How to Interpret Tympanogram Results
Tympanogram interpretation centers on three key parameters: the presence or absence of a peak, the peak pressure location, and tympanic membrane compliance—with a flat tracing indicating middle ear effusion, a shifted peak suggesting eustachian tube dysfunction, and normal tracings showing a peak near atmospheric pressure.
Core Tympanometric Parameters
A tympanogram objectively measures how easily the tympanic membrane vibrates and at what pressure it does so most easily (pressure admittance function). 1 The test provides three critical pieces of information:
Traditional Classification System (Types A, B, C)
While historically useful, the conventional ABC classification system has limitations and should be understood in context:
Type A (Normal)
- Peak present near atmospheric pressure (0 daPa) 2
- Normal tympanic membrane mobility 2
- Indicates air-filled middle ear at normal pressure 2
Type B (Flat Tracing)
- No discernible peak or minimal/sluggish movement 1
- Most commonly indicates middle ear effusion (fluid behind the tympanic membrane) 2
- When middle ear is fluid-filled, vibration is impaired, resulting in a flat or nearly flat tracing 1
- Can also indicate tympanic membrane perforation or other middle ear abnormalities 1
Type C (Negative Pressure)
- Peak shifted to negative pressure range 2
- Traditionally defined as peak less than -100 daPa 2
- Indicates eustachian tube dysfunction 2
Critical Limitation of Traditional Classification
The conventional Type C threshold of -100 daPa is likely insensitive to detect subtle abnormalities, including some presentations of obstructive eustachian tube dysfunction. 3 The existing definition relies on large and arbitrary threshold shifts that were valuable for diagnosing severe, chronic middle ear disease requiring surgery but may not be well-suited for evaluating less severe disease. 3
Modern Interpretation Approach
Clinical accuracy improves by reporting absolute values of tympanometric peak pressure (TPP) rather than relying solely on arbitrary classification thresholds. 3 This approach allows detection of more subtle middle ear pathology.
Specific Pressure Values to Note:
- Peak at 0 daPa: Normal middle ear pressure 1
- Peak shifted left (negative): Indicates negative middle ear pressure; if the middle ear is air-filled but at lower pressure than atmosphere, the peak shifts left 1
- Peak shifted right (positive): Indicates positive middle ear pressure; if middle ear pressure exceeds atmospheric pressure, the peak shifts right 1
- High positive pressure (≥50 mm H2O or approximately ≥50 daPa): Uncommon finding that may be associated with normal ears (90% when compliance is high) or acute infection (43% when compliance is low) 4
Important Technical Considerations and Pitfalls
Measurement Artifacts:
- The tympanometry procedure itself can alter measurements in ears with negative middle ear pressure 5
- Peak compliance increases by approximately 0.10 mmhos under negative pressures greater than -190 daPa (hypercorrection phenomenon) 5
- Tympanogram width increases up to 50 daPa for middle ear pressure ≤-130 daPa 5
- External ear canal volume decreases when middle ear pressure ≤-190 daPa 5
Directional Tracing Differences:
- Forward tracing (decreasing pressure) peaks tend to form in the negative pressure area 6
- Backward tracing (increasing pressure) peaks tend to form in the positive pressure area 6
- Unidirectional tympanograms cannot indicate precise middle ear pressure 6
- Valid middle ear pressure is better estimated by averaging forward and backward tracing peak pressures 6
- Peak shift magnitude is influenced by the speed of external auditory meatus pressure change, with linear increase up to 70 mmH2O/sec 6
Clinical Context Integration
Tympanometry provides objective data that serves as a useful adjunct—not a replacement—for clinical diagnosis of middle ear disease. 2 The test should be interpreted alongside:
- Pneumatic otoscopy findings: Normal eardrums move briskly with applied pressure; minimal or sluggish movement indicates middle ear fluid 1
- Clinical symptoms and history 1
- Hearing assessment results 1
Common Clinical Scenarios:
- Flat tympanogram with conductive hearing loss: Strongly suggests otitis media with effusion 1
- Type C pattern with symptoms: May warrant closer follow-up even if pressure is only mildly negative (e.g., -50 to -100 daPa) 3
- Discordant findings: When tympanometry conflicts with otoscopy, consider technical factors, repeat testing, or refer for audiologic assessment 1