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TENs (Transcutaneous Electrical Nerve
Stimulation) TENS
is a method of electrical stimulation which primarily aims to provide a
degree of pain relief by specifically exciting sensory nerves and
thereby stimulating either the pain gate mechanism.
The different methods of applying TENS relate to these different
physiological mechanisms. Success is not guaranteed with TENS, and the
percentage of patients who obtain pain relief will vary, but would
typically be in the region of 65%+ for acute pains and 50%+ for more
chronic pains. Both of these are better than the placebo effect. The
technique is non invasive and has few side effects when compared with
drug therapy.
How It Works :
The type of stimulation delivered
by the TENS unit aims to excite (stimulate) the sensory nerves, and by
so doing, activate specific natural pain relief mechanisms. For
convenience, if one considers that there are two primary pain relief
mechanisms which can be activated : the Pain Gate Mechanism and the
Endogenous Opioid System, the variation in stimulation parameters used
to activate these two systems will be briefly considered.
Pain relief by means
of the pain gate mechanism involves activation (excitation) of the A
beta sensory fibres, and by doing so, reduces the transmission of
the noxious stimulus from the c fibres, through the spinal cord and
hence on to the higher centres. The A beta fibres appear to appreciate
being stimulated at a relatively high rate (in the order of 90 - 130 Hz
or pps). It is difficult to find support for the concept that there is a
single frequency that works best for every patient, but this range
appears to cover the majority of individuals.
An alternative
approach is to stimulate the A delta fibres which respond
preferentially to a much lower rate of stimulation (in the order of 2 -
5 Hz), which will activate the opioid mechanisms, and provide pain
relief by causing the release of an endogenous opiate (encephalin) in
the spinal cord which will reduce the activation of the noxious sensory
pathways.
A third possibility is
to stimulate both nerve types at the same time by employing a burst mode
stimulation. In this instance, the higher frequency stimulation output
(typically at about 100Hz) is interrupted (or burst) at the rate of
about 2 - 3 bursts per second. When the machine is on, it will deliver
pulses at the 100Hz rate, thereby activating the A beta fibres and the
pain gate mechanism, but by virtue of the rate of the burst, each burst
will produce excitation in the A delta fibres, therefore stimulating the
opioid mechanisms. For some patients this is by far the most effective
approach to pain relief, though s a sensation, numerous patients find it
less acceptable than the other forms of TENS.
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