TENS Therapy (Transcutaneous Electrical Nerve Stimulation) Exam Guide
💡 The Core Concept: TENS is a non-invasive modality that uses pulsed low-frequency electrical currents to stimulate nerves across the skin surface to provide symptomatic pain relief. It works primarily on two principles: Pain Gate Mechanism and Opioid Release.
1. The Technical Arsenal (Modes & Parameters)
Different frequencies activate different physiological mechanisms. This table is the backbone of TENS questions in AIIMS.
| Parameter | High TENS (Conventional) | Low TENS (Acupuncture) |
|---|---|---|
| Frequency (Hz) | 80 - 150 Hz (High) | 1 - 10 Hz (Low) |
| Pulse Duration (µs) | Short (50 - 100 µs) | Long (200 - 300+ µs) |
| Intensity | Sensory Only (Tingling / Paraesthesia) |
Motor / Twitch (Visible Contraction) |
| Mechanism | Pain Gate Theory (Pre-synaptic inhibition) |
Endogenous Opioid (Endorphin Release) |
| Onset & Relief | Rapid Onset, Short Relief | Slow Onset (20-40 min), Long Relief |
Other Important Modes
- Burst Mode: Delivers "packets" (bursts) of high frequency (100Hz) at a low frequency rate (1-5 Hz).
Result: Combines comfort of High TENS with Opioid release of Low TENS. - Brief Intense TENS: High Freq (100Hz) + Long Pulse (200µs) + High Intensity (Tolerance). Used for rapid analgesia during procedures (e.g., debridement).
2. Mechanism: The "Pain Gate" Explained
Location: Substantia Gelatinosa (SG) in the Dorsal Horn of the Spinal Cord (Lamina II & III).
🚪 The Simplified Analogy:
Imagine a gate that lets pain signals through to the brain.
Imagine a gate that lets pain signals through to the brain.
- Pain Fibers (A-delta, C): These are small and slow. They open the gate.
- Touch/TENS Fibers (A-beta): These are large and fast. They close the gate.
3. Indications & Contraindications
Indications
- Acute Pain: Post-operative, Labour pain (High TENS preferred).
- Chronic Pain: Low back pain, Arthritis, Neuralgia (Low TENS/Burst often effective).
- Phantom Limb Pain.
⚠️ Absolute Contraindications
- Pacemakers: Electrical interference can cause cardiac arrest.
- Carotid Sinus: (Anterior Neck) Stimulation here can trigger a vagal reflex, causing a rapid drop in BP (Hypotension) and fainting.
- Pregnancy: Over the abdomen/pelvis (may induce contractions). Safe on extremities/upper back.
- Epilepsy: Head/Neck placement may trigger seizures.
- Internal bleeding/Hemorrhage.
🏆 AIIMS "Golden Points"
- Accommodation: Nerves get used to constant stimuli (threshold rises). To prevent this, use Modulation Mode (machine varies freq/width/intensity automatically).
- Electrode Placement: Most common method is Dermatomal (placing electrodes on the skin area supplied by the same spinal nerve root as the painful structure).
- Pregnancy Safety: TENS is often used during labor (T10-L1 for Stage 1, S2-S4 for Stage 2) but strictly contraindicated on the abdomen before labor.
📝 20 High-Yield MCQs (Practice Now)
Click an option to check your answer immediately.
Q1. Conventional (High) TENS works primarily through which mechanism?
Rationale: High frequency TENS (80-100Hz) selectively stimulates large diameter A-beta fibers, which activate inhibitory interneurons in the Substantia Gelatinosa, closing the gate to pain.
Q2. What is the typical frequency range for Acupuncture-like (Low) TENS?
Rationale: Low frequency TENS uses 1-10 Hz (usually 2-4 Hz) to stimulate muscle twitches, facilitating the release of endorphins and enkephalins.
Q3. Which nerve fibers are primarily stimulated by Conventional TENS to close the pain gate?
Rationale: A-beta fibers are large, myelinated, fast-conducting mechanoreceptors (touch/pressure). Their activation inhibits the transmission of pain from A-delta and C fibers.
Q4. What is the correct intensity (amplitude) setting for Low Frequency (Acupuncture) TENS?
Rationale: To trigger the release of endogenous opioids, Low TENS requires a repetitive muscle twitch (Motor level intensity). Conventional TENS is Sensory only.
Q5. Why is TENS application over the Carotid Sinus (anterior neck) contraindicated?
Rationale: The Carotid Sinus contains baroreceptors. Electrical stimulation fools the body into thinking BP is high, triggering a vagal response that rapidly drops heart rate and BP, potentially causing fainting.
Q6. Which TENS mode combines high frequency pulses in low frequency packets?
Rationale: Burst mode delivers a "train" or burst of high frequency pulses (e.g., 100Hz) delivered at a rate of 2-5 bursts per second. It is more comfortable than pure Low TENS but has similar effects.
Q7. "Accommodation" of nerves to TENS current can be prevented by:
Rationale: Accommodation occurs when nerves stop firing in response to a constant stimulus. Modulation varies the frequency, pulse width, or amplitude automatically to keep the nerves responsive.
Q8. The Pain Gate mechanism is located in which part of the spinal cord?
Rationale: The Substantia Gelatinosa (Lamina II & III) of the dorsal horn acts as the "gate" where pain modulation occurs before signals ascend to the brain.
Q9. Which TENS parameter setting corresponds to "Brief Intense TENS"?
Rationale: Brief Intense TENS uses High Frequency (>100Hz) and Long Pulse (>150µs) at the highest tolerable intensity to produce a rapid "nerve block" effect for painful procedures (e.g., wound dressing, suture removal).
Q10. Which type of TENS typically provides pain relief that lasts for hours after the treatment stops?
Rationale: Low TENS releases endorphins (natural painkillers) which have a half-life of several hours in the body, providing long-lasting relief. High TENS relief usually stops shortly after the machine is turned off.
Q11. What is the pulse duration range for High Frequency (Conventional) TENS?
Rationale: Short pulse durations (50-80µs) are used in Conventional TENS to selectively stimulate sensory A-beta fibers without triggering motor nerves.
Q12. Which electrode placement is most effective for radiating pain (e.g., Sciatica)?
Rationale: For radiculopathy, placing electrodes along the relevant dermatome or at the spinal nerve root exit effectively blocks pain transmission at the spinal level.
Q13. Opioid peptides released by Low TENS include:
Rationale: Enkephalins and Endorphins are the body's natural pain-killing chemicals (endogenous opioids) released from the pituitary and spinal cord.
Q14. Why is TENS generally safe to use during labor (delivery) but not before?
Rationale: During active labor, TENS (applied to the back T10-L1) provides significant pain relief (Gate Control) without drugs that might affect the baby. Before term, abdominal application is avoided as it might trigger premature contractions.
Q15. A patient with a pacemaker has severe knee pain. Can you apply TENS to the knee?
Rationale: Pacemakers are an absolute contraindication for TENS anywhere on the body because electrical currents travel through body fluids and may interfere with the device's sensing mechanism.
Q16. If a patient reports the TENS sensation is fading during treatment (without machine change), this is called:
Rationale: Neural accommodation/adaptation is the decrease in perceived sensation over time to a constant stimulus. The intensity must be increased, or modulation used.
Q17. Which fiber type transmits sharp, localized, "fast" pain?
Rationale: A-delta fibers are myelinated and transmit fast, sharp pain. C fibers are unmyelinated and transmit slow, dull/aching pain.
Q18. How long does it typically take for Acupuncture TENS to produce pain relief?
Rationale: Endorphin release takes time to build up in the bloodstream/CSF. High TENS provides immediate relief, whereas Low TENS has a latency period.
Q19. Is it safe to use TENS over a metal implant (e.g., Total Knee Replacement)?
Rationale: Unlike SWD/MWD, TENS does not create deep electromagnetic heating. It is generally considered safe over metal implants, though direct placement over superficial metal hardware should be done with care.
Q20. TENS is a type of:
Rationale: TENS operates in the Low Frequency range (1-150 Hz). Medium frequency is IFT (2000-5000 Hz), and High frequency is SWD/MWD (>1 MHz).
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