Hearing Loss

Question 1

What are the main structures of the inner ear?

Answer 1

• semicircular canals → for balance
• cochlea → for hearing
• cochlea present with hair cells helping to generate electrical signals for hearing, sent via auditory nerve

Question 2

How are sound waves transduced to nerve impulses?

Answer 2

• tympanic membrane converts waves of sound in air to pressure waves in the fluid of the inner ear
• displacement of the basilar membrane causes movement of hair cells
• generates action potentials → different bits of cochlea respond to different frequencies
• vestibulocochlear nerve sends impulses to the cortex where sound is perceived

Outer hair cells amplify sounds whereas inner hair cells transmit sounds

Question 3

What is the neurological pathway between the cochlea and the auditory cortex?

Answer 3

• the primary auditory pathway
• primary auditory cortex is located in the temporal area within the lateral sulcus

auditory nerve → cochlear nuclei (brainstem) → superior olivary complex (brainstem) → superior colliculus (mesencephalus) → medial geniculate body (thalamus) → auditory cortex

Question 4

What is the difference between sensorineural hearing loss, conductive hearing loss, and mixed hearing loss?

Answer 4

• sensorineural → stems from either cochlea (inner ear), the nerve that runs from cochlea to brain or a combination of both, sometimes termed ‘permanent’ hearing loss
• conductive → stems from fluid, tissue or bony growth that blocks or reduces incoming sound - can involve ear canal, middle ear, ear drum or bones in middle ear, often referred to as ‘temporary’ or ‘transient’
• mixed → both types present in same ear

Question 5

Rinner’s and Weber’s are bedside tests to determine if it is a conductive or sensorineural deafness (if you don’t have access to audiometry). Must use a 512Hz tuning fork.

What is Rinne’s test?

Answer 5

• hold tuning fork adjacent to patient’s external acoustic meatus and then against mastoid process - “is it louder in front of behind?”
• if louder in front ie. AC > BC:
  
  • Rinne’s +ve = normal or mild/mod SNHL on that side
• if louder behind ie. AC < BC:
  
  • Rinne’s -ve = conductive hearing loss on that side OR severe/profound SNHL on that side

Question 6

What is Weber’s test?

Answer 6

• hold tuning fork against patient’s forehead in midline
• ask patient where they hear the sound
• ‘do you hear loudest in: left, right or both?’
• if it doesn’t lateralise → normal (or symmetrical mild/mod hearing loss)
• lateralises →
  
  • loudest in affected ear = conductive hearing loss
  • loudest in contralateral ear = SNHL

Question 7

We have mentioned that pure tone audiometry is a subjective hearing test.

What are examples of objective hearing tests?

Answer 7

• otoacoustic emissions (OAE) → test of outer hair cell cochlear function, forms part of newborn hearing screen
• auditory brainstem responses (ABR) → test of auditory nerve function + forms part of newborn hearing screen testing

Question 8

What are examples of hearing tactics?

Answer 8

• get their attention before speaking
• face them
• get to the point
• don’t cover your mouth
• don’t turn away while talking
• don’t shout - distorts mouth + lip patterns
• don’t speak too quickly
• cut down ambient noise
• reduce distance to 1.5m

Question 9

What are the different types of hearing aids?

Answer 9

• Electronic aids → pt needs to have some residual hearing, can sit in ear canal
• Bone achored hearing aids → good for conductive deafness or where out-the-ear-aids are unsuitable (eg. eczema), utilises bone conduction
• Cochlear implants → where auditory nerve is functional, and pt has already developed speech + language skills (or infants), usually only for profoundly SNHL

Question 10

What is the difference between conductive and sensorineural hearing loss?

Answer 10

• Conductive → the conduction of the sound to cochlea is impaired, can be caused by external and middle ear disease
• Sensorineural → due to a defect in the conversion of sound into neural signals or in the transmission of those signals to the cortex, can be caused by disease of the cochlea, acoustic nerve (CN VIII), brainstem or cortex

Question 11

What are the causes of conductive hearing loss?

Answer 11

• wax (cerumen)
• otitis externa
• foreign body
• acute/chronic otitis media
• trauma → tympanic membrane or ossicles
• otosclerosis
• middle ear tumours
• cholesteatoma

Question 12

What are the causes of sensorineural hearing loss?

Answer 12

• presbycusis
• noise-induced traumatic loss
• acoustic neuroma
• Meniere’s
• trauma
• infection → meningitis, measles, mumps
• intra-uterine infection → TORCH
• birth injury
• genetic → Alport’s, Jervell-Lange-Nielson
• autoimmune
• ototoxic drug exposure

Question 13

What is presbycusis?

Answer 13

• common cause of hearing loss
• affecting more than half of all adults by age 75
• multiple factors influence onset: genetic predisposition, low socioeconomic status, noise exposure, smoking, hypertension, diabetes, vascular disease
• hallmark is progressive, symmetric loss of high-frequency hearing loss over many years
• Rx → hearing aids, hearing tactics

Question 14

Ototoxicity → what drugs cause permanent and temporary hearing loss?

Answer 14

• antineoplastics (cisplatin, carboplatin) → permanent HL
• loop diaretics → rapid reversible oedema in stria vascularis
• salicylates → temp threshold shifts during dosing across most frequencies + tinnitus
• aminoglycosides → permanent progressive loss of hair cells, affects high freq first, progressing to lower freqs, affects vestibular organs, may become dizzy/imbalanced
• antimalarials → temporary HL + tinnitus
• macrolides (-mycin) → temporary HL

Question 15

What are the features of a vestibular schwannoma (acoustic neuroma)?

Answer 15

• benign, usually slow growing tumour from an overproduction of Schwann cells
• annual incidence ~1 per 100,000
• presentation → asymmetrical hearing loss, tinnitus, dizziness, CN VII symptoms large
• Dx → MRI of internal acoustic meatus
• Rx → dependent on size + symptoms - monitoring, radiotherapy + surgery

Question 16

What are the features of Meniere’s Disease?

Answer 16

• recurrent vertigo lasting 20mins to several hours
• w/ unilateral tinnitus, aural fullness + fluctuating hearing loss
• rare → prevalence ~20-200/100,000
• likely 2^o to endolymphatic hydrops
• in early stages → vestibular/cochlear symptoms may occur in isolation, hearing loss (low freq) + tinnitus recede fully
• after 1-2 years complete set of symptoms usually established
• later stages → progressive deterioration of hearing superimposed on fluctuations
• Rx → symptomatic relief, numerous treatments for prophylaxis (betahistine, low-salt diet, bendroflumethiazide)

Question 17

What is sudden hearing loss?

Answer 17

• defined as 30dB hearing loss over 3 consecutive frequencies, in less than 3 days
• annual incidence 5-20/100,000
• 95% unilateral (~90% idiopathic) → 30-65% spontaneous recovery
• 5% bilateral (most identified aetiologies) → ~20% complete/partial recovery

Question 18

Sudden conductive causes of hearing loss tend to include trauma.

What are the causes of sudden sensorineural hearing loss?

Answer 18

• TOXIC → alcohol, cocaine, heroin, ecstasy, opioid, benzo, antivirals, retinoid, chemo, NSAIDs, immunosuppressives, bisphosphonates, insecticides, gentamicin
• AUTOIMMUNE → AI inner ear disease, cogan’s, kawasaki, guillain-barre, scleroderma, anti-phospholipid syndrome, crohn’s, polychondritis
• INFECTIOUS → mumps, HIV, HSV, cryptococcal meningitis, bacterial meningitis, viral URTI
• IATROGENIC → micro-emboli surg complications, GA haemodynamic complication, GA ototoxicity
• VASCULAR → CV accident, migraine-associated vasospasm, sickle cell
• NEOPLASTIC → CPA or petrous meningioma, neuolymphatosis, leptomeningeal carcinoma, vestibular schwannoma, acoustic neurofibroma, meningeal carcinoma

Question 19

Bilateral sudden sensorineural hearing loss is a medical emergency.

What are the immediate investigations + management?

Answer 19

• investigations →
  
  • audiometric testing
  • MRI internal auditory meatus / CT petrous bones
  • blood tests
• management →
  
  • treat underlying cause (if found)
  • steroids

Question 20

What is pre-lingual deafness and its impact?

Answer 20

• hearing loss that occurs before development of speech + language
• severity will determine outcome
• those who cannot hear <90dB will acquire comprehension difficulties
• children who become deaf after the development of some language (even if v basic) will demonstrate a much higher level of linguistic achievement than those who have had no language exposure
• congenital causes → genetic, prenatal infections, prematurity
• acquired causes → infections (meningitis, mumps), trauma
• 90% of pre-lingually deaf children are unlikely to ever develop good speech + speech reception (even w/ hearing aids and early speech intervention)
• reading skills are impaired
• delayed social development + social isolation (less of a problem if child identified w/ ‘deaf culture’)

Question 21

What is post-lingual deafness and its impact?

Answer 21

• also causes communication difficulties, but bc speech is already established, communication can be augmented by measures discussed previously
• can cause social isolation, anger, depression + denial
• as well as interventions to augment hearing, supportive measures are also required