Hearing Defects and Hearing Tests

Introduction

Hearing depends on the efficient transmission of sound through the external, middle, and inner ear, followed by accurate neural processing. Understanding the types of hearing loss and the principles of hearing tests is essential for identifying the site and cause of auditory impairment.

Hearing Defects

Deafness

• Deafness is a reduction or complete loss of hearing and is classified into conductive, sensorineural, and mixed hearing loss.
• The type of hearing loss depends on the anatomical site and underlying pathological process.

Conductive Deafness

• Conductive hearing loss results from impaired transmission of sound through the external or middle ear.
• The cochlea and auditory nerve remain functional, so bone conduction is relatively preserved.
• Hearing loss is usually partial because sound can still reach the cochlea through vibrations of the skull.
• Common causes include impacted earwax, foreign bodies, congenital narrowing or absence of the external auditory canal, and tumors obstructing the canal.
• Recurrent infections may thicken or perforate the tympanic membrane, reducing its ability to vibrate.
• Acute and chronic otitis media interfere with normal sound transmission through the middle ear.
• Repeated inflammation may produce adhesions that restrict movement of the auditory ossicles.
• Otosclerosis, characterized by abnormal fixation of the stapes at the oval window, is a common cause of progressive conductive hearing loss in adults.
• Trauma may disrupt the ossicular chain and impair sound conduction.
• Eustachian tube dysfunction creates negative middle ear pressure, causing retraction of the tympanic membrane and reduced hearing.

Sensorineural Deafness

• Sensorineural hearing loss results from disorders affecting the cochlea, auditory nerve, or central auditory pathways.
• Damage to cochlear nuclei or higher auditory centers produces central hearing loss, although this is relatively uncommon.
• Ototoxic medications, particularly aminoglycoside antibiotics, may permanently damage cochlear hair cells.
• Other drugs, including high-dose salicylates, quinine, certain chemotherapeutic agents, and loop diuretics, may also impair hearing.
• Degeneration of cochlear hair cells with aging leads to presbycusis, a common cause of progressive bilateral hearing loss in older adults.
• Chronic exposure to loud noise damages outer hair cells, initially reducing sensitivity to high-frequency sounds.
• Tumors involving the vestibulocochlear nerve, such as vestibular schwannoma, may cause unilateral hearing loss, tinnitus, and balance disturbances.
• Skull base fractures and head trauma can injure the cochlea or auditory nerve.
• Labyrinthitis may produce hearing impairment accompanied by vertigo.
• Ménière disease typically presents with episodic vertigo, fluctuating hearing loss, tinnitus, and a sensation of ear fullness.

Mixed Deafness

• Mixed hearing loss combines conductive and sensorineural components.
• It may result from trauma, chronic infection, tumors, or other disorders affecting both the middle and inner ear.

Genetic Causes

• Hereditary hearing loss may be nonsyndromic, where hearing loss occurs alone, or syndromic, where it is associated with abnormalities in other organs.
• Mutations affecting proteins involved in potassium recycling, stereociliary structure, motor proteins, or the tectorial membrane can impair cochlear function.
• Certain inherited disorders combine deafness with systemic abnormalities, including thyroid disease, cardiac conduction defects, or renal disorders.

Tinnitus

• Tinnitus is the perception of sound without an external acoustic stimulus.
• Patients commonly describe ringing, buzzing, roaring, or hissing sounds.
• Tinnitus may occur with either conductive or sensorineural hearing loss and can significantly affect quality of life.

Treatment of Deafness

• Management depends on identifying and correcting the underlying cause whenever possible.
• Removal of impacted wax, extraction of foreign bodies, treatment of middle ear infections, and surgical repair of tympanic membrane or ossicular defects can restore hearing in many cases of conductive loss.
• Hearing aids amplify incoming sounds and improve communication in individuals with mild to severe hearing impairment when residual hearing is present.
• Digital hearing aids can be programmed according to the patient’s audiometric profile for optimal sound amplification.
• Cochlear implants are indicated for selected individuals with severe to profound sensorineural hearing loss who obtain limited benefit from conventional hearing aids.
• These devices convert sound into electrical impulses that directly stimulate the cochlear nerve, bypassing damaged hair cells.
• Early implantation, particularly in children with congenital deafness, supports better speech and language development through timely auditory rehabilitation.
• Comprehensive management also includes hearing rehabilitation, speech therapy when required, and regular audiological follow-up to optimize long-term outcomes.

Tests for Hearing

• Evaluation of hearing begins with a detailed clinical examination to identify the site and cause of hearing impairment.
• The external ear is inspected for impacted wax, foreign bodies, inflammation, or structural abnormalities.
• The tympanic membrane is examined for perforation, scarring, retraction, fluid collection, or middle ear infection.
• Reversible conditions, such as impacted cerumen or acute otitis media, should be treated before formal hearing assessment.
• Hearing may be screened initially by assessing the ability to hear a conversational voice with each ear tested separately at a distance of approximately 3–4 meters.
• Bedside assessment is followed by tuning fork tests, usually performed with 256-hertz or 512-hertz tuning forks.

Tuning Fork Tests

Rinne’s Test

• Rinne test compares air conduction with bone conduction in the same ear.
• Bone conduction is tested first by placing the vibrating tuning fork on the mastoid process.
• When the patient no longer hears the sound, the vibrating fork is immediately held close to the external auditory canal to test air conduction.
• In individuals with normal hearing, air conduction exceeds bone conduction because sound transmission through the external and middle ear is more efficient.
• In conductive hearing loss, bone conduction becomes better than air conduction because transmission through the external or middle ear is impaired.
• In sensorineural hearing loss, both air and bone conduction are reduced, but air conduction remains greater than bone conduction if hearing loss is incomplete.
• In profound sensorineural deafness, neither air nor bone conduction may be perceived.

Weber’s Test

• Weber test compares bone-conducted sound perception between the two ears.
• The base of a vibrating tuning fork is placed on the midline of the forehead or the vertex of the skull.
• A person with normal hearing perceives the sound equally in both ears.
• In conductive hearing loss, the sound is heard more loudly in the affected ear because environmental sounds are reduced, decreasing masking.
• In sensorineural hearing loss, the sound is perceived more strongly in the normal ear because neural transmission is reduced in the affected ear.

Schwabach’s Test

• Schwabach test compares the patient’s bone conduction with that of an examiner who has normal hearing.
• Prolonged bone conduction suggests conductive hearing loss.
• Shortened bone conduction indicates sensorineural hearing loss.
• This test is now used less frequently because modern audiological methods provide greater accuracy.

Audiometry

• Audiometry provides quantitative assessment of hearing using specialized electronic equipment.
• Hearing tests are broadly classified as subjective and objective methods.
• Pure tone audiometry:
• Pure tone audiometry is the most widely used subjective hearing test.
• The instrument used is an audiometer, and the graphical record obtained is called an audiogram.
• Pure tones of different frequencies are presented separately to each ear.
• Air conduction is assessed using earphones, whereas bone conduction is evaluated with a bone vibrator placed over the mastoid process.
• Testing is performed in a soundproof room to eliminate interference from environmental noise.
• Each ear is examined individually, usually beginning with the better-hearing ear.
• Frequencies between 250 and 8,000 hertz are commonly tested because they encompass the speech frequency range and clinically important hearing thresholds.
• The lowest intensity detected at each frequency is recorded in decibels, generating an audiogram.
• Comparison of air and bone conduction thresholds helps distinguish conductive, sensorineural, and mixed hearing loss.
• When one ear has significantly better hearing, masking noise is delivered to the non-test ear to prevent it from detecting the test stimulus.
• During bone conduction testing, narrow-band masking centered around the test frequency is commonly used because skull vibrations easily reach both cochleae.

Brainstem Auditory Evoked Responses (BAER)

• Brainstem auditory evoked responses record electrical activity generated along the auditory pathway after brief sound stimulation.
• The test evaluates neural conduction from the cochlea to the midbrain.
• Five major waveforms, labeled I to V, are routinely analyzed.
• Wave I arises from the distal auditory nerve near the cochlea.
• Wave II originates mainly from the cochlear nucleus.
• Wave III reflects activity within the superior olivary complex.
• Wave IV is generated predominantly by the lateral lemniscus.
• Wave V originates primarily from the inferior colliculus and is the most clinically reliable waveform.
• Delayed latency, reduced amplitude, or absence of specific waves helps localize lesions within the auditory pathway.
• Brainstem auditory evoked responses are valuable for assessing hearing in newborns, infants, and uncooperative patients.
• They also assist in diagnosing brainstem lesions, vestibular schwannoma, demyelinating disorders, stroke, coma, and confirmation of brain death.

Otoacoustic Emissions

• Otoacoustic emissions are low-intensity sounds generated by healthy outer hair cells within the cochlea.
• Their presence indicates normal outer hair cell function, whereas their absence suggests cochlear dysfunction.
• This test is widely used in newborn hearing screening and helps distinguish cochlear hearing loss from disorders affecting the auditory nerve or central auditory pathways.