Pure Tone Audiometry (PTA) | Types of Hearing Loss & Degree of Hearing Loss : An Overview

PURE TONE AUDIOMETRY -

• A pure tone is atone having a single specific frequency. The frequency of the tone is determined by the rate or speed at which the sound source vibrates
• Pure tone audiometry is the use of pure tone to assess an individual 's hearing.
• The result of this testing are plotted on the audiogram.
• Pure tone are generated by an audiometer and presented to the patient via headphones or, in some cases, through loudspeakers.
• Can be air conduction audiometry or bone conduction audiometry

AIR CONDUCTION MEASUREMENT 

• To determine the type of hearing loss present by comparison of the air & bone conduction thresholds

• To determine the magnitude of hearing loss.

• To detect the presence of functional hearing loss.

• To monitor the effectiveness of medical intervention by

• comparing pre & post treatment AC thresholds.

• To assess the need for & the benefit from amplification

• Bone Conduction measurement

• To determine the type of hearing loss present by comparison of the air & bone conduction thresholds.

Bone Conduction Testing Measures The Sensorineural Sensitivity -

Degree & Severity of Loss                     Degree Of Hearing Loss (dbHL)

     NORMAL                                                           -10 to 15

     MINIMAL                                                            16 to 25

     MILD                                                                  26 to 40

  MODERATELY                                                         41 to 55      

  
MODERATELY SEVERE                                               56 to 70 

     SEVERE                                                              71 to 90

     PROFOUND                                                             90+
       

TYPES OF HEARING LOSS

1. Conductive Hearing Loss

2. Sensorineural Hearing Loss

3. Mixed Hearing Loss

4.Functional Hearing loss

5. Central Hearing Loss

6. Conductive Hearing loss

*Patient shows losses by air conduction but normal hearing by bone conduction.

  Air Bone gap(ABG) at least 10DBHL.

*AC  loss generally does not exceed 60dBHL.

  If the threshold level exceeds 60 dbHL , the problem cannot be purely conductive.

CONDUCTIVE HL :-

• AC threshold > 15 dB                        
• BC threshold < 15 dB
• Sensorineural Hearing Loss
• AC and BC thresholds are outside the normal limits.
• Air Bone gap(ABG) does not exceed 10 dB (<=10)
• “Sensory” hearing loss is applied when the damage is localised in the inner ear
• “Neural” hearing loss is the correct term to use when the damage is in auditory nerve.

SENSORINEURAL HL :-

• AC threshold > 15 dB
• BC threshold > 15 dB
• ABG <= 15 dB
• Mixed Hearing loss
• AC and BC both are outside the normal limits.
• Air Bone Gap(ABG) greater than 10dB(>10).

MIXED HL :-

AC threshold > 15 dB

BC threshold > 15 dB

ABG > 15 dB

FUNCTIONAL HEARING LOSS :-

• The patient may neither seem to hear or respond ;yet the handicapped may  not caused by any organic pathology in the peripheral or central auditory pathways.
• The hearing difficulty may have an entirely emotional /psychological etiology.
• In some cases it may be feigned or simulated and the patient is conscious of the deception.

CENTRAL HEARING LOSS :-

Some patient cannot interpret or understand what is being said and that the cause of the difficulty is not in the peripheral mechanism but some where in the central nervous system.

In this loss, the problem is not in lowered pure tone thresholds but in the patient’s ability to interpret what he hears.

PROCEDURE

Instructions

Threshold: 

Differential threshold

Absolute threshold

Method: Modified method of limits.

Modified Hughson – Westlake down-up procedure (Hughson & Westlake, 1944; Carhart & Jerger, 1959; ASHA, 2005)

Modified Hughson – Westlake down-up procedure

Testing begins with better ear (right ear if both ears are identical). Tonal duration is 1 - 2 seconds for puretone stimulation.

If patient complaints of reduced hearing, start the test  at 70 dBHL otherwise start at 30 dBHL.

 

If listener doesn't respond raise the tone in 20-dB steps until a response is obtained.

After every response to tone, the level is decreased in 10-dB steps until there is no response (NR).

For subsequent  steps when there is NR, raise the level of tone by 5-dB steps until a response is obtained.

Follow this “down-10/up-5” rule few times for threshold estimate.

ASHA (2005) recommends that threshold should correspond to the level at which responses were obtained for two ascending runs out of three ascending runs.

Plot the threshold on audiogram as per ASHA (1990) guidelines for audiometric symbols.

INTERPRETATION :-

Degree of hearing loss: calculate pure tone average (PTA) of AC thresholds at 500, 1000 & 2000 Hz.

Type of hearing loss: compare the amount of hearing loss for AC & BC thresholds at same frequency.

Configurations of hearing loss.

The range of frequencies for air conduction testing generally 250 to 8000Hz while that for bone conduction testing is 250-4000Hz.

Normal range of AC is -10 to 15 dB

Normal Range of BC is -10 to 10dB

Types of Audiogram :-

1.Flat Audiogram

2.Gradually sloping Audiogram

3.Sharply sloping Audiogram

4.Precipitous sloping Audiogram

5.Rising Audiogram

6.Trough Audiogram

7.Saucer Audiogram

Flat Audiogram

Equal degree of hearing in all test

frequencies

Magnitude of difference not exceeding 5-

10dB

Usually associated with conductive

hearing loss such as Serous Otitis Media,

collapsed ear canal, moderately advanced

condition of Meniere’s Disease.

Gradually Sloping Audiogram

Loss begins at low frequency with a

gradual increase in the high frequencies.

■

At 500 Hz , a threshold of 25dBHL or

greater with an increase in threshold of

around 5-12 dB per octave.

■

The difference between the highest and

the lowest being no more than 35 dB

Sharply Sloping Audiogram

Normal or near to normal hearing in low

frequency with a threshold of 30 dBHL or

better at 500 Hz.

■

Between 500 and 1000 or 1000 and

2000 there is a drop in threshold of at

least 20 dB and the difference between

the highest and the lowest threshold is greater than 40dB.

Precipitous Sloping Audiogram

■ Initially shows a flat or gradually sloping

pattern with the threshold suddenly

increasing at rate of 25+dB per octave.

Rising Audiogram

Significant loss at low and mid –test

frequencies with relatively normal or near

to normal hearing in the high frequency

region

■

Usually seen inn conductive hearing loss

cases

■

May be seen in SN loss conditions such as

Meniere’s disease at early stage.

Trough Audiogram

There is a greater loss at the mid

frequencies of around 20dB or greater than

at the extreme frequencies.

Typically seen in some children with Rubella 

Saucer Audiogram

Opposite to trough pattern in that

there is 20dB or greater loss is

extreme frequencies than at the

mid frequencies.

Often associated with malingerers.