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 (Aβ) 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
Aβ 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. Clinically it is important to enable the patient to find
their optimal treatment frequency – which will almost certainly vary
between individuals. Setting the machine and telling the patient that
this is the ‘right’ setting is almost certainly not going to be the
maximally effective treatment, though of course, some pain relief may
well be achieved.An alternative approach is to stimulate the A delta (Aδ) 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. In a similar way to the pain gate physiology, it is unlikely that there is a single (magic) frequency in this range that works best for everybody – patients should be encouraged to explore the options where possible.
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 Aβ fibres and the pain gate mechanism, but by virtue of the rate of the burst, each burst will produce excitation in the Aδ fibres, therefore stimulating the opioid mechanisms. For some patients this is by far the most effective approach to pain relief, though as a sensation, numerous patients find it less acceptable than some other forms of TENS as there is more of a ‘grabbing’, ‘clawing’ type sensation and usually more by way of muscle twitching than with the high or low frequency mod