Tuning fork hearing tests measure hearing by which of the following mechanisms?

As a friend and former medical school colleague of Dr. Weatherall, it
gives the senior author, in collaboration with colleagues, great pleasure
in proffering some assistance in his quest for an answer to the Weber
test1.

Classically the Weber test can be useful in detecting a unilateral
sensorineural hearing loss or a unilateral conductive hearing loss. In
patients with a unilateral sensorineural hearing loss, a positive Weber
result is obtained in the contralateral normal ear, with sounds being
heard louder on this side. When a unilateral conductive hearing loss is
present a positive test result is obtained in the affected ear, where
sounds are perceived louder despite the hearing deficit. However, as Dr.
Weatherall points out, the intensity of sounds reaching the cochlea
transmitted via the bony skull should be equal in both ears when inner ear
function is normal. In addition the normal ear should be receiving
additional sound energy via the air conduction route. So how can one
explain a positive Weber test in patients with a unilateral conductive
loss?

There are four explanations for the Weber phenomenon. The first
suggests it is due to a masking effect. That is, sound heard in the
blocked ear seems louder than in the normal ear as there is less
background noise reaching the cochlea via air conduction. Thus sound
heard in a background of silence seems louder than the same sound obscured
by excessive ambient noise. The second more physiological explanation is
based on the fact that there are more efferent fibres than afferent fibres
in the cochlear nerve. Air conduction of sound into the blocked ear is
reduced, resulting in a discrepancy between the sound energy reaching the
two cochleas. This inequality is detected in the brainstem nuclei, and a
compensatory mechanism is activated which increases the innate gain of the
cochlea on the side receiving less stimulation. Any sound energy reaching
the cochlea directly via skull vibration is consequently amplified.
Thirdly, it is now known that when sound enters the ear most is
transmitted via the cochlea along the central pathways to the auditory
cortex, but outer hair cell vibrations cause a significant amount of
"reflected" sound, the so-called "cochlear echo" described by Kemp in
19782. This transient evoked otoacoustic emission can be recorded in the
ear canal using a microphone, and would normally dissipate outwards into
the air. A conductive hearing loss due, for example, to fixation of the
ossicular chain or occlusion of the external meatus will prevent this
external dissipation of sound energy, resulting in increased cochlea
stimulation and apparent increased loudness. Lastly, the skull acts as a
resonating chamber. Any change in the properties of this chamber, such as
occlusion of the external canal with wax, will change the resonance of
sound within the medial part of the canal.

Although these explanations give some credence to the Weber
phenomenon, clinical trials have demonstrated that the test is highly
unreliable. Bilateral hearing impairment understandably causes major
difficulties in interpretation. However, even in patients with a
documented unilateral hearing impairment 30 per cent will refer the Weber
to the midline. Moreover, of 70 per cent of patients with a positive
Weber about 25 per cent have been shown to lateralise the test result to
the incorrect ear 3,4. Furthermore, patients with cochlear hearing losses
display the phenomenon of recruitment, in which sounds are perceived as
abnormally louder in the affected ear due to a defect of the inner ear
dampening mechanisms. Providing the level of sensory hearing impairment
is only modest, a positive Weber result may in fact be obtained with an
early sensorineural hearing loss as well.

In reality, the Weber test is mainly used in our department as a
crude method of excluding a post-operative dead ear on the morning ward
round, and of course for testing the ingenuity of the Senior House Officer
in finding a tuning fork on the surgical ward. Patients presenting to the
Out-Patient department undergo a pure tone audiogram for more accurate
assessment of their hearing loss.

Kay Seymour
Senior SHO

Natalie Brookes
Specialist Registrar,

Simon Lloyd
Specialist Registrar,

Hesham Saleh
Consultant.

Department of Otolaryngology/Head & Neck Surgery,
Charing Cross Hospital, Fulham Palace Road,
London SW6 8RF.

Contact e-mail address:

No competing interests.

References

1. Weatherall MW. The mysterious Weber's test. BMJ 2002;325:26.

2. Kemp DT. Stimulated acoustic emissions from within the human auditory
system. J Acoust Soc Amer 1978;64:1386-91.

3. Stankiewicz JA, Mowry HJ. Clinical accuracy of tuning fork tests.
Laryngoscope 1979;89(12):1956-63.

4. Browning GG. Clinical Otology and Audiology (2nd Ed). 1998 Butterworth
Heinmann: 12-13.

Which tuning fork to use for hearing test?

Which tuning fork test evaluates conductive and sensorineural hearing loss?

How is the mechanism of hearing different if the handle of the tuning fork is held on the mastoid process instead of holding the tuning fork tines near the ear canal quizlet?

How is the sound transmitted from the tuning fork to your inner ear when you place it on the mastoid process?