicular chain amplifies the sound energy that is transmitted to the cochlea through the oval window.

The cochlea is a spiral shaped structure that contains the fluid compartments of endolymph and perilymph, the basilar membrane and the neuronal elements that transduce sound energy into neuronal impulses.

The sound energy travels through the fluid of the inner ear and moves the basilar membrane in a wave that travels from the base where high frequency sounds are encoded, to the apex where low frequency sounds are encoded. The movement of the basilar membrane is amplified by the outer hair cells within the organ of Corti.

The amplified basilar membrane motion then deflects the stereocilia of the inner ear hair cells. This triggers release of glutamate at the primary afferent synapse between the inner hair cell and the neurons of the cochlear nerve. The signal is then transmitted to the cochlear nucleus within the brainstem via the cochlear division of the eighth nerve.

We can classify hearing loss into Conductive and Sensorineural hearing loss.

Conductive hearing loss occurs when the conduction of sound to the cochlea is interrupted by a mechanical disturbance between the external auditory canal and the inner ear.

Common examples of conductive hearing loss include:

Build up of cerumen
Otitis externa
Acute or chronic otitis media
Perforation of the tympanic membrane
Ossicular chain disruption from trauma, infection, Eustachian tube dysfunction or cholesteatoma
Ossicular chain fixation from otosclerosis or tympanosclerosis that occurs with chronic infection of the ear

Sensorineural hearing loss occurs when there is a failure to convert sound into neural signals, or a defect in the transmitting these neural signals to the cerebral cortex.

Common causes of sensorineural hearing loss include:

Noise-induced hearing loss
Presbycusis (age-associated hearing loss)

Less common examples of sensorineural hearing loss that are very important to recognize include:

Sudden sensorineural hearing loss
Acoustic neuroma (more accurately, vestibular schwannoma)
Drug exposure (loop diuretics, aminoglycosides, chemotherapy drugs)
Meniere’s disease

A detailed history and physical examination are instrumental to determine the correct diagnosis. Always remove excess cerumen to clearly visualize the tympanic membrane during otoscopy.

Tuning fork tests (Weber and Rinne) can be helpful in distinguishing conductive from sensorineural hearing loss. Hearing should be tested by an audiologist with assessment of pure tone thresholds with air and bone conduction, speech detection threshold and speech discrimination. Further investigation with CT scanning and MRI may be necessary to exclude or confirm pathology from the eighth nerve to the auditory cortex.