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Test of the Month

Several readers have asked Bandolier to start a 'test of the month' series, looking at the evidence and practice of diagnostic and imaging tests and their value. Source material for such a series is not widely available, and preparing the evidence is not easy. Nevertheless, Bandolier feels that a start should be made, if only to encourage readers to point out where information is available, or to induce them to start their own reviews.

The series starts with a description of the electrocochleogram (ECochG) for Ménière's disease from Mr Grant Bates, an Oxford ENT surgeon.

Electrocochleogram

Ménière's disease was covered in Bandolier 13 . A health Authority of 250,000 patients would expect to see some 60 cases a year. Treatment is usually with betahistine, which trials have shown to be effective. The usual doses of 48 to 64 mg a day costs upwards of £235 a year, and it may also be effective for patients with benign paroxysmal positional vertigo, but not where balance disorders have other causes.

There are rarely physical signs and many patients with unsteadiness and/or vertigo are undiagnosable and usually improve with time. Electrocochleography is an objective test for endolymphatic hydrops - and by far the most common cause of endolymphatic hydrops is Ménière's disease. It is a simple test that takes 20 minutes to perform, and though a fine needle is passed through the tympanic membrane it is not unpleasant for the patient. It allows the diagnosis of Ménière's disease to be confirmed or refuted so that appropriate prognostic advice can be given together with medical or surgical treatments if indicated.

What it involves

With the affected ear uppermost, and the tympanic membrane anaesthetised, a fine needle connected to a recording wire is passed through the anterior part of the membrane until it meets the medial wall of the inner ear. Here the needle is very close to the cochlea hair cells and is recording the electrical activity directly from these hair cells.

Tonal clicks are then applied to the ear through headphones, usually at about 1000Hz, but of variable frequency and intensity. The needle is connected to an oscilloscope which records the activity from the cochlea and provides an electrical record of events in the inner ear. The electrocochleogram (ECochG) records particularly the summating potential, thought to arrive from electrical potential surrounding the basilar membrane. This alters with the degree of distension of the basilar membrane and thus measures endolymphatic hydrops and confirms Ménière's disease.

About 1000 clicks are performed and the activity recorded for 10 milliseconds after each click. The information is summated by computer. Subjects with Ménière's disease have abnormal wave forms [1].

Three stages

There are three main stages of Ménière's disease:-
  1. Early: hearing returns to normal between attacks of vertigo. ECochGs suggest hydrops is increased before an attack and gradually lessens during an attack, and returns to normal afterwards.
  2. Mid stages: hearing fluctuates but does not completely return to normal between attacks. The ECochG may show changes when the patient notices the feeling of fullness of ear before an attack of vertigo.
  3. End stage: hearing is poor and no longer fluctuates the ECochG almost always abnormal and there is a 50% reduction in vestibular function in the affected ear.

When to do an ECochG

In all patients who have unilateral persistent otological symptoms an MRI is required to exclude acoustic neuroma, which can mimic the presentation of Ménière's disease. It is sensible thereafter to confirm Ménière's with an ECochG so that appropriate effective treatments can be applied.

Electrocochleography is available in virtually all ENT departments, takes only 20 minutes or so and requires an ENT surgeon and usually an audiologist. The equipment has many other uses in otolaryngology.
Grant Bates
Radcliffe Infirmary, Oxford

Reference:

  1. WPR Gibson, RTR Ramsden, DA Moffat. Clinical electrocochleography and diagnosis and management of Ménière's disorder. Audiology 1977 16: 389-401.

Useful Internet addresses:

Vestibular Disorders Association:
http://www.teleport.com/~veda/index.html
Otolaryngology On-line:
http://vumclib.mc.vanderbilt.edu/~floyd/ent.html
Primary care Teaching Module:
http://www-med.stanford.edu/MedSchool/DGIM/Teaching/Modules





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