Letters

Weber's test demystified

BMJ 2002; 325 doi: https://doi.org/10.1136/bmj.325.7372.1117 (Published 09 November 2002) Cite this as: BMJ 2002;325:1117

Physics renders Weber's test not so mysterious …

  1. Chima E Mbubaegbu, consultant orthopaedic surgeon. (chima.mbubaegbu{at}ntlworld.com)
  1. Homerton University Hospital NHS Trust, London E9 6SR
  2. Department of Otorhinolaryngology, Faculty of Health Sciences, University of Stellensbosch, 7505 South Africa

    EDITOR—Weatherall's mystery—a positive Weber's test in the normal ear in unilateral sensorineural hearing loss but in the affected ear in unilateral conductive hearing loss—has baffled many neurologists and ear, nose, and throat surgeons for some time; I wonder whether my explanation would convince him.1

    The word conduction is used confusingly by ear, nose, and throat surgeons and neurologists to describe normal transmission of sound from the outside world to the ear. Everything apart from the sensorineural aspect of the hearing is thought to be conductive. The sound is normally conducted (transmitted) through the air through the external ear into the middle ear. This makes air a better sound conductor (when defined this way) than a solid object.

    It is, however, clear to any engineering student that bone or any denser object is a better conductor of sound than is air. When the tuning fork is placed directly on the bone, there is no significant sound transmission from the tuning fork directly through the air. The conduction being tested is that through bone to the inner ear.

    The air medium in the ear, being a less efficient transmitter of sound, results in sound energy loss at the interface of bone and air. The resultant sound energy to the inner ear is therefore less. If you have a more solid (denser) object in the ear (which would have resulted in conduction deafness (as defined by doctors) the sound conduction is actually better. Less energy is lost, and the sound is localised to that side in Weber's test.

    If both ears are blocked but with different materials with different conductive properties, positive results in Weber's test would localise to the side with the denser and therefore better sound conducting material. You could test this by blocking your ear with one finger and the other with another material, comparing the sounds and comparing each with air. Nice little study for a neurologist, I say.

    References

    … and a collaborative group of otorhinolaryngologists reports its findings

    1. Gary Kroukamp, registrar On behalf of Carel Van Wyk, Adriaan Pentz, Ola Basson, Zak Mansab, Amal Alabdulla, Rory Attwood, James Loock, and George Browning (visiting professor), all of the ear, nose, and throat department.
    1. Homerton University Hospital NHS Trust, London E9 6SR
    2. Department of Otorhinolaryngology, Faculty of Health Sciences, University of Stellensbosch, 7505 South Africa

      EDITOR—Weber's test is indeed mysterious, and the filler by Weatherall had our ear, nose, and throat department, as well as an eminent visiting professor from Glasgow, occupied for the best part of a week.1 Literature searches and audiological experiments were conducted. Arcane, dusty journals were dusted off.

      The simulation of conductive hearing impairment by occluding the ear with a finger suggested by Weatherall is the basis of the Bing test. A tuning fork is placed on the skull, and the patient indicates when the sound disappears. A normal conductive mechanism is demonstrated if the sound is heard again when the ear canal is occluded.2

      The so called occlusion effect described by Tonndorf et al in 1966 is responsible for this phenomenon.3 Sound conducted through bone causes the cochlea, the ossicular chain, and the air in the external auditory canal to vibrate. Some lower frequency sound, as produced by the 512 Hz tuning fork, escapes from the canal. When the ear is occluded, these frequencies cannot escape and the sound seems to become louder.

      We conducted an experiment to determine whether the exclusion of environmental noise or the occlusion effect was responsible for Weatherall's observation. Baseline binaural air and bone thresholds were obtained at 500 Hz on a subject in a soundproof audiometric booth. Both ears were occluded with modified tympanometry earpieces (blocked with plasticine), without causing a measurable conductive loss. The ears were then occluded with foam “ear defenders,” which caused a 25 dB conductive impairment. In both instances the bone conduction thresholds improved by 15 dB.

      Having proved that an occlusion effect occurs independently of a conductive loss, we then introduced background noise to assess its influence. Binaural masking noise (similar to environmental noise) was presented over headphones, first occluding with modified tympanometry earpieces and then with ear defenders. Bone conduction thresholds were reassessed when the masking noise was audible despite the conductive impairment. In both occluded ear conditions the masking noise did not affect the improvement in the bone conduction thresholds.

      We therefore concluded that it is the occlusion effect, rather than elimination of environmental sound, that is responsible for the improved bone conduction threshold when occluding a normal ear. Other explanations are required for aetiologies of conductive loss other than occlusion. Middle ear effusion and ossicular chain disruptions cause a “mass loaded” middle ear, with lowering of the inherent resonant frequency. Ossicular chain fixation causes a phase shift in the sound wave. Both cause preferential transmission of lower frequencies to the cochlea.2

      References

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