Hearing Loss Flashcards
What are the main structures of the inner ear?
- semicircular canals → for balance
- cochlea → for hearing
- cochlea present with hair cells helping to generate electrical signals for hearing, sent via auditory nerve
How are sound waves transduced to nerve impulses?
- 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
What is the neurological pathway between the cochlea and the auditory cortex?
- 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
What is the difference between sensorineural hearing loss, conductive hearing loss, and mixed hearing loss?
- 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
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?
- 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
What is Weber’s test?
- 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
We have mentioned that pure tone audiometry is a subjective hearing test.
What are examples of objective hearing tests?
- 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
What are examples of hearing tactics?
- 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
What are the different types of hearing aids?
- 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
What is the difference between conductive and sensorineural hearing loss?
- 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
What are the causes of conductive hearing loss?
- wax (cerumen)
- otitis externa
- foreign body
- acute/chronic otitis media
- trauma → tympanic membrane or ossicles
- otosclerosis
- middle ear tumours
- cholesteatoma
What are the causes of sensorineural hearing loss?
- 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
What is presbycusis?
- 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
Ototoxicity → what drugs cause permanent and temporary hearing loss?
- 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
What are the features of a vestibular schwannoma (acoustic neuroma)?
- 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
