Summary
Highlights
TENS (Transcutaneous Electrical Nerve Stimulation) is a form of electroanalgesia that uses low-frequency currents. It primarily produces a sedative effect by stimulating A-beta fibers, which in turn activate T-cells in the substantia gelatinosa. These T-cells prevent painful stimuli from reaching the cerebral cortex, making the pain imperceptible. This effect is temporary, inhibiting pain rather than eliminating its cause.
Beyond the gate control theory, TENS promotes the release of endorphins and enkephalins, neurotransmitters that prolong the analgesic effect. Additionally, psychological factors contribute to its effectiveness: a suggestive effect (telling the patient it won't hurt), a distracting effect (the sensation of the current distracts from the pain), and a pleasurable effect (larger currents can induce a sense of well-being).
TENS typically uses square, rectangular, and spike waveforms, often in bipolar and symmetrical/asymmetrical biphasic forms. The two basic modalities are high-frequency (60-100 Hz, low intensity), known as conventional TENS, and low-frequency (under 10 Hz, higher intensity), which causes visible muscle contractions. High-frequency TENS is often pre-programmed in devices.
Stimulation below the sensitive level (sub-threshold) produces no sensation for the patient, typically at 1 mA. Sensitive level stimulation, or conventional TENS, causes paresthesias that should be pleasant, not sharp or stinging. This modality primarily activates the gate control mechanism, and its analgesic effect ceases once stimulation stops.
Motor level stimulation, including electroacupuncture and invasive methods, uses high intensity and low frequency to produce strong, rhythmic muscle contractions. Brief intense stimulation uses high intensity and high frequency, leading to contractions and paresthesias. High intensities can cause titanic contractions, while low intensities result in muscle fasciculations. This type of stimulation provides a more prolonged analgesic effect by combining gate control with endorphin release.
Burst stimulation, a programmable modality, delivers 2 to 5 bursts of pulses, each containing frequencies between 40 and 100 Hz. High amplitudes yield analgesia, while low amplitudes produce pulsating paresthesia. Painful level stimulation, using monophasic pulses, creates a burning or pinching sensation. This is rarely used as it treats pain with more pain, employed only in cases of accommodation. Modulated stimulation, a third programmable type, prevents accommodation by varying frequency (and sometimes intensity) in a non-linear fluctuating pattern.
A basic TENS device usually features conventional, burst, and modulated modalities, with controls for frequency and intensity for each channel.