TENS

TENS physiological effects

TENS – Physiological mechanisms

• Pain gate mechanism:

  • Activates Aβ fibres → competing impulses → “close the gate”

  • Activates Aδ fibres → stimulates endogenous opioid system → ↑ threshold in pain gate

• Activation of descending inhibitory pathways

• Physiological block of nerve conduction

  • Stim. peripheral nociceptors above max conduction frequency

  • → cessation of AP propagation

Events at spinal cord

• Bang elbow + rub it → stimulates faster nerves (large-diameter fibres) → activates pain gate

• Look at elbow + see it’s OK → ↑ firing threshold of T-cell → nociception coming in doesn’t get through → feels better

• See that it’s bleeding → brain ↓ firing threshold of T-cell → ↑ nociception gets through → feels worse

Events at the spinal cord

Events at the Spinal Cord

• Message from periphery + changes in spinal cord are NOT “one-way”

• Pain gate → downward modulation of nociceptive input at spinal cord

Clinical implications

Physio can impact pain by influencing processes in the spinal cord:

• ↓ input into spinal cord → HOW?

• Provide competing input at spinal cord → HOW?

• ↓ activation of spinal excitatory circuits → HOW?

Physiological mechanisms of TENS

TENS primarily works through pain gate mechanism:

• Aβ fibres → competing impulses → “close the gate”

• Aδ fibres → stimulate endogenous opioid system → ↑ threshold in pain gate

Where do we feel pain?

• In the BRAIN!

• EPA like TENS can prevent the pain message reaching the brain by stimulating the pain gate mechanism in the spinal cord.

Evidence for TENS

Evidence for TENS

• Moderate evidence to support use of TENS

• Non-RCTs: show TENS is effective

• RCTs: show TENS is not effective

  • → possible large placebo effect?

• Acute pain: evidence inconclusive

  • RCTs had poor methodology

• Postoperative pain: limited evidence of effectiveness

• Labour pain: limited evidence of effectiveness

• Chronic pain: inconclusive

  • → very poor methodology in the field

Placebo for TENS consideration and approach

TENS-specific considerations

• Research quality: adequate intensity → better outcomes

• Patient understanding: sensation ≠ effectiveness

• Expectations: realistic expectations re: duration of pain relief

• Active rehabilitation: integrate TENS with active rehab, not passive reliance

• Fear: address fear of electrical stimulation

• Individual response: understand that response varies between patients

Evidence-based approach

• Explain mechanism without overpromising

• Set appropriate trial period: 2–3 weeks

• Monitor + adjust parameters based on response

• Combine with movement + education

TENS and tissue healing

Proposed mechanisms

• ↓ Inflammatory cytokines: IL-1β, IL-6, TNF-α

• ↑ Growth factor production: PDGF-A, EGF

• ↑ Local blood flow + tissue oxygenation

• Accelerated wound healing phases

Current evidence level

• Promising, but limited high-quality studies

• Consider as a potential secondary benefit

• Primary indication = pain management

TENS effectiveness

Strongest evidence

• Fibromyalgia: significant improvements in pain, function + QoL

• Postoperative pain: 36% ↓ analgesic consumption

• Movement-evoked pain + fatigue

Mixed evidence

• Knee OA: individual response varies

• Chronic low back pain: benefits when properly applied

• Neuropathic pain: limited but promising findings

Individual factors

• Pain phenotype + characteristics

• Patient preferences + beliefs

• Concurrent treatments + comorbidities

TENS parameters

• intensity (80-100mA)

• frequency (2-150Hz)

• pulse width (length of time each pulse is delivered for)

• MODES

  • burst mode → impulses = bursts (2-3 per second)

  • modulation mode → less regular, prevents tolerance

High frequency tens

Mechanism

• Activates pain-gate mechanism → stimulation of Aβ fibres

Dosage

• Frequency: start at 100 Hz for most patients

• Pulse width: set for comfort

• Intensity: definite sensation (“normal”), not uncomfortable

• Time: ≥30 min

Indications

• Best for acute pain

• Pain relief during administration

  • Little “carry-over” / latent period

Low frequency tens

Mechanism

• Activates endogenous opioid mechanism → stimulation of Aδ fibres

Dosage

• Frequency: 2–5 Hz, up to 10 Hz

• Pulse width: longer than traditional → 200–250 μs

• Intensity: stronger → definite, strong sensation

• Time: ≥30 min

Indications

• Takes longer for opioid release → better carry-over effect

• Useful for chronic pain / sustained pain relief

• Can be used at intervals throughout the day

TENS Modes

Brief Intense TENS

Indication

• Rapid pain relief required

Dosage

• Frequency: 90–130 Hz

• Pulse width: 200 μs

• Intensity: at tolerance

• Time: 15–30 min

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Burst Mode TENS

• TENS machine set for traditional TENS, BUT burst mode activated

• → Interrupts stimulation

Dosage

• Frequency: start at 100 Hz for most patients

• Pulse width: set for comfort

• Intensity: near tolerance

• Time: ≥30 min

Indications

• Theorised to activate both pain-gate + opioid mechanisms

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Modulation Mode TENS

• Varies the pattern of stimulation delivery

• Pattern can vary using:

  • Frequency

  • Intensity

  • Pulse

Indications

• Patients using TENS for extended periods

  • → seem to get less accommodation

• No evidence about which parameter to set is available

TENS electrode placement

Electrode placement

• Stimulate: nerve root, peripheral nerve, motor point, dermatome/myotome/sclerotome

• 2 channels → local + referred pain

Hazards & detrimental effects

• Shock

• Skin reaction/dermatitis

• Distraction → don’t drive/operate hazardous equipment

• Avoid water

Contraindications

Absolute:

• Pregnancy → around foetus/1st trimester

• Active implants e.g. pacemakers

• Epilepsy → don’t apply to neck

• Actively bleeding tissue

Localised:

• Malignancy

• Eyes

• Testis

• Active epiphysis

Caution

• Local circulatory insufficiency

• Devitalised tissue

Precautions

• Sharp/blunt skin test

• Check skin integrity → allergy, eczema, lesions

• Don’t place electrodes on anterior neck

• Set all parameters before switching on/connecting electrodes