Tympanometry
Tympanometry is an objective diagnostic test that assesses middle ear function by measuring the acoustic immittance—a combination of admittance and impedance—of the ear as a function of varying air pressure in the external ear canal.[1] This procedure evaluates the mobility of the tympanic membrane (eardrum), the presence of middle ear effusion, and the integrity of the ossicular chain, providing critical insights into conditions such as otitis media with effusion, otosclerosis, or eustachian tube dysfunction.[2] Introduced in the 1950s and standardized in the 1970s, tympanometry has become a cornerstone of audiological evaluation, particularly for non-verbal patients like infants and young children.[1] The test is performed using a probe inserted into the sealed ear canal, which emits a low-frequency pure tone—typically 226 Hz for adults and children over 6 months, or higher frequencies like 1000 Hz for newborns—and systematically alters the air pressure from positive to negative values (usually ±200 to -400 daPa).[1] The resulting tympanogram, a graphical representation of immittance changes, is classified into types such as Type A (normal, single-peaked with peak pressure near 0 daPa), Type B (flat, indicating fluid or perforation), Type C (shifted peak, suggesting negative middle ear pressure), Type As (shallow peak, stiff system), and Type Ad (deep peak, flaccid or discontinuous system).[2] Multi-frequency tympanometry, using probe tones from 220 to 2000 Hz, enhances diagnostic precision by identifying resonant frequencies and patterns like 1B1G or 3B3G, which help differentiate subtle pathologies such as ossicular fixation or inner ear involvement.[2] Clinically, tympanometry is integral to hearing screenings and differential diagnoses, often combined with otoscopy, pure-tone audiometry, and acoustic reflex testing.[3] In pediatric populations, it serves as a second-stage tool following failed otoacoustic emissions or pure-tone screens, with referral criteria including equivalent width greater than 250 daPa or static admittance below 0.2 mmho to detect conductive hearing loss risks.[3] For adults, it aids in preoperative assessments for conditions like large vestibular aqueduct syndrome and monitors treatment efficacy in chronic middle ear diseases.[2] Its non-invasive nature, quick administration (under 1 minute per ear), and reliability across ages make it indispensable, though interpretation requires consideration of age-specific norms and potential confounders like cerumen occlusion.[4]Overview
Definition and Purpose
Tympanometry is an objective, non-invasive diagnostic test that measures the acoustic immittance, specifically the admittance or compliance, of the tympanic membrane and middle ear system as a function of varying air pressure introduced into the external ear canal.[1] This assessment is achieved by delivering a low-intensity probe tone through a sealed ear probe while systematically altering the pressure, allowing quantification of the middle ear's mechanical response without requiring active patient participation.[5] The primary purpose of tympanometry is to evaluate key aspects of middle ear function, including static middle ear pressure, eardrum stiffness or flaccidity, the presence of fluid or effusion, Eustachian tube patency, and the integrity of the ossicular chain.[1] Clinically, it aids in diagnosing conditions such as otitis media with effusion, Eustachian tube dysfunction, and conductive hearing loss by identifying deviations from normal middle ear mechanics that may contribute to these disorders.[6] For instance, a normal Type A tympanogram, characterized by a sharp peak at ambient pressure, indicates healthy eardrum mobility and middle ear function.[1] Unlike subjective hearing tests such as audiometry, which measure auditory thresholds and neural responses to sound, tympanometry focuses exclusively on the mechanical and acoustic properties of the middle ear, providing complementary objective data independent of patient cooperation or behavioral responses. Typically, the test employs a 226 Hz probe tone frequency for adults and children over 6 months of age to assess compliance effectively in this population.[7] For neonates and infants under 6 months, multifrequency tympanometry variants, often using higher frequencies like 1000 Hz, are preferred to account for the immature middle ear structure and improve diagnostic accuracy.[8]History
The physical principles underlying tympanometry originated in 19th-century physics, with the concept of impedance first coined by Oliver Heaviside in electrical engineering in 1886. This was later extended to acoustic systems by John Webster in 1919, conceptualizing impedance as opposition to sound flow in analogous mechanical terms.[9] This foundational analogy laid the groundwork for later measurements of middle ear function, though clinical applications remained undeveloped for decades.[10] In the 1950s, impedance audiometry emerged in Sweden through the work of Knut Terkildsen, who investigated pressure variations' effects on ear drum impedance using early electroacoustic bridges.[11] Terkildsen coined the term "tympanometry" in 1964 to describe the measurement of impedance changes with ear canal pressure variations.[12] The first commercial tympanometers appeared in the early 1960s, enabling routine clinical use of these devices.[13] During the 1960s, James Jerger refined the technique in the United States, introducing the influential classification of tympanogram types A, B, and C based on peak compliance and pressure patterns in 1970.[10] By the 1970s, tympanometry achieved widespread adoption in audiology for screening otitis media, with the American Speech-Language-Hearing Association (ASHA) establishing guidelines that promoted its routine integration into clinical protocols and addressed instrumentation standards.[1] The decade also saw initial explorations of multi-frequency approaches, though full clinical implementation lagged.[13] In the 1980s, the field evolved from manual electroacoustic impedance bridges to automated, computer-controlled tympanometers, improving measurement precision and efficiency.[14] The 1990s introduced multifrequency and high-frequency tympanometry, particularly using 1000 Hz probe tones, enhancing assessment accuracy for neonates and infants by better accounting for their immature middle ear systems.[15]Principles and Equipment
Underlying Physics
Tympanometry assesses middle ear function by measuring the acoustic immittance of the ear, which includes acoustic impedance (Z), defined as the opposition to acoustic energy flow, and its reciprocal, acoustic admittance (Y = 1/Z), which quantifies the ease of sound energy conduction into the middle ear.[16] Compliance, a component of admittance, specifically describes the mobility or ease of movement of the tympanic membrane and ossicles in response to pressure variations, typically expressed in equivalent volume units such as acoustic millimhos (mmho), where 1 mmho approximates 1 mL at low frequencies.[17] This measurement relies on the physical properties of the middle ear as a transmission system that couples airborne sound to the cochlea via mechanical vibrations. The core physics involves delivering a low-frequency probe tone, commonly 226 Hz at moderate intensity (≤90 dB SPL), into the sealed ear canal, where it partially reflects off the tympanic membrane; the reflected sound is captured by a microphone to compute immittance based on the ratio of sound pressure to volume velocity.[16] Variations in ear canal air pressure, typically swept from +200 daPa to -400 daPa, modify the trans-tympanic pressure gradient, altering middle ear volume and the stiffness or mass loading on the eardrum, which in turn affects sound transmission and reflection.[17] The resulting admittance is a complex quantity, Y_a = G + jB, where G (conductance) represents the real part associated with energy dissipation and absorption, and B (susceptance) the imaginary part linked to reactive energy storage due to stiffness and mass reactance; the magnitude |Y_a| peaks at the pressure yielding maximum compliance, often near ambient levels in healthy ears.[17] The middle ear operates as a resonant system, with natural resonance around 1,100 Hz in adults, where impedance minimizes and sound transmission maximizes (up to 22 dB gain).[17] Negative middle ear pressure, arising from Eustachian tube dysfunction that impairs pressure equalization between the middle ear and nasopharynx, increases eardrum stiffness, shifts the admittance peak toward negative pressures, and reduces low-frequency energy transfer.[17] Fluid accumulation in the middle ear adds mass, diminishing tympanic membrane mobility and producing a flat admittance curve without a peak, as the increased inertial loading dampens pressure-induced motion.[17] Stiffness in the ossicular chain, such as from fixation disorders, elevates baseline impedance and conductance, particularly at low-to-mid frequencies, thereby lowering overall compliance and altering the resonant frequency upward.[17] For instance, a normal Type A tympanogram peak at ambient pressure exemplifies these balanced acoustic-mechanical principles.[18]Components of a Tympanometer
A tympanometer consists of several key hardware components designed to measure middle ear function through controlled variations in ear canal pressure and sound transmission. The probe assembly is the primary interface, featuring a soft rubber tip that forms an airtight seal in the external auditory canal to prevent pressure leaks; it integrates a loudspeaker (or driver) to generate the probe tone, a sensitive microphone to detect reflected sound pressure levels, and a pressure port connected to the pneumatic system for air delivery and extraction.[1][19] The pump, typically an electromechanical device, systematically varies air pressure within the sealed canal, often across a range of -400 to +200 daPa to simulate physiological conditions without discomfort.[19][1] Supporting these are an amplifier to maintain stable probe tone output via automatic gain control and a digital processor that analyzes microphone signals to compute acoustic admittance (or immittance) as a function of pressure, generating graphical outputs such as tympanograms plotting admittance against pressure in decaPascals (daPa).[19] Tympanometers are categorized by probe tone frequency and form factor to suit different clinical needs. Conventional devices use a single-frequency probe tone, most commonly 226 Hz for adults and older children, which effectively assesses tympanic membrane mobility in typical middle ear systems.[1] Multifrequency tympanometers, including wideband variants, employ multiple tones such as 226 Hz, 678 Hz, and 1000 Hz (spanning up to 2000 Hz) to evaluate middle ear resonance and detect subtle ossicular or fluid-related abnormalities with greater sensitivity than single-frequency models.[2] Devices also vary in portability: handheld, battery-operated units offer flexibility for bedside or field use, while desktop models provide enhanced processing power, larger displays, and integration with audiometric systems for comprehensive testing.[20] Operational specifics ensure reliable measurements, with the probe tone typically delivered at an intensity of 70-85 dB sound pressure level (SPL) to probe the middle ear without exceeding safe limits (≤90 dB SPL).[1][20] Modern units incorporate automated seal detection, which monitors initial pressure stability or sound reflection to confirm an airtight fit before testing proceeds, reducing errors from poor probe placement.[19] Outputs are digitally rendered, displaying curves of acoustic admittance versus pressure to quantify parameters like peak compliance and equivalent ear canal volume. Calibration is essential for accuracy and is performed regularly to verify component performance. Daily checks involve inserting the probe into standard calibration cavities (e.g., 0.5 cm³, 2.0 cm³, 5.0 cm³) to confirm equivalent volume measurements within ±5% or ±0.1 cm³, with adult ear canal volumes typically ranging 0.6-2.2 cm³ (often calibrated against a 1.5-2.0 cm³ standard).[21][19] Pressure accuracy is validated annually using manometers, ensuring variations within ±10-15 daPa across the operational range, while probe tone levels are assessed in a 2 cm³ coupler with a sound-level meter.[1][21] These procedures align with standards like ANSI S3.39 and BS EN 60645-5 to maintain clinical precision.[1]Procedure
Patient Preparation
Prior to undergoing tympanometry, patients receive clear instructions to ensure cooperation and accurate results. The procedure is explained as a quick, non-invasive test that involves inserting a soft probe tip into the ear canal for 10-30 seconds per ear, during which they should remain still and quiet without swallowing or speaking.[22][23] Informed consent is obtained, emphasizing the test's painless nature and the patient's right to stop if discomfort occurs.[23] The external auditory canal must be clear and dry, with otoscopy performed beforehand to assess for cerumen impaction or other obstructions. If cerumen is present and not excessive, it may be removed by a qualified professional to avoid invalid results; however, impacted cerumen requires referral for safe removal prior to testing.[19][24] Patients are advised to avoid testing if they have had recent ear surgery (within 2 months) or active infection, unless cleared by an otolaryngologist.[23] Positioning is tailored to promote stability and minimize movement. Adults and older children are seated comfortably with their head stabilized, while young children may sit on a caregiver's lap for reassurance, with the head gently held against the caregiver's chest.[23][25] For infants, testing occurs shortly after feeding or during natural sleep to reduce fussiness, often with swaddling or a pacifier to maintain quietness; a supine position may be used if needed for calm.[25] Contraindications include active ear drainage (otorrhoea), tympanic membrane perforation (due to risk of pressure imbalance), acute otitis media, severe ear pain or tenderness, and foreign bodies in the canal.[23][19] An allergy check for probe tip materials, such as latex, is recommended if the patient has known sensitivities.[25] A proper probe seal is essential, as a poor seal can lead to invalid measurements.[19]Step-by-Step Process
The tympanometry procedure involves a series of automated steps to measure middle ear function by varying air pressure in the ear canal while monitoring acoustic admittance. The test is conducted using a handheld tympanometer with a probe that delivers a low-intensity probe tone and controls pressure changes.[26][23] Guidelines, such as the British Society of Audiology's 2025 recommended procedure (minor revision of 2024), specify variations like faster pressure sweep rates up to 600 daPa/s for infant screening.[23] To begin, the clinician selects an appropriately sized disposable probe tip based on the patient's ear canal and inserts the probe gently into the external ear canal, pulling the pinna up and back in adults or down and back in children to straighten the canal. An airtight seal is confirmed when the device displays a green light, emits a tone, or shows a stable pressure indicator and full movement of an on-screen arrow; if a red light or error appears indicating a leak or poor fit, the probe is repositioned or a different tip size is used. Once sealed, the probe tone—typically 226 Hz for patients over 6 months—is activated to emit a continuous sound into the ear canal.[26][23] The pump then automatically varies the air pressure in the sealed canal, starting from ambient pressure and sweeping positively to +200 daPa before decreasing to -300 daPa in adults or -400 to -600 daPa in children and neonates, at a rate of 50 daPa per second in adults or up to 200 daPa per second in some protocols. This pressure variation alters eardrum mobility and middle ear admittance, which the device records continuously as a tympanogram tracing the changes in acoustic admittance (measured in mmho or ml) relative to pressure. The sweep typically lasts 5-10 seconds per ear, during which the patient is instructed to remain still and may swallow if pressure feels uncomfortable, though the procedure is generally painless.[26][23] Upon completion, the pressure automatically returns to ambient levels, signaled by a flashing probe light, and the probe is removed carefully to avoid discomfort. The process is then repeated for the contralateral ear. If movement or noise artifacts occur during the test, indicated by error codes or an unstable tracing, the measurement is repeated after repositioning. Variations include using a high-frequency probe tone of 1000 Hz for neonates under 6 months to better assess immature middle ear systems, and an optional acoustic reflex add-on, where loud broadband or tonal stimuli (starting at 60-70 dB HL and increasing in 5 dB steps) are presented to measure stapedius muscle contraction via admittance changes of at least 0.02 ml.[26][23]Interpretation
Types of Tympanograms
Tympanograms plot the relationship between changes in air pressure in the external ear canal and the mobility (compliance or admittance) of the tympanic membrane and middle ear system. The x-axis represents pressure in decaPascals (daPa), typically ranging from -400 to +200 daPa, while the y-axis measures acoustic admittance in milliliters (mL) or millimhos (mmho), reflecting the ease with which sound energy passes through the middle ear. Key quantitative features include the height of the peak (maximum compliance, normally 0.3-1.6 mL), the pressure at the peak (tympanometric peak pressure, TPP), and the width of the peak, which indicates the sharpness of the response.[27][28] The most widely used classification system for tympanograms was developed by Jerger in 1970, categorizing curves based on peak location and shape to identify middle ear status. Type A represents a normal configuration, featuring a sharp peak near ambient pressure (TPP between -100 and +100 daPa, often -20 to +10 daPa in healthy ears) with compliance in the 0.3-1.6 mL range, indicating balanced middle ear pressure and normal tympanic membrane mobility. Type B shows a flat curve with no discernible peak across the pressure range, reflecting absent or minimal compliance changes due to middle ear fluid or tympanic membrane perforation; this pattern occurs in approximately 80% of cases of otitis media with effusion. Type C displays a peaked curve similar to Type A but shifted to negative pressures (e.g., TPP around -200 daPa), suggesting retracted tympanic membrane from Eustachian tube dysfunction.[29][30][31] Variations on the normal Type A include Type As and Type Ad. Type As exhibits a shallow peak (compliance below 0.3 mL) at near-normal pressure, indicating a stiffened middle ear system with reduced mobility. Type Ad features a deep, exaggerated peak (compliance above 1.6 mL) at normal pressure, signifying excessive compliance from a flaccid tympanic membrane or ossicular chain discontinuity. These subtypes provide nuanced insights into middle ear mechanics beyond the basic A, B, and C patterns.[29][27][30] Multifrequency tympanometry extends traditional single-frequency (226 Hz) testing by using multiple probe tones (e.g., 226-678 Hz) to generate frequency-specific curves, enabling calculation of tympanometric gradients—the width of the curve at half its peak height, which quantifies the sharpness of the response and helps detect subtle abnormalities like ossicular fixation or high-frequency stiffness not evident in low-frequency tests.[32][2]| Type | Description | Key Features |
|---|---|---|
| A | Normal middle ear function | Peaked at 0 daPa (±100 daPa), compliance 0.3-1.6 mL |
| B | Flat curve, no peak | Indicates fluid or perforation; common in ~80% of otitis media with effusion cases |
| C | Shifted negative peak | TPP e.g., -200 daPa; suggests Eustachian tube dysfunction |
| As | Shallow normal peak | Compliance <0.3 mL; stiff system |
| Ad | Deep normal peak | Compliance >1.6 mL; flaccid or disarticulated ossicles |