Objective Tests of Hearing Tympanometry & Acoustic Reflexes.

Objective Tests of Hearing

Tympanometry & Acoustic Reflexes

Introduction

The clinical value of measuring middle ear function with aural impedance techniques dates back to Otto Metz in 1946

Today, Measurements of acoustic immittance have become a routine test as are puretone audiometry and speech audiometry.

Acoustic immittance testing primarily includes tympanometry and acoustic reflex audiometry.

Immitance battery

Administration of the complete immittance test battery provides a quick and relatively inexpensive objective assessment of the status of the middle ear, cochlea, VIIth and VIIIth nerves, and lower brainstem.

Some consider the immittance test battery is much more sensitive to middle ear disorders than the determination of air-bone gaps.

If the results on all the middle ear immittance subtests are within normal limits in the presence of a hearing loss, the hearing loss is sensorineural in nature, and some feel that BC testing does not need to be performed.

Tympanometry

Is a valuable diagnostic tool.Is a simple, reliable, non-invasive, and

objective means of assessing the function of the ossicular chain, Eustachian tube, tympanic membrane and the interrelation of these parts.

Can be used with young children and uncooperative adults.

What is tympanometry?An objective measurement of middle-ear

mobility and middle-ear pressure through the use of sound (probe tone) and air pressure.

Is a way of assessing the manner in which energy flows through the outer and middle ears to the Cochlea.

Plotted as a tympanogram.A tympanogram is a graphic representation of

the change in compliance (Y or vertical axis) of the middle-ear system as air pressure (X or horizontal axis) is varied.

Tympanometer

There are different types of tympanometry...

Vector tympanometry Multi-frequency tympanometry Multi-component tympanometry

Vector tympanometry is the most commonly used. Employs a single, low frequency (220 or 226

Hz) probe tone.

Normal Tympanogram

Procedure for vector tympanometry

The test is administered after appropriate otoscopic examination is performed to rule out any external ear canal pathologies, ear discharge, tympanic membrane perforation, etc.

Instructions to the patient

Include information regarding what you are about to do, what you expect them to do, what they might hear, and that they should inform you when intolerable pain or intense discomfort is experienced.

Inform them that this test tells us how well your eardrum, the bones in your middle ear, and the muscles in their middle ear work.

Instructions to the patient

Instruct them that they will feel some pressure in their ear similar to when they go up in an elevator.

Tell them that this test does not require any kind of responses from them and that they should try to sit very still and try not to talk, as any movement or sound can affect the test results.

Testing

To begin testing, a probe is fitted with an appropriately sized soft ear tip and is positioned at the opening of the ear canal to obtain an air-tight seal.

For this the tester gently pulls up and back on the pinna to straighten the ear canal and inserts the probe tip into the opening of the ear canal. Proceed by directing the probe tip inwards with a

slight twisting motion.

Vector Tympanometry

By introducing a known amount of sound energy to the ear, a measurement of the energy not transmitted (i.e., reflected back into the canal) into the middle-ear system is recorded.

The amount of sound energy transmitted (admittance) into the middle ear is equal to the amount of sound energy introduced, minus the amount of sound energy that returns to the probe microphone.

Thus more the sound reflected back, less the sound admitted into the middle ear.

This sound energy reflected back is calculated while the air pressure in the ear canal is varied (typically from + 200 dapa to -400 dapa).

Major components of a clinical acoustic immitance device

Maximal sound transmission through the middle ear system occurs when the air pressure in the ear canal is the same as the air pressure in the middle ear space. In a normal middle ear system, this is typically at 0 dapa.

When the air pressure in the ear canal is more (+ pressure) than or less (- pressure) than the air pressure in the middle ear space, the transmission of sound through the middle ear is significantly reduced.

This is depicted at the tails (200 and –200 daPa) of a normal tympanogram.

Vector Tympanometry

Normal Tympanogram

Vector Tympanometry

The presence of middle ear disorders alters the tympanogram in predictable ways so that the various middle ear pathologies correspond to various tympanometric shapes.

Low admittance measurements (low peak) indicate a stiff or obstructed middle ear (as seen in cases with middle ear fluid).

Of course, the opposite is true for a high amount of energy transmission, indicating a flaccid or highly mobile system (Ex, fractured or discontinuous ossicle chain).

Diagnostic Indices in Tympanometry

Three main indices - Static Acoustic Admittance Ear Canal Volume Gradient or Width

Another common index that is used by a few Middle-ear Peak Pressure (TPP)

Static Acoustic Admittance

Energy flow into the system is reported as static acoustic admittance (YTM).

Static admittance is an estimate of the admittance at the lateral surface of the tympanic membrane, excluding the effects of the ear canal volume.

Static admittance is sensitive to many middle ear conditions.

What affects YTM?

Gender (women have lower admittance).

Age (children are lower than adults).Norms:

Children (3 - 10 Yrs): 0.25 to 1.05 mmho or ml

Adults: 0.3 to 1.7 mmho or ml

(where mmho is the unit for acoustic admittance)

How does pathology affect YTM?

Reduced: less energy is flowing into the middle ear system. Something has made the system stiff. Otitis media (fluid in the middle ear), Otoschlerosis,

etc.

Increased: abnormally more energy is flowing in. debris on TM (bugs!), external otitis, ossicular chain

discontinuity, scarring of tympanic membrane.

Examples of tympanogram for ears with low admittance

Example of a tympanogram showing high admittance

Static Acoustic Admittance

Tympanometric Width or Gradient

Is a measure of how broad the tympanogram is.

Tympanometric Width Measured in daPa. Breadth of tympanogram at 1/2 distance from

peak to base.Norms

Children (3 - 10 Yrs): 80 - 159 dapa Adults : 51 - 114 dapa

Ear canal volume ( Vea or ECV )

Estimate of volume between probe tip and tympanic membrane.

In the presence of a flat tympanogram, an estimate of the volume of air in front of the probe can be useful for detecting eardrum perforations, evaluating the patency of tympanostomy tubes, and evaluating the effects of cerumen in the ear canal.

The ear canal volume values may be abnormally large, abnormally small, or within normal limits.

What is a normal ear canal volume?

Children: 0.3 to 0.9 cm3

Adults: 0.9 to 2.0 cm3

What if you see ear canal volumes that aren’t WNL?

What does an excessively large volume indicate? Tympanic membrane perforation. In children, if they have tubes (PE tubes –

pressure equalization tubes)

What if you see ear canal volumes that aren’t WNL?

What does a reduced measure indicate? Mass in the external auditory canal (such as tumors,

excessive cerumen, foreign bodies, etc.) Could also be because the probe tip is not placed

correctly & is close to the external auditory canal wall.

In the presence of a flat tympanogram, normal or near normal ear canal volume measurements may be indicative of a fluid filled or tissue-filled middle ear cavity.

Tympanic Peak Pressure (TPP)

The TPP is the ear canal air pressure at which the peak of the tympanogram occurs.

Ideally, TPP for a normal ear is 0 dapa (i.e., pressure is equal in both sides of the tympanic membrane. Is typically within +50 and –50 dapa for normal ears

Although used to confirm the results of other tests, in the absence of other tympanometric, audiometric, or otoscopic abnormality, negative middle ear pressure alone is not an indication of a significant middle ear disorder.

Tympanic Peak Pressure (TPP)

Abnormally negative tympanometric peak pressures are associated with eustachian tube disorders that can occur either with or without the presence of middle ear fluid.

Ears with Otitis media have a significant negative pressure (> -100 dapa).

A negative pressure is also seen with other eustachian tube pathologies (closed eustachian tubes due to some upper respiratory tract infection, inflamed tonsils/adenoids)

Negative TPP

Normal Tympanogram with indices

Summary

Tymanometry is a valuable diagnositic tool for assessing middle ear status.

The indices we use in tympanometry are: static acoustic admittance tympanometric width ear canal volume

Acoustic Reflexes -Introduction

The action of the two middle ear muscles (tensor tympani & stapedius) affect the immittance of the middle ear system.

Although the role of the tensor tympani is still not clearly understood, the effect of reflexive contraction of the stapedius muscle for acoustic stimuli is generally accepted. Acoustic reflex

Acoustic reflex

The acoustic reflex is controlled by a reflex arc or pathway.

The simplest stapedius reflex arc involves the following structures – spiral ganglion neurons (in the cochlea), the auditory nerve, the cochlear nucleus, the superior olivary complex, the facial nerve nucleus, the facial nerve, and the stapedius muscle.

Acoustic Reflex Pathway

Acoustic reflex

Because the stapedius reflex involves several nerves and brainstem connections, it can be abnormal in a variety of situations that may not necessarily impair hearing.

Acoustic reflex

Most normal hearing individuals will effect a bilateral acoustic reflex when pure tones are introduced to either ear at 70 - 100 dB above threshold (~ 85 dB SL).

This acoustic reflex results in stiffening of the tympanic membrane. Most commercial immittance

instruments allow for measuring the ipsilateral and contralateral acoustic reflexes.

Acoustic reflex

Acoustic reflexes are recorded at a single air pressure setting (ie, the pressure setting that provided the peak immittance reading for that particular ear on the tympanogram).

Ear canal pressure is maintained at that specific setting, while tones of various intensities are presented into the ear canal and immittance is recorded. pure tones 500 Hz - 4000 Hz A significant change (>0.02 ml) in middle ear immittance

immediately after the stimulus is considered an acoustic reflex. Acoustic reflex threshold – lowest intensity that results in an

acoustic reflex.

Interpreting the Acoustic Reflex

Different types of auditory pathologies affect the acoustic thresholds differently. Conductive hearing impairment - Acoustic

reflex threshold (ART) is either elevated or absent.

Senorineural pathologies - ART is elevated, decreased or absent.Cochlear pathologies generally decrease the

threshold while neural pathologies elevate or abolish the acoustic reflex.

Interpreting the Acoustic Reflex

Acoustic reflex testing can help us cross check our findings in a pediatric evaluation or as an indication of normal hearing or potential hearing loss in difficult to test populations.

It is important to remember that the absence of an acoustic reflex by itself cannot be interpreted to confirm the presence of a hearing loss. Acoustic reflexes may not be present in some

otherwise normal hearing individuals.