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 TEN |
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