Case 4 - Hearing Flashcards
What components make up the external ear?
Pinna and external auditory meatus
What is the function of the external ear?
Direct sound waves into auditory canal
What does the middle ear include?
Tympanic membrane and chain of ossicles (malleus, incus, stapes)
List the functions of the middle ear
Protection
Amplification
Conduction
Transduction - converts energy
What does the inner ear include?
Bony labyrinth: 3 semi-circular canals, cochlear
Membraneous labyrinth: vestibule, cochlear duct, 3 semi-circular ducts
What fluid does the bony and membraneous labyrinth contain? Give the characteristics of the fluid
Bony labyrinth - perilymph, low K+
Membraneous labyrinth - endolymph, high K+/ low Na+
Where is the tensor tympani and stapedius muscles found and what is their role?
Tensor tympani - around tympanic membrane
Stapedius - around stapes
They contract when the sound is very loud to reduce sound conduction (limits vibration) = attenuation reflex (or ‘acoustic’ reflex). Only lasts for around 40 mins before fatigue
What innervates the tensor tympani and stapedius?
Tensor tympani = trigeminal V3
Stapedius = facial nerve
What is the role of the pharyngotympanic tube?
Connects nasopharynx to the middle ear - equalises pressure on either side of the tympanic membrane
What does the vestibule contain and what is its role?
Utricle and saccule - have equilbirium receptors called maculae to measure static / linear acceleration
Describe the function of the 2 otolith organs
Saccule: cochlear duct empties here, responsible for vertical acceleration (i.e. sensation of going up and down, like a lift)
Utricle: where semi-circular ducts empty into, responsible for horizontal acceleration (i.e. sensation of forward and back, like a car)
What makes otolith organs so sensitive to changes in acceleration?
Otoconia (top layer) with striola resting on the otolithic membrane - formed by calcium carbonate crystals. These add weight to increase sensitivity to gravity.
Think of it as a very wobbly layer
What is the role of the semi-circular canals?
Forms the dynamic system - detects angular acceleration
How do semi-circular canals respond to movement and detect positioning of the head in space?
Have dilated ends called ampullae which contain crista (sense organs). The ampulla have hair cells covered by cupula (a gelatinous mass).
Turning head = moves the fluid layer which displaces the cupula. This stains the stereocilia within it:
- Deflection towards kinocilium = depolarises and increases firing in afferent nerves
- Deflection away from kinocilium = hyperpolarises and decreased firing
List the 3 semi-circular canals and what movement of the head they detect
Anterior SCC= nodding of head
Posterior SCC= head to shoulder
Lateral SCC = rotation of head
What neurotransmitter is released when there is deflection towards kinocilium?
Glutamate - through mechanically gated ion channels
What are the functions of the vestibular system?
- Detects forces generated by movement, translates into balance
- Provides information about the position of head in space
- Allows other CNS structures to rapidly compensate for changes
What is the kinocilium?
The tallest cilium (vestibular hair cells have 1)
How is sound amplified?
Lever action of ossicles
Concentration of sound waves from the large tympanic membrane onto the small oval window (20:1)
Describe how sound waves lead to a relay of information to the CNS (physiology of hearing)
1: sound waves travel through external auditory meatus to the tympanic membrane
2: Movement of TM displaces the ossicles, and the membrane vibrates in resonance to sound waves - vibrates the malleus, incus then stapes
3: Movement at stapes at oval window produces pressure waves in perilymph of scala vestibuli
4: Depending on frequency/ amplitude of sound waves, they can vibrate the basilar membrane: location of distortion varies with sound frequency (high frequency = short wavelength, vibrates membrane nearer oval window) and louder sound = higher vibration
5: vibration of basilar membrane causes sterocilia to beat against the tectorial membrane and alters K+ conductance:
- Towards kinocilium = depolarised (increased K+ conductance)
- Away from kinocilium= hyperpolarised (decreased K+ conductance)
6: depolarised hair cells cause voltage-gated Ca2+ channels to open = glutamate release
7: activates spiral ganglion, relays info to cochlear branch of CN VIII (relays info to CNS)
Describe the auditory pathway from the cochlear branch of CN VIII to the cortex
1: cochlear branch travels through the petrous part of temporal bone (via internal auditory meatus with CN VII) to reach pons-medullary junction
2: Synapses at cochlear nuclei
3: Information is sent off to superior olivary nucleus of pons
4: information to inferior colliculi of midbrain
5: axons continue upwards in the lateral lemniscus to the medial geniculate nucleus of the thalamus
6: projection fibres deliver information to superior temporal gyrus (area in primary auditory cortex) - high frequency sounds activate a different area to low frequency
** At each step, axons decussate so 2 tracts from each nucleus going up
Why is a lesion in the brainstem unlikely to cause hearing loss?
The decussation and cross-talk between nuclei aids in bilateral processing and localisation of hearing, i.e. still will have info from the other side
What is the role of the inferior colliculi?
Unconscious motor responses to acoustic stimuli, i.e. turning your head when you hear a noise
What areas are located nearby to the primary auditory cortex?
Brocas area - control muscles of speech
Wernickes area - speech comprehension
Secondary auditory cortex (association area)
Who can benefit from a cochlear implant?
Post-lingually deafened adults: already developed speech so can use hearing aids to optimise hearing
Pre-lingually deaf children: implant before age 4, ideally by age 12 months (or hearing aids given before 3 months old - encourages auditory inputs)
Why would a cochlear implant not be beneficial for an pre-lingually deafened 6 year old?
After the period of neuroplasticity, the cochlear implant would not improve hearing or speech
What is the vestibulo-ocular reflex?
When eye movements counter head movements, so permit the gaze to remain fixed on a particular point. They compensate for the head movement to produce a stable image, preventing retinal slip
What are the phases to VOR?
Semi-circular canals in one ear sense rotation of head
Slow phase: eyes slowly rotate in opposite direction
Fast phase: eyes rapidly reset to centre of gaze
Describe what would happen to the eyes (inc. extra ocular muscles) if the head rotated left (VOR)
Eyes rotate right:
- Left eye = stimulation of oculomotor nuclei to contract the MR, inhibits abducens nuclei to relax LR
- Right eye = contract LR and relax MR
Essentially the contralateral LR and ipsilateral MR contracts (to the direction of movement)
What is nystagmus?
Element of VOR involving alternate slow eye movement with rapid saccadic movement
What controls the slow and fast phases of the VOR?
Slow phase = vestibulo-ocular pathway
Fast phase = cerebral cortex
What part of VOR may be absent/ present in a patient in a coma?
Slow phase is still present, but fast phase isn’t as it is controlled by higher cortical centres
What does a dysfunctional slow phase of the VOR indicate?
Lesion in the vestibulo-ocular pathway
How can nsytagmus be tested?
Introduce cold/ warm water into the external auditory meatus. The warming/cooling causes convention currents in endolymph, which stimulates hair cells as if the head was moving. This should produce nystagmus (if healthy)
How is eye movement affected by the introduction of cold water vs warm water?
Cold water = mimicks head movement to Opposite side (that the water is introduced) - i.e. if to L ear, head is turning to R so then eyes will turn to the L
Warm water = mimicks head movement to the Same side
What is otitis media?
Umbrella term for diseases of inflammation of the middle ear. This leads to dysfunction of the Eustachian tube due to inflamed adenoid tonsils in the nasopharynx.
How can otitis media lead to middle ear effusion?
The gas volume in the middle ear is trapped so there’s negative pressure - fluid from surrounding tissues is then ‘sucked in’
What is the sign of otitis media?
Erythematous bulging tympanic membrane
- If opaque = indicates something behind TM (i.e. otitis media with effusion)
What is the most common cause of otitis media? How can it be treated?
Streptococcus pneumonia
Amoxicillin
What hearing loss can otitis media with effusion lead to?
Conductive hearing loss due to impaired transduction of sound waves
Why is otitis media (with/ without effusion) so common in children?
- Shorter and narrower Eustachian tube
- Haven’t developed T cells yet - pathogens can grow
How can tonsilitis contribute to otitis media with effusion?
Leads to swelling of adenoids, which can push on the Eustachian tube and reduce ventilation
Why are children with cleft palate at increased risk of OM with effusion?
Shorter eustachian tube
What tools might be used to diagnose ear infections?
Otoscopy - look at TM colour, opacity, position, whether its bulging
Tympanometry
Audiometry
How is otitis media treated?
3 months watchful waiting - only if not at risk for speech/ language problems or learning difficulties
IF LASTING >3 MONTHS, and associated with bilateral hearing loss:
- Grommets or ventrilation tubes (tympanostomy) can be inserted into TM, stay for 6-12 months then extruded out
How do ventilation tubes work?
Hold a hole open in the ear drum so air can move and pressure equalises, helps drain fluid due to the ventilation
What is not recommended for treating otitis media?
Decongestants
Anti-histamines
Intranasal corticosteroids
When would adenoidectomy be recommended?
Persistent upper respiratory infections and if child > 4 years
(remove adenoids as can obstruct Eustachian tube)
What connects Wernicke’s and Broca’s areas?
Acruate fasciculus
Describe the embryology of the ear
Structure of ear by 20 wks gestation
By 25 wks the fetus will have a functional auditory system (including a cochlear and auditory cortex)
From 25 wks to 5/6 months, structures are tuned - hair cells of cochlear, axons of auditory nerve, neurons of temporal lobe etc.
What is required for development of the auditory system?
Outside auditory stimulation, i.e. speech, music, meaningful sounds
> First 3 years of life is most intensive period for acquiring speech and language
Describe how speech develops from birth to age 10
Birth to 3 months: reacts to loud sounds, makes pleasure sounds
4-6 months: responds to change in tone, follows sound with eyes, laughs
7 months-1 year: listens when spoken to, 1 or 2 words by first birthday
1-2 years: simple commands and questions, short sentences by around 18 months
2.5 years: simple sentences, lacks tenses, errors in syntax (word arrangement in sentences)
2.5-5 years: improvement in phonemes, pronunciation and articulation
6-10 years: master syllabel stress
What is a phoneme? When can they normally be detected?
Distinct units of sound in a word that can distinguish it from another word, i.e. ‘p’ in tap seperates it from tab
By 6-12 months
What is a morphome? When are they normally recognised?
Smallest meaningful unit in a language, not identical to a word, i.e. worker = 2 morphomes (work and er), girl = 1 morphome
Developed at around 1 year
How does decibels of sound relate to sound intensity?
10 dB = 10 fold increase in sound intensity
20 db = 100 fold increase
What is the scale of hearing loss from normal hearing to profound?
Normal: can hear less than 20 dB
Mild: 20-40 dB
Moderate: 40-70 dB
Severe: 70-90 dB
Profound: > 90dB
Are most constanants heard at low or high frequencies?
High
What can be the impact of hearing loss on children?
Aggressive behaviour
withdrawn into silence
reading and educational difficulties
What is conductive hearing loss?
Middle or external ear hearing loss, occurs when there is a problem with conducting sound waves through the ear
What can cause conductive hearing loss?
- External ear: ear wax (cerumen), otitis externa (swells and occludes)
- Tympanic membrane: perforation or retraction, i.e. middle ear effusion, upper respiratory infection
- Middle ear: acute otitis media and serous otitis media, damaged ossicles
How can conductive hearing loss be treated?
Hearing aids, antibiotics, cochlear implants
What is sensorineural hearing loss and how can it be caused?
Inner ear hearing loss, i.e. damage to cochlear or CN VIII
- Acquired causes= noise trauma, infection
- Congenital causes= deafness genes, ageing loss of outer hair cells
Persistent exposure to what level of sound can cause hearing loss?
85 db or higher
How is Webers test performed?
Vibrate a 256 Hz tuning fork, place on middle of forehead and then the patient reports which ear hears it louder or if its equal (normal)
How are results from Weber’s test interpreted?
- If defective ear hears sound louder = conductive hearing loss
- If normal ear hears sound louder = sensorineural hearing loss
How is Rinne’s test performed?
Vibrate a 512 Hz tuning fork and place the base on the mastoid process of the ear, when the sound is no longer heard then the tongs of the fork are placed outside the ear, and patient reports when they can no longer hear
How are results from Rinne’s test interpreted?
Normal (‘POSITIVE’ Rinne’s): sound outside the ear is heard for longer than when placed on the mastoid process, i.e. air conduction > bone conduction
Negative Rinne’s: bone conduction > air conduction (defective ear)
How are the results of Weber’s and Rinne’s test interpreted?
Weber’s test - shows whether it is conductive hearing loss or sensorineural hearing loss of another ear
Rinne’s test - used to diagnose which ear is affected for the hearing loss
What is the newborn screening protocol?
Otoacoustic emissions - can be done <3 months, 2 frequency sound stimuli into ear and the microphone detects presence of OAE’s from outer hair cells. Presence shows the outer hair cells of Organ of Corti are intact
IF failed - repeat in 4 wks. 2nd failure - Auditory brain response
How does auditory brain response test hearing?
Sound signals are sent into the ear, stimualtes inner hair cells. A map is then made of the auditory pathway and an absence of signal indicates deafness.
Measures: amplitude (number of neurons), latency (speed of transmision), interpeak lacency (time between peaks), interaural lacency (difference in wave V between ears)
What hearing test is used for children between 6-24 months?
Visual reinforcement audiometry: child distracted with toys whilst sound is played, if they turn the head to the sound then child is rewarded
What hearing test is used for children >24 months (and usually under 4 yrs)?
Conditioned play audiometry: conditioned to perform a task when sound signals
What hearing test is used for children > 4 years?
Pure tone audiogram: detect sound stimulus at different frequencies and amplitudes
How can hearing loss lead to an unhealthy lifestyle?
Impairs language skills and educational achievement = lower health literacy = more unhealthy lifestyle
What is the intersectional theory?
People are disadvantaged by multiple sources of oppression, i.e. race, gender identity etc., and there is an interactive component of how they engage with each other
What are the stages to listening development?
1: detection at 25-27 wks gestation. Maturation of cochlear allows us to use OAEs during neonatal screening
2: discrimination: learn key features associated with sounds/ phoneme/ words, both in isolation and speech
3: identification: requires memory to attribute meaning, can differentiate sounds and suprasegmental features of speech
4: comprehension: listening becomes closely linked with language, child has to remember respond and recall (important they hear and hear clearly)
What are the stages to developing attention?
1: fleeting attention (0-1 yrs) - highly distracted by sounds and movement
2: inflexible and rigid attention (age 1-2 yrs) - child focuses on activity of choice
3: flexible single channeled attention (age 2-3 yrs) - childs attention can be transferred from task when directed by an adult
4: focus single channeled attention (age 3-4 yrs): childs attention can be directed from task to adult under childs control
5: two channeled attention (age 4-5 yrs): child can switch between auditory and visual stimuli for short periods of time
6: sustained attention (age 5-6 yrs): child can control their attention
What age do children gain adult attention levels?
14 years of age
What is the difference in the role of dorsal vs ventral cochlear nuclei?
Dorsal cochlear nuclei = pitch of sound and quality of sound
Ventral cochlear nuclei = intensity of sound and timing
What is the function of the superior olivary nucleus?
Localisation of sounds in space, responds differently to intensity and timing
Where is the primary auditory cortex?
Angular gyrus
What would damage to the arcuate fasciculus cause?
Conduction aphasia
