Impedance Audiometry

IMPEDANCE AUDIOMETRY

• Objective test
• The following tests are included in the category Impedance Audiometry tests:
• Tympanometry (most commonly used)
• To determine the status of the tympanic membrane and middle ear
• Eustachian Tube Function
• Acoustic reflex thresholds
• To evaluate acoustic reflex pathways, which include cranial nerves (CN) VII and VIII and the auditory brainstem.
• Acoustic reflex decay.

INDICATIONS OF IMPEDANCE AUDIOMETRY TESTS

Impedance Audiometry test can assist in identifying the following pathologies of the peripheral or central auditory systems:

• Middle ear effusion
• Perforation of the tympanic membrane, including patency of pressure equalization tubes
• Tympanosclerosis
• Hypermobility of the tympanic membrane
• Eustachian tube dysfunction
• Glue ear
• Otosclerosis
• Ossicular discontinuity
• Acoustic Neuroma or other CN VIII disorder
• CN VII disorder
• Hearing loss (a correlative, but not a direct measure)
• Certain brainstem disorders.

TYMPANOMETRY

• Tympanogram is the graphic representation of compliance and impendance of tympanic ossicular system with air pressure changes.
• Tone is delivered at 220 Hz
• By charting the compliance of tympano-ossicular system against various pressure changes, different tympanograms are obtained
• Low-frequency tympanometry is not recommended for infants younger than 7 months because their ear canal cartilage is so soft that misleading tympanometric results can occur.

TYPES OF TYMPANOGRAM

• Type A – normal middle ear function.
• Type As – (Low compliance) otosclerosis, malleus fixation
• Type Ad (High compliance) ossicular discontinuity or thin and lax tympanic membrane
• Type B – (No change in compliance with pressure changes) middle ear fluid as in Secretory Otits Media, thick tympanic membrane
• Most common cause of Type B tympanogram in an old aged patient with neck nodes is Nasopharyngeal Carcinoma.
• Type C – (Maximum compliance occurs with negative pressure in excess of 100 mm of H20) retracted tympanic membrane.indicates a malfunctioning eustachian tube 

EUSTACHIAN TUBE FUNCTION TESTING

• Eustachian tube function testing can be implied by tympanograms.
• Type A tympanograms reflect normal middle ear pressure with an intact tympanic membrane. This situation only occurs if the eustachian tube is ventilating the middle ear.
• Type C tympanograms show significant negative middle ear pressure, implying eustachian tube dysfunction.
• In patients with perforated tympanic membranes, type B tympanograms disclose high-volume readings and no information on eustachian tube dysfunction.
• In these patients pressure in the ear canal can be increased via the probe in the ear canal. Instruct the patient to swallow several times or yawn to determine if the eustachian tube can clear the pressure.