TENS (TRANSCUTANEOUS ELECTRICAL NERVE STIMULATION)
TENS (Transcutaneous Electrical Nerve Stimulation)
Transcutaneous electrical nerve stimulation devices are most frequently referred to as TENS. TENS devices can be used to assist with short-term or long-term pain relief. These systems are used frequently for acute or post surgical pain.
TENS units all produce analgesia of pain and reduce responses of dorsal horn neurons to painful stimuli. The TENS systems activate the descending inhibitory pathway from the brain stem to the spinal cord. However, the means of reducing pain varies between the specific types of systems and includes activating spinal cord gating mechanisms, endogenous opiates, serotonin receptors, noradrenalin receptors and muscarinic receptors.
The initial development of TENS technology was based on the gate-control theory of pain developed by Melzack and Wall in 1965. The researchers found that pain transmission could be stopped when the “gate” closed, therefore the pain signal would not be felt by the patient. TENS units were developed to send an electrical impulse that confuses the pathways in sensory nerves “gating” or blocking the sensation of pain. However, continued research has shown that there are many different methods the body uses to modulate pain and therefore reducing pain requires a multifaceted approach. The understanding of how TENS devices work continues to evolve, and one current piece of research has found that the blocking of pain may occur at deep tissue afferents.
Types of TENS Units
TENS units are typically about the size of a pack of cigarettes. There are usually several predetermined stimulation options available with preset variables. This variety of parameters is used to reduce the problem of adaptation to one stimulus parameter, therefore reducing the effectiveness of the TENS units. There are typically two channels and the user has the ability to control each channel separately, however in most situations the channels are set to be identical.
TENS devices are characterized based on the intensity of the stimulation. Intensity is basically a combination of amplitude, duration and frequency of the pulses. There are three major forms of TENS which are commonly referred to as high frequency, low frequency and noxious. Within these three categories, there are sensory-level stimulators and motor-level stimulators. During sensory-level stimulation, the client will feel a slight tingling or buzzing to the skin, and during motor-level stimulation the client will feel a muscle contraction. Motor-level stimulation is typically used for chronic pain.
High-frequency (or conventional) TENS (sensory and motor)
Example of High-Frequency TENS Stimulation Pattern
These TENS units stimulate sensory nerves and can cause an increase in the pain threshold. At the highest frequencies, the TENS devices can stimulate motor nerves. The analgesic effect is thought to occur due to the activation of acetylcholine, a neurotransmitter. In addition, there is some current research showing that the release of endogenous opiates occurs with high-frequency TENS. The release of opiates was originally thought to only occur with low-frequency stimulation.
High-frequency TENS uses short pulse durations of around 20 microseconds combined with a 50-200 pps frequency of stimulation. The client can tolerate the stimulation for hours, but the resultant pain relief generally lasts for a short period of time.
Low-frequency TENS (sensory and motor)
Example of Low-Frequency TENS Stimulation Pattern
Low-frequency TENS will cause pain relief due to the release of endogenous opiates, activation of serotonin and acetylcholine. The frequency is less than 10 pps (typically 1 to 4 pps) with long pulse durations of 150-200 microseconds. The amplitude can be adjusted so that a muscular contraction is elicited. This form takes longer to provide pain relief after initiation of treatment, and analgesia lasts for some time after the stimulation has ended. The treatment is typically less tolerated by the client and lasts 20-30 minutes.
Noxious also referred to as Brief or Intense TENS (high-frequency and low-frequency)