Case 4 - Extra Flashcards
the external ear summary
- consists of the pinna, concha and external auditory meatus
- these structures collect sound waves and focus them on the tympani membrane
- the configuration of the external auditory meatus selectively boosts the sound pressure for frequencies around via passive resonance effects
- this amplification makes humans especially sensitives to frequencies in the range of 2-5Hz
- the ear recognises the elevation of a sound source because the vertically asymmetrical convolutions of the pinna are shaped so that the external ear transmits more high-frequency components form an elevated source than from the same source at ear level
the middle ear summary
- the middle ear converts vibrations in air pressure into movements of the ossicles
- the middle ear ensures transmission of the sound energy across the air fluid boundary by boosting the pressure measured at the tympanic membrane greatly by the time it reaches the inner ear
what are the two mechanical processes that occur within the middle ear to achieve the large pressure gain
- focusing the force impinging on the relatively large diameter tympanic membrane on to the much smaller diameter oval window
- a second and related process relies on the mechanical advantage gained by the level action of the three ossicles which connect the tympani membrane which are transformed into smaller but stronger vibrations of the oval window
why is the pressure at the oval window great
because it has a smaller surface are to the tympanic membrane and because the force on the oval window membrane is greater than that on the tympanic membrane
why is the force applied to the oval window greater than that on the tympanic membrane
because behind the tympanic membrane is air, whereas begins the oval window is fluid. therefore more force needs to be applied at the oval window
what are the small muscles in the middle ear:
the tensor tympani - innervated by mandibular nerve V3
the stapdeius - innervated by the facial nerve CNVII
what does contraction of these muscles in the middle ear cause
the chain of ossicles to become more rigid and sound condition to the inner ear is greatly diminished
what does the onset of a Loud sound trigger
a neural response called the attentuation reflex
what is Meniere’s disease
disease of the inner ear characterised by recurring episodes of rotatory vertigo, deafness, and tinnitus.
atttacs are recurrent over months or years. typically the attacks are preceded by a sensation of fullness in the ear
classically it is associated with a low frequency sensorineural hearing loss, feeling of fullness in the eat, loss of balance and tinnitus
there is a build up of end lymphatic fluid in the inner ear
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what does treatment of Meniere’s include
vestibular sedatives. preventative measures, such as a low salt diet and avoidance of caffeine are useful. if the disease cannot be controlled in this way, then a chemical labyrinthectomy which involves perfusing the round window orifice with ototoxic drugs such as gentamicin is possible
what does gentamicin do
it destroys the vestibular epithelium. therefore the patient has severe vertigo for around 2 weeks util the body compensates for the lack of vestibular input on that side
what is the Weber test
in the weber test a vibrating tuning for (256Hz) is placed in the middle of the forehead, above the upper lip under the nose over the teeth, or on top of the head qui-distant from the patient’s ears on top of thin skin in contact with the bone
The patient is asked to report in which ear the sound is heard louder
A normal weber test has a patient reporting the sound heard equally in both sides
In an affected patient, if the defective ear hears the Weber tuning fork louder, the finding indicates a conductive hearing loss in the defective ear
In an affected patient, if the normal ear hears the tuning fork sound better, there is sensorineural hearing loss on the other defective ear
Example:
If the right ear is defective, the results will be as follows:
- sound hear in right (defective) ear louder - conductive loss in right ear
- sound hear in left (normal) ear louder - sensorineural loss in right ear
what is the Rinne’s test
for the Rinne test, a vibrating tuning fork (typically 512Hz) is placed initially on the mastoid process behind each ear Intel sound is no longer heard
Then, the fork is immediately placed just outside the ear with the patient asked to report when the sound caused by the vibration is no longer heard
A normaal or positive Rinne test is when the sound hear outside fo the ear is louder than that heard when the tuning fork was placed against the skin on top of the mastoid process behind the ear
A negative Rinne test indicates conductive hearing loss of that ear
what is the weber and rinne test together
air conduction thresholds are determined by presenting the stimulus in air with the use of headphones. Bone conduction thresholds are determined by placing the stem of a vibrating tuning for in contact with the head. In the presence of a hearing loss, broad-spectrum noise is presented to the non-test ear for masking purposes so that responses are based on perception from the ear under test
The responses are measured in decibels. An audiogram is a plot of intensity in decibels of hearing threshold versus frequency
the difference between these two lines plotted on your audiogram is the air-bone gap
An air-bone gap indicates problems somewhere in your outer or middle ears
what is tympanometry
test of middle ear compliance
this test measures the staginess of the eardrum and estimates middle ear pressure and the volume of the external canal.
it is useful in detecting middle ear effusions in glue ear, under ventilation of the middle ear space in Eustachian tube dysfunction and small perforations that are not seen on otoscope
