The role of the ME

Pathology

Outer Ear Disorders: deformities.

Microtia-small pinna

Atresia-absence or closure of the external auditory meatus. May also exhibit ME deformities, hearing loss can be relatively flat and even up to a 40 dB loss.

Foreign objects/cerumen/cysts/bacteria (swimmers ear) are frequently encountered.

Ear mites in cats.

Serous otitus media

The most frequently seen pathologies in the ENT clinic are middle-ear infections.

85% of all children have one episode by age 6.

Prevalence is increasing. A result of day care? More upper respiratory infections likely a result of eustachian tube obstructions.

 

Important economic problem with possible long term consequences, Miragotony tubes.

Acute (AOM) last 3 weeks with bulging, reddening TM, pain.

chronic (COM)- perforation of eardrum with fluid discharge.

 

Guinea pigs- have high rate of middle-ear problems turns into tissue. Ear mites in cats.

eustachian tube provides pressure equalization path. Airplane takeoff/landing experience.

A muscle is responsible for opening and closing the ‘tube’. The muscle is weaker in children. Plus tube is at 10° in children and – 45° in adults.

Audiometric profile: 20 – 30 dB haring loss over the entire frequency range.

COM could lead to perforations of the TM,

cholesteotoma could lead to facial paralysis. Why?

 

Otosclerosis- hardening of the ear reported to affect 5-10% of population common cause of conductive hearing loss.

Fig. 5.

Management of otosclerosis is surgery of stapes

 

 

Middle-ear reflex:

see also Geisler Ch 3.

 

2 muscles- tensor tympani and stapedius muscle.

Contraction is an involuntary reflex arising from loud acoustic stimuli (> 85 dB SPL), puff of air to eyes or due to voluntary movements of mouth muscles, as in chewing, swallowing,

When muscles contract the ME chain is stiffened and the sound pressure reaching the inner ear is decreased (15 to 20 dB in a frequency dependent fashion. The greatest effect is on low frequencies (f < 2000 Hz), and appears to apply only to high level stimuli (low level are not attenuated).

The reflex arc.

Transmission of energy into cochlea: air to water

Za= 42; ZW = 1.54x105

Consider the ratio ZW/ Za = 1/4000: need ~ 30 dB gain in order to overcome the impedance mismatch, however, the transformer ratio is frequency dependent, as such the coupling of energy into the TM varies with frequency.

If one lost their ME and sound reached both the RW and OW with the same intensity the hearing loss could be 50-60 dB.

 

P. 29 Møller.

Negative pressures in ME have a greater effect in reducing transmission and have positive pressure.

P. 37, p. 39.

 

J.J. Rosowski (1991, JASA, 90: 124-135).

External and middle ear; play a major role in determining the shape of the audiogram.

Fig. 1

Some authors have suggested that the cochlea performs as a sound power detector at threshold.

Little difference in threshold shapes predicted by either the constant power or constant stapes-velocity model.

frequency range of largest magnitudes in external-ear transfer function (Pt/Ppw) corresponds to the greatest sensitivity region and also the region most likely cochlear region to be damaged.