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Device Watch: NNTS & Tinnitus

Medical devices tend to have much less attention than drugs or other therapies. A recent article has explained beautifully the rationale behind applying the same methods to medical devices as are used for drugs to measure their effectiveness, and what needs to be done [1].

RCTs of medical devices can be found, and the calculation of numbers-needed-to-treat (NNT) can be applied to these also. NNT is a useful measure because it can be applied to any clinically relevant and useful outcome.


Tinnitus is a prevalent condition of varying unpleasantness. About 8% of the population (nearly 4 million people) claim to experience tinnitus which causes moderate to severe annoyance and interferes with sleep. In about 0.5% of the adult population (200,000 people) tinnitus prevents them from leading a normal life.

There is a literature which indicates that electrical stimulation can reduce or abolish tinnitus. A relatively recent RCT of an external device (Therapak) which delivers a variable low frequency pulsed electromagnetic signal rich in harmonics seems to show efficacy.

The study in Liverpool examined 58 patients with severe tinnitus of at least one year's duration and with an unselected aetiology. Patients were given either an active or a placebo device - the active device producing no discernible sensation. They used the device for one week, and had the nature of the tinnitus examined after treatment in terms of subjective response and objectively by a repeat pure tone audiogram and tinnitus pitch and loudness matching.



The ideal solution for people with tinnitus is for it to be abolished - and failing that for it to be improved. A simple calculation for a simple device in a short but elegant trial indicates that, compared with placebo, at least twenty people will benefit for every 100 with tinnitus who use the device. In the absence of any other successful treatment, this could be a clinically useful yield.


  1. RW Morgan. Medical device trials of the future. Applied Clinical Trials 1994 3:37-41.
  2. NJ Roland, JB Hughes, MB Daley, JA Cook, AS Jones, MS McCormick. Electromagnetic stimulation as a treatment of tinnitus: a pilot study. Clinical Otolaryngology 1993 18: 278-81.

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