Pain Definition and Types Overview

Pain Definition

• Pain is broadly defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.

Types of Pain (Review)

• Acute: Associated with distinct tissue trauma and serving a biologic function. Often nociceptive or neuropathic.
• Chronic: May or may not be associated with tissue damage or trauma. Often nociplastic.
• Referred: Painful sensation in a location other than the site of the biologic injury.

Basic Concepts for the Transmission of Pain

• Cutaneous afferents (A-delta): Transmit sensory pain information rapidly. First response. Often sharp, stabbing, or pricking pain. Heavily myelinated.
• C polymodal afferents: Transmit pain slowly. Often described as dull, aching, throbbing, or burning pain. Unmyelinated. 80-90% of pain transmission.
• Visceral nociceptive afferents: Carry the message of referred pain.

C Polymodal Nociceptive Afferents

• Small, unmyelinated nerve fibers.
• Transmit information (action potentials) more slowly than A-delta fibers causing a slower onset after the stimulus.
• Pain sensations are often longer lasting and less localized.
• Responsive to multiple stimuli (mechanical, thermal, chemical).
• Can be mediated by opiates.
• May be sensitized and involved in hyperalgesia.

Spinal Pathways

• Dorsal Horn Lamina 5 is responsible for a wide range of pain transmission.
• Dorsal Horn Lamina 1 is responsible for non-specific nociception.
• Spinothalamic tracts carry pain input to the thalamus and somatosensory cortex, as well as the reticular formation and limbic system.
• Transmission of pain information can impact affect and emotion.

Modulation of Pain

• Endogenous Opiate System reduces pain.
• Descending Inhibition modulates pain messages.
• Gate Control is a classic explanation for pain modulation where sensory input from A beta fibers can inhibit pain transmission through C fibers.

Endogenous Opiates

• Pain perception can be altered endogenously when peptides (endorphins) bind to opiate receptors.
• Endogenous peptides act similar to exogenous opiates.
• Opiate receptors reduce the release of Substance P, thus reducing pain transmission.
• These peptides circulate through the bloodstream to affect pain throughout the body.

Primary Endogenous Opiates

• Beta Endorphin
• Enkephalins
• Serotonin

Endogenous Opiates (Nice to know)

• Beta endorphin is released from the pituitary and hypothalamus into the circulatory and cerebrospinal fluid.
• Enkephalins are a 5-amino acid peptide released from the adrenal gland and other organs, inhibiting action potentials and release of Substance P.
• Serotonin is often considered part of this class of substance due to its effects on pain.

Descending Inhibition

• General concept of pain control/modulation.
• Pain messages can be modulated by descending neurons in the brain.
• This influences pain perception, motor responses, and emotional aspects of pain.
• This is now thought to influence the gate control method of pain modulation.

Gate Control

• Sensory input from A beta fibers can inhibit pain transmission through C fibers.
• This reduces overall pain sensation.

Pain Modalities

• Techniques to interfere with normal pain processes in the body to modulate pain transmission.
  • Gate Control Theory (with descending inhibition)
  • Endogenous Opiate Theory

Electric Stimulation for Pain Modulation

• Transcutaneous electrical nerve stimulation (TENS)
• Interferential current therapy (IFC)

TENS

• Sensory level stimulation to produce analgesia.
• Low frequency device with three common application modes, including:
  • Conventional
  • Motor
  • Noxious (hyperstimulation)

Review of Literature

• Common problems plaguing physical agent literature comparisons:
  • Type of pain treated
  • Type of parameters used
  • Type of electrode placement
  • Type of measurement of pain or dysfunction
  • Types of subjects

Conventional TENS

• Shown to be effective in reducing low back pain, pancreatitis, post herpetic neuralgia, rheumatoid arthritis, pain during jaw exercises, phantom limb pain, and improving stretching outcomes.
• Subjects should feel a comfortable sub-motor paraesthesia (buzzing/tingling).
• Pulse duration should be short.
• Rapid activation of sensory, afferent nerve fibers.
• Electrodes placed around the pain site or related spinal segment.
• Time: 30-60 minutes typical, up to all day.

Motor Low Rate TENS

• Subject should feel a strong, rhythmic muscle contraction.
• Pulse duration should be long.
• Strength duration curve: must reach motor threshold.
• Pulse rate should be low.
• Electrodes should be placed at the pain site or at a segmentally related myotome.
• Time: 40-60 minutes, typically up to several hours per day.

Hyperstimulation/Noxious TENS

• Subject should feel a noxious, pricking sensation.
• Pulse duration should be long.
• Electrodes placed at the pain site (trigger or acupuncture point).
• Time: Short bouts since stimulus is painful.

Advantages and Disadvantages of Hyperstimulation TENS

• Advantages: fast acting relief, long lasting relief, wide location of relief, unlikely to accommodate
• Disadvantages: skill/ease of application, lack of patient control, unpleasant to patient

TENS Effectiveness in Aging Patients

• Evidence is mixed regarding the effectiveness of TENS as patients age.

TENS and Low Back Pain

• Systematic review shows TENS consistently better than placebo in treating low back pain.
• Treatment plans using TENS showed no significant difference in functional outcomes.

TENS and Labor Pain Relief

• Cochrane database review (2009) shows no significant difference in pain scores, need for intervention, or length of labor between women using TENS and controls.

TENS and Fibromyalgia

• Systematic review found TENS significantly reduced visual analog scale (VAS) scores.

TENS and Sympathetic Nervous System

• Transcutaneous electrical nerve stimulation (TENS) at different frequencies can reduce heart rate variability. This is associated with a change in the balance between sympathetic and parasympathetic nervous systems.
• TENS had an inhibitory effect on SSR.

Genetic Predisposition for TENS

• Genotype influences how well patients respond to different TENS frequencies.

Further Implications for TENS

• Possible applications to prevent post-operative nausea and vomiting.
• Efficacy in reducing spasticity, constipation in children with lower urinary tract dysfunction, and improvement in voiding symptoms in those with dementia.

Interferential Current Therapy (IFC)

• Combines two applied electric fields to produce a third distinct electric field.
• Medium frequency stimulation, producing deeper pain relief.
• Less impedance (resistance) compared to other methods.
• Produced by two distinct current generators in the machine.

Beat Frequency

• The outcome of combining two electrical fields with slightly different frequencies.
• Values of the beat frequency depend on the difference between the frequencies of the two wave trains.

Frequency Difference

• Frequency difference is created by using one current generator at a constant frequency and the other at a variable frequency.
• Allows modulation of interferential current.
• Higher beat frequencies are used for sensory stimulation.
• Lower beat frequencies are used for motor stimulation.

Application of Interferential Stimulation

• Quadripolar electrode set-up around the pain area.
• Set channel 1 carrier frequency.
• Channel 2 is set to produce a variable carrier frequency.
• Depending on the desired outcome of treatment (paraesthesia/muscle contraction), frequency can be adjusted.
• Treatment duration of 20-30 minutes, up to several hours per day depending on the specific need.

Literature Review (IFC)

• Early work by Nemec (1948, 1950, 1959) laid the foundation of research into IFC.
• Little literature compared IFC to other methods, such as TENS.
• Early work focused on older equipment.

Brief Overview of Literature (various conditions)

• Kloth (1982), Hobler (1991), and Snyder-Mackler et al. have supported IFC effectiveness in various painful conditions like post-traumatic edema, pain, and strengthening of muscles.
• Some articles support IFC being effective in urinary incontinence, chronic back pain, and bone healing.

Home Interferential Therapy

• Studies have shown home IFC to be effective in reducing pain, inflammation, and improving range of motion for various conditions.

Advantages of IFC

• Pain relief is generally quick (within minutes), large, and deep. Often carries over to the following session.

Premodulated Interferential Current

• Current generated by the intersection of two distinct wave trains.
• Occurring at the level of the machine, not in the body. This is a single channel, bipolar electrode technique.
• The rest of the principles of IFC still apply.

Indications for TENS/IFC

• Indications: Acute trauma, post-operative pain, local inflammation, and painful treatments.

Contraindications for TENS/IFC

• Application in certain anatomic locations
• Transthoracically, transcerebrally, and abdominally use cautiously, especially in pregnant females.
• Avoid application to the anterior neck.
• Do not use on patients using demand cardiac pacemakers.
• Use cautiously or avoid in patients with unknown pain origins.

Additional Precautions

• Potential contact dermatitis is common with TENS users due to prolonged electrode use.

Review of Pain Modalities (Comparison)

A table outlining the comparison of various pain modalities.