Tonometry is used to assess the middle ear status.
It complements the typical audiogram, which tests pure tone hearing sensitivity.
Particularly useful when a conductive hearing loss is indicated in an audiogram.
Difference between air conduction and bone conduction sensitivity suggests outer/middle ear issues, known as the air-bone gap.
Non-behavioral and quick test, often used in children to detect issues like otitis media.
How Tonometry Works
Equal Air Pressure: Efficiency of sound transmission in the middle ear is optimal when air pressure is equal on both sides of the tympanic membrane (ear drum).
Equipment Used:
Probe with three parts: speaker, microphone, and air pressure changer.
Tests response to a steady tone while fluctuating air pressure between positive, normal, and negative.
Testing Process:
Emits a steady tone and measures sound bouncing back from the eardrum.
Determines the air pressure at which the least amount of sound bounces back, indicating maximum sound transmission.
Tempanogram Interpretation
Tempanogram Shape:
Negative and positive air pressure on horizontal axis.
Middle ear compliance on vertical axis.
Compliance: Indicates the stiffness of the middle ear system; less sound bouncing back means higher compliance.
Stages:
Positive Air Pressure: Low compliance, high sound bounce-back due to increased stiffness.
Normal Air Pressure: High compliance, least sound bounce-back, optimal condition.
Negative Air Pressure: Low compliance, high sound bounce-back as system stiffens again.
Types of Tempanograms
Type A (Normal): Peak over normal air pressure, indicating even air pressure.
Type C (Early Otitis Media): Peak over negative air pressure, indicating inward retraction of the eardrum.
Type B (Advanced Otitis Media): Flat, no peak, indicating fluid-filled middle ear.
Transition from Type A to C to B as otitis media progresses.
Special Conditions
Otosclerosis:
Soft porous bone growth around stapes footplate.
Causes artificial stiffening of the middle ear.
Type As Tempanogram: Shallow peak but still over normal air pressure due to increased stiffness.
Conclusion
Tonometry provides useful insights into the middle ear's efficiency and can detect conditions like otitis media and otosclerosis.
It's a quick, non-invasive, and non-behavioral test beneficial for both diagnosis and monitoring.