🧠Neurology🧠 - Auditory & Vestibular Systems Flashcards
What is the function of the vestibular organ?
Capture low frequency motion (movement)
What is the function of the hearing organ?
Capture high frequency motion (sound)
What is amplitude and frequency?
Amplitude - loudness, sound pressure, subjective attribute correlated with physical strength
Frequency - pitch, oscillations per second, perceived tone
What is the range of human hearing?
20-20000Hz
0dB to 120dB
What are the 3 parts of the ear?
Outer ear
Middle ear
Inner ear
What is the function of the outer ear?
Capture sound, and focus it onto the tympanic membrane
Modest amplification (10dB) of upper range speech frequencies via resonance
Protect the ear from external threats
What is the function of the middle ear?
Mechanical amplification (20-30dB)
What is the inner ear, and give its function
The cochlea
Transduces vibration into nervous impulses
Does so in a way that captures both frequency and intensity of the sound
Outline the structure of the cochlea
3 compartments:
Scala vestibuli
Scala media
Scala tympani
Vibrations travel into the scala vestibuli via the oval window, through the scala media, where it is translated to nervous impulses, to the scala tympani and out the round window
What are the compartments of the cochlea?
Scala vestibuli, scala media, scala tympani
Scala vestibuli and tympani both bone structure containing perilymph (high in Na)
Scala media is a membranous structure containing endolymph (high in K), where the Organ of Corti (hearing organ) is located
Where does the organ of Corti lie?
Lies in the basilar membrane
How does the basilar membrane allow for different frequencies to be interpretated?
Arranged tonotopically, meaning different parts of the membrane will oscillate according to the frequency of the incoming vibrations
What does the organ of Corti consist of?
Inner hair cells
Outer hair cells
Tectorial membrane
(Hair cells attach to the auditory nerve)
What is the function of the inner hair cells?
Carry afferent information on the auditory nerve
Transduction of sound into nerve impulses
What is the function of the outer hair cells?
Carry efferent information on the auditory nerve
Modulation of the sensitivity of the response
Which hair cells are in constant contact with the tectorial membrane?
Outer hair cells
How do the hair cells achieve transduction?
Stereocilia of the hair cells are deflected (moved) when the tectorial membrane comes into contact with them (during a sound vibration)
This deflection opens the K+ channels, causing the cell to depolarise, releasing neurotransmitters to the afferent nerve, which itself then depolarises
Higher amplitudes cause greater deflection, and so more K+ channel opening
How does the direction of deflection effect the signal produced?
Deflection in one direction causes depolarisation
The other way causes hyperpolarisation
Deflection towards the kinocilium will cause depolarisation
What is the longest stereocilia called?
Kinocilium
What is the first part of the auditory pathway?
Spiral ganglions via the vestibulo-cochlear nerve (VIII) travel to the ipsilateral cochlear nuclei in the brainstem (pons)
What happens after auditory information reaches the cochlear nucleus?
Auditory information crosses at the superior olive level
All connections after this point are bilateral
Where does auditory information go after it leaves the superior olive?
To the inferior colliculus, then the medial geniculate body
Terminates at the auditory cortex in the parietal lobe
What are the different types of hearing loss?
Anatomical:
Conductive
Sensorineural
Central
Timing:
Sudden hearing loss
Progressive hearing loss
What is conductive hearing loss?
Problem in the outer or middle ear - areas of sound conduction
What is sensorineural hearing loss?
The sensory organ (cochlea) or nerve (auditory nerve) is affected
90% of all hearing loss
What is central hearing loss?
Problem originates in the brain or brainstem
Very rare
What is sudden vs progressive hearing loss?
Sudden hearing loss occurs over minutes to days
Progressive hearing loss occurs over months to many years
What are the causes of conductive hearing loss?
Outer ear - cerumen impaction (too much wax), foreign body
Middle ear - Otitis (inflammation/infection of the ear) including potential fluid buildup, otosclerosis
What is otosclerosis?
Abnormal bone growth in the middle air, impairing bones ability to vibrate
What are the bones of the middle ear?
Malleus
Incus
Stapes
What are the causes of sensorineural hearing loss?
Inner ear - noise, presbycusis (ager-related deterioration). ototoxicity
Nerve - Acoustic neuroma (vestibular schwannoma) - unilateral
What bedside tests can be used to assess hearing?
Tuning fork
Weber test and Rinne test
How can hearing thresholds be accurately measured?
Audiogram
A normal hearing threshold is 0-20dB
How does conductive hearing loss compare to sensorineural loss in terms of patients hearing?
Conductive hearing loss is along all frequencies while sensorineural hearing loss affects some frequencies more than others
What are OAEs?
The normal cochlea produces low-intensity sounds called OAEs (otoacoustic emissions)
These sounds are produced specifically by the outer hair cells as they expand and contract (acts to sensitise)
This test is often part of the newborn hearing screening and hearing loss monitoring
What are the treatment options for hearing loss?
Underlying cause
Hearing aids
Cochlear implants
Brainstem implants
What is the function of the vestibular system?
To detect head motion and gravity and transmit this information to the CNS
The outputs are reflexes that maintain posture and gaze
What structures make up the vestibular organ?
Anterior, lateral and posterior canals connected to the utricle, have an ampulla on one side
Utricle
Saccule
What are the otolith organs?
Utricle and saccule are the otolith organs
Their cells are located on the maculae, placed horizontally in the utricle and vertically in the saccule
What is the maculae?
The maculae contain the hair cells, a gelatinous matrix and the otoliths on top
Otoliths are carbonate crystals that help the deflection of the hairs
What is the function of the utricule vs saccule?
Utricule detects horizontal movement
Saccule detects vertical movement
Outline the structure of the canals
The hair cells in the canals are located in the ampulla
Rest of canal has high potassium endolymph
The ampulla has the crista, where the hair cells are located
What are the semi-circular canal planes?
The orientation of the canals in the head defines three planes.
Anterior and posterior canals form a 90° angle. Lateral canals are horizontal to the other canals.
Therefore they work in pairs to evaluate head movement
How do vestibular hair cells compare to cochlear hair cells?
Same concept of hair deflection resulting in depolarisation/hyperpolarisation
How do hair cells generate a signal?
Hair cells have a resting potential which has a basal discharge to the nerve
Movements towards kinocilium causes depolarization and an increase in nerve discharge
Hairs moving away from the kinocilium generates hyperpolarization and a reduction in nerve discharge
Where do nerve impulses from the vestibular organs go?
Primary afferents end in vestibular nuclei in the brainstem (pons)
Impulses travel to nuclei for eye movements, and down the vestibulospinal tract (for posture related reflexes)
Which major blood vessel supplies the vestibular system?
What are the overall functions of the vestibular system?
To detect and inform about head movements
Keep images fixed in the retina during head movements
Balance
What is the vestibulo-ocular reflex?
Eye remains fixed on a point even with rapid head movements
How does the vestibulo-ocular reflex come about?
Keeps images fixed in the retina
Connection between vestibular nuclei and the abducens and oculomotor nuclei
Eye movement in opposite direction to head movement, but same velocity and amplitude
How can vestibular disorders be categorised?
Timing and laterality
Acute and unilateral
Slow and unilateral/bilateral
What are the usual complaints in acute, unilateral vestibular issues?
Imbalance
Dizziness
Vertigo
Nausea
What are the usual complains in slow and unilateral/bilateral vestibular issues?
Imbalance
Nausea
NO vertigo
How can vestibular disorders by categorised by location?
Peripheral vestibular disorders
Central vestibular disorders
What do peripheral vestibular disorders effect, and give some examples
Vestibular organ and/or VIII nerve
Examples:
Vestibular neuritis
BPPV
Meniere’s disease
What do central vestibular disorders effect, and give some examples
CNS (brainstem/cerebellum)
Examples:
Stroke
Vestibular migraine
Multiple sclerosis
Tumours
What are the main diagnoses for someone presenting with vestibular symptoms?
BPPV
Vestibular Neuritis
Vestibular Migraine
Stroke (cerebellar)
What are the key red flags with vestibular symptoms?
Gait problems
Hyper-acute onset
Hearing loss
Why are gait problems a red flag symptom for vestibular symptoms?
Suggest cerebellar dysfunction - stroke
Why is a hyper-acute onset a red flag for vestibular symptoms?
Suggests vascular cause (i.e. stroke)
In neurology, hyper-acute onset = think vascular cause
Why is hearing loss a red flag for vestibular symptoms?
Suggests a range of conditions, such as labyrinthitis, neuritis, stroke/TIA, vestibular schwannoma and Meniere’s disease, all of which can lead to serious/permanent damage or life-threatening complications
What is a nystagmus
Nystagmus is a jerky eye movement that involves a slow phase (pathology) and a fast phase (saccadic response to keep image focused, i.e. a correction from the brain)
How can balance disorders by separated?
Timing (evolution)
What is likely to be the cause of an acute balance disorder?
Vestibular neuritis
Stroke
What is likely to be the cause of an intermittent balance disorder?
Benign paroxysmal positional vertigo (BPPV)
What is likely to be the cause of a recurrent balance disorder?
Migraine
(Can also be Meniere’s disease)
What is likely to be the cause of a progressive balance disorder?
Vestibular schwannoma
Degenerative conditions (e.g. MS)
What is the HINTS exam?
Clinical exam for acute dizziness
Distinguish between vestibular neuritis or stroke?
Outline the HINTS exam
Head Impulse test - normal in stroke, abnormal in VN
Nystagmus - direction changing in stroke, unidirectional away from a side in VN
Test of Skew deviation - Vertical misalignment - abnormal in stroke
