Case 16- ears Flashcards
Common bacterial causes of acute otitis media
- Streptococcus pneumoniae (30-35%)
- Haemophilus influenza (20-25%)
- Moraxella catarrhalis (10-15)
- Up to 30% of AOM cases are viral, viral infections make bacterial infections more likely as they reduce mucocillary clearance
Otitis media and measles
Otitis media occurs in about 8% of measles cases, more common in children as there eustacian tube has less of an angle also due to immature immune system.
Otitis media with effusion
Fluid behind the tympanic membrane, appears dull, the cone of light is ill defined. Can occur after acute otits media but there may be no known cause. Often sterile with no surrounding bacteria. Fluid forms behind the tympanic membrane causing pressure changes, may mean the tympanic membrane does not vibrate correctly causing hearing loss. Can have a significant effect if the child is first learning speech.
Treatment for Otitis media effusion
No particular treatment tends to resolve. May use a grommet which is a tube that goes through the tympanic membrane and causes the pressure to equilibrate.
Perforation of the tympanic membrane
Can occur if there is a severe infection or from barrow trauma (big difference in pressure between inside and outside). Tend to be small and resolve spontaneously but occasionally need to be repaired in an operation called a tympanic plasty.
Oto-acoustic emission (OAE)
The faint echo that occurs when you play a sound in the ear. This emission shows that the cochlea is healthy i.e. the biological amplifies is working. Easy way to screen for hearing in babies. Used in the NHS neonatal screening programme
Key processing stages in the auditory pathway
• Auditory nerve connects the cochlea to the brain
• First synapse in the auditory pathway is the Cochlear nucleus
• Superior olivary complex- the information from the two ears interact, this is important for localising sound
• Inferior colliculus- midbrain auditory centre
• Medial geniculate body- thalamic auditory nucleus
• Auditory cortex- in the temporal lobe
Connections are mirrored in both ears
The auditory pathway- lots of information
1) The auditory nerve fibre enters the brain at the Ventral and Dorsal cochlear nucleus.
2) This is at the junction between the medulla and the pons.
3) Information travels to the right and left Superior olive complex.
4) Information then travels up a bundle of fibres known as the Lateral lemniscus and synapses in the midbrain at the Inferior colliculus.
5) From here fibres go to the Medial Geniculate body in the Thalamus of the forebrain.
6) Fibres then go to the Auditory cortex in the temporal lobe of the Cerebral cortex.
Location of the auditory complex
The auditory cortex is in the superior surface of the temporal lobe. The Sylvian fissure runs between the temporal and parietal lobes. The primary auditory cortex is in Heschel’s gyrus, Planum temporale is also part of the primary auditory complex, receives the strongest input from the primary auditory thalamus.
How brain damage can cause speech defecits
Dysphasias are deficits in speech
1) Receptive dysphasia- problems in speech comprehension. Associated with the back of the temporal lobe in Wernicke’s area
2) Expressive dysphasia- problems in speech production. Associated with Broca’s area
Cues for sound localisation
- The position of sound is not represented on the basilar membrane in the cochlea
- You compare the inputs from the two ears and contrast with eyes
- Because the ears are separate, sounds can cause differences in timing and intensity (loudness) between the ears. The left ear hears differently from the right ear
- these are termed intra-aural (between the ears differences)
Method of sound localisation- path difference
The extra distance and time that the sound has to travel in order for the same point in the sound wave to reach the opposite ear. Causes a continuous difference in the timing between the ears. At any given point the phase of the sound will be different between the two ears. The path difference changes as the source of the sound changes location. This only works for low frequency sounds <1500 Hz.
Localising high frequency sounds >1500 Hz
- Because the waves are close together the head casts a sound shadow for high frequencies, this can block how much sound gets to the other ear. Less sound will get to the ear away from the sound source
- The brain detects differences in sound levels between the ears
- This difference changes as the sound source changes position
Benefits of Binaural hearing- having two ear
Improves speech detection in noisy environments i.e. the ability to listen selectively to one person in a noisy room. Speech is easier to detect when the competing noise comes from a different location, Binaural hearing aids are better then just one. We can do this due to our ability to separate sound sources spatially.
Sound
Longitudinal pressure waves travelling through air or another medium
What is sound defined by
- Frequency- pitch of sound, measured in Hertz (Hz) cycles per second
- Amplitude (intensity)- loudness, measured in a log scale (because the ear can detect a wide range of sound pressure). Its measured in dB (deciBels).
Pressure wave- sound
Alternating compression (closer together) and rarefraction (further apart) of air molecules causing sound
Frequency, Intensity and T
T= period (s), the time between the peak of one wave and the next.
Frequency= 1/t Hertz (s-1)
A high frequency means the peaks are closer together. Intensity measures the pressure fluctuations i.e. the height and how loud it is
How is intensity measured
How loud it is, its measured using the log of the ratio of sound pressures, one of them being the reference level (20) which is the quietest sound humans can hear
20*log (measured pressure / reference pressure).
A dB of 0 doesn’t mean there is no sound but that it is at the reference level
Frequency range
20Hz to 21kHz (for a young person)
At a really high or low frequency you need a louder sound in order to hear it. Frequenzy is measured on a log scale, as you age the upper frequency limit decreases
Nerve fibres and hair
The nerve fibres are from the 8th cranial nerve, 30,000 fibres 95% of nerve fibres are type 1 and go to the inner hair cells. Each inner hair cell receives up to ten type 1 fibres. Each nerve fibre only goes to one hair cell. Type 1 nerve fibres carry the most amount of information. Type 2 nerve cells are associated with the outer hair cells. Each type 2 nerve fibre makes contact with a number of outer hair cells. Unclear role. Most of the information we use to analyse sound comes from the type 1 nerve fibres.
How many inner and outer hair cells are there
There are 12,000 outer hair cells in 3-5 rows and 3,500 inner hair cells