Transcutaneous Electrical Nerve STimulation (TENS) (WEEK 9) Flashcards
What is TENS?
electrical stimulation: alternating electrical current is delivered to the surface of the skin through electrodes
Goal: stimulate nerve fibers (sensory, motor, and pain).
How does TENS work? (Flow of current)
- current flow between anode (+) and cathode (-) produces depolarization under the cathode and hyperpolarization under the anode
Physiological effect occurring
- At rest: nerve has resting potential of -40mV to -90mV (inside more negative than outside)
- Cathode: draws positively charged particles away from the nerves surface and the inside of nerve becomes relatively less negative until threshold potential is reached
- Action Potential: triggered anywhere along the nerve and will propagate down the nerve
- Motor nerve fires to muscle; sensory nerves fire to CNS
Pulse Duration - Current Strength curve
Left to right
- AB sensory
- Motor
- AD sharp pain
- C Dull pain
- denervated muscle
pulse duration: 40us
Amp little: 30mA
Fibers stimulated?
AB Sensory fibers
Pulse duration 200us
Amplitude 20mA
Fibers stimulated?
Sensory fibers
If sensory fibers are activated, and
- we use a longer pulse duration, how do get the same response?
- Keep pulse duration but increase amplitude
- Further increase amplitude?
- decrease/less amplitude
- Stimulate sensory and motor nerves
- Stimulate sensory, motor, and pain fibers
if you wanted to stimulate nociceptive fibers what pulse duration would allow you to do so at a lower amplitude (current strength)
> 300 usec
if you wanted to stimulate sensory fibers (a-beta) and no other fibers, what pulse duration would be the best option?
< 100usec
Parameters: Frequency
influence is not very clear
- High: paired with sensory stimulation - “close the gate”
- Low: paired with motor/pain stimulation - slow enough to get muscle contraction but not so high to fatigue the muscle too quickly
Therapeutic goal ofTENS (how does it work)?
CONTROL PAIN
- Pain gate mechanism
- Descending endogenous opioid system
- Endogenous opioid release at thr level of SC
Body’s response to pain
- Brain does not passively receive nociceptive signals
- CNS receives signal, interprets, and responds
- block pain (pain gate)
Pain gate (Gate Control Theory) Mechanism.
Selectively stimulate A-B fibers to close the gate
- Nociceptive signals are transmitted to the dorsal horn of the spinal cord via A-D and C fiber’s (small diameter fibers)
- In the spinal cord, these peripheral receptors synapse with transmitter neurons and sends the pain signals to the brain
- Larger diameter A-B fibers (mechanoreceptos) also enter dorsal horn to synapse with transmitter neurons
- Large diameters overwhelm the smaller diameter and blocks pain signs from reaching the brain
Descending Endogenous Opiod System (DEOS): what is it?
Group of different opiod peptides and receptors that modulate pain
Descending Endogenous Opiod System (DEOS): mechanism
- Pain triggers release of opioid peptides bind by interneurons in CNS
- Peptides bind to receptors on pre-synaptic neurons = pre-synaptic inhibitions
- fewer NT released, nociceptive single becomes weaker - Peptides bind to post-synaptic neuron = post-synaptic inhibition
- hyperpolarize nerve, more NT needed to trigger an action potential
Nociceptive interpretation (pathway)
- Nociceptive signal to the brain via 1st order afferent neutron to the dorsal horn of the spinal cord
- 2nd order afferent neutrons takes the message to the brain via the spinothalamic tract
DEOS response
The brain stem responds
- pain results in stimulation of PAG matter
- PAG sends signal down to raphe nucleus
- raphe nucleus sends efferent signals down dorsolateral funiculus (tract) in the spinal cord
- efferent neurons release opioids into the synapse between the 1st and 2nd order afferent neurons
- opioids bind to the neurones and cause pre- and post-synaptic inhibition
Endogenous opined release: at the spinal cord
- pain triggers the release of endogenous opioid peptides by interneurons in the spinal cord
- results in further pre- and post-synaptic inhibition at the 1st and 2nd order afferent neurones
Pulse Duration:
- Conventional (High):
- Acupuncture (Low):
- Noxious (Brief):
- Conventional (High): 50-80us
- Acupuncture (Low): >150us
- Noxious (Brief): > 150us
Pulse rate:
- Conventional (High):
- Acupuncture (Low):
- Noxious (Brief):
- Conventional (High): 80-120Hz
- Acupuncture (Low): 10Hz
- Noxious (Brief): 120-200Hz
Intensity:
- Conventional (High):
- Acupuncture (Low):
- Noxious (Brief):
- Conventional (High): Sensory & Comfortable
- Acupuncture (Low): Sensory & Motor; generally less comfortable
- Noxious (Brief): sensory, motor, and pain; uncomfortable/pain
Pain mechanism:
- Conventional (High):
- Acupuncture (Low):
- Noxious (Brief):
- Conventional (High): Pain-gate
- Acupuncture (Low): DEOS & Opioid release at SC
- Noxious (Brief): DEOS & Opioid release at SC
Onset and Duration of pain relief:
- Conventional (High):
- Acupuncture (Low):
- Noxious (Brief):
- Conventional (High):
O: fast
D: short - Acupuncture (Low):
O: slow
D: long - Noxious (Brief):
O: fast
D: long
Conventional TENS: primary mechanism, what do you feel?
- pain gate: immediate pain relief but pain relief returns shortly after treatment ends
- intensity is set to produce sensory stimulation so that patient feels a tingle (fast enough to “close the gate”)
Motor TENS: primary mechanism, what do you feel?
- release of endogenous opioids (Both spinal cord and DEOS) will not provide immediate pain relief but can get relief that continues for hours after treatment
- intensity: just at motor threshold - sensation of pinching
Noxious TENS: primary mechanism, what do you feel?
- release of endogenous opioids (Both spinal cord and DEOS) just enough to bring on pain
- intensity will provide pain relief that continues for hours
Clinical trials:
- fMRI of High frequency TENS
- Type of pain
- decreased activity of brain areas linked to pain perception
- Pressure pain, not experimental
Clinical trials: fMRI of Low frequency TENS
- decreased area of brain linked to pain perception
Clinical trials: fMRI of Noxious TENS
- effective for all types of experimental pain
Clinical trials: acute pain
- more effective than placebo
parameters:
- intensity: strong, near noxious
- frequency 1-150Hz
Clinical trials: neuropathic pain (SCI and amputees)
SCI: low frequency tens more effective than placebo for SCI
Amputee: high frequency TENS x 60 minutes reduce pain intensity at rest and movement
Clinical trials: knee OA
inconclusive (Low quality studies)
Clinical trials: neck pain
inconsistent results, weak evidence TENS is better than placebo
Clinical trials: LBP
Poor evidence
- no difference with TENS and active treatment
Clinically meaningful change in pain
2 point change = clinically meaningful
