Immittance Testing and Tympanometry

Acoustic immittance testing objectively assesses the middle ear by measuring how readily sound energy passes into it. Tympanometry, its central component, varies air pressure in the sealed ear canal and records the resulting change in admittance, producing a tympanogram whose shape and peak pressure reflect the mobility of the eardrum and the state of the middle-ear space. Acoustic reflex measurement complements it by recording the middle-ear muscle response to loud sound.

Definition

Acoustic immittance testing measures the ease or opposition with which acoustic energy flows into the middle ear; tympanometry records middle-ear admittance as ear-canal pressure is varied, and acoustic reflex measures record the stapedius muscle response to intense sound.

Scope

This entry covers the immittance concept (admittance and impedance), the tympanogram and its common patterns, ear-canal volume, the acoustic reflex and its thresholds and decay, and the newer wideband acoustic immittance measures. It is a reference description of the methods and does not provide clinical interpretation for individuals.

Core questions

• How mobile is the tympanic membrane and middle-ear system across changes in ear-canal pressure?
• Is the middle-ear pressure normal, negative, or is there evidence of effusion?
• Are the acoustic reflexes present, and at what levels, on each side?
• How does ear-canal volume help interpret a flat tympanogram?

Key concepts

• Acoustic admittance and impedance
• Tympanogram
• Peak pressure and static admittance
• Ear-canal volume
• Probe tone frequency
• Acoustic (stapedial) reflex threshold
• Acoustic reflex decay
• Wideband acoustic immittance

Mechanisms

A probe sealed in the ear canal delivers a probe tone and varies canal pressure across a range while a microphone tracks the sound level, from which the instrument computes admittance. Admittance is greatest when pressure on both sides of the eardrum is equal, so the tympanogram peaks near the prevailing middle-ear pressure; a shifted peak suggests altered middle-ear pressure and a flat trace suggests reduced mobility, with ear-canal volume distinguishing a sealed effusion from an open perforation or patent tube. The acoustic reflex is the contraction of the stapedius muscle to loud sound, which stiffens the ossicular chain and reduces admittance; its presence, threshold level, and pattern across ipsilateral and contralateral stimulation give information about the conductive system and the reflex arc (Jerger 1970). Wideband acoustic immittance extends the principle across a broad range of frequencies rather than a single probe tone, improving sensitivity to middle-ear conditions including in infants (Hunter et al. 2013).

Clinical relevance

Immittance testing provides an objective, quick assessment of middle-ear function that complements behavioural audiometry and helps localize a conductive component identified on the audiogram. Because it does not require a behavioural response, it is useful across ages, including in young children. This entry describes how the measures work; it is not a basis for individual diagnosis or treatment.

Epidemiology

Middle-ear conditions, especially effusion in children, are extremely common, and tympanometry is a primary objective screen for them; immittance measures are accordingly a routine part of audiologic screening and diagnostic batteries. Wideband measures have expanded the approach, particularly for infant assessment (Hunter et al. 2013).

History

Clinical measurement of middle-ear acoustic impedance grew from mid-twentieth-century work on the acoustic reflex, and Jerger's 1970 report established impedance (immittance) audiometry, including standard tympanogram classifications, in routine clinical practice. Guidelines later incorporated immittance into screening protocols (ASHA 1997), and wideband acoustic immittance emerged as a frequency-rich extension of the method (Hunter et al. 2013).

Key figures

• James Jerger
• Otto Metz
• Lisa Hunter

Seminal works

• jerger-1970

Frequently asked questions

What does a tympanogram show?

It plots how the admittance of the middle ear changes as air pressure in the ear canal is varied, and the location of its peak and its overall shape reflect the mobility of the eardrum and the pressure within the middle-ear space.

What is the acoustic reflex?

It is the reflexive contraction of the stapedius muscle in response to a loud sound, which stiffens the middle-ear system and momentarily reduces its admittance; its presence and threshold give information about the conductive system and the reflex pathway.