Referred Pain, Radiating Pain, and the Gate Control Theory of Pain

INTRODUCTION

PART I: REFERRED PAIN

Definition

"Sometimes, a person feels pain in a part of the body that is fairly remote from the tissue causing the pain. This phenomenon is called referred pain. Knowledge of the different types of referred pain is important in clinical diagnosis because, in many visceral ailments, the only clinical sign is referred pain." - Guyton & Hall Textbook of Medical Physiology

Characteristics of Referred Pain

1. Visceral origin - most commonly arises from diseased visceral (internal) organs
2. Poorly localized - the patient cannot pinpoint the exact location
3. Dull, aching quality - transmitted via slow unmyelinated C fibers
4. Associated with cutaneous hyperalgesia - the skin over the referred area may become tender and hypersensitive
5. Predictable patterns - referral follows dermatomal maps based on embryological origin

Classic Examples of Referred Pain

Mechanism of Referred Pain - Convergence-Projection Theory

• Both visceral pain fibers and somatic (skin) pain fibers from the same spinal segment synapse on the same second-order neurons (projection neurons) in the dorsal horn of the spinal cord
• These second-order neurons then project up the spinothalamic tract to the thalamus, and finally to the somatosensory cortex
• When visceral pain fibers are activated, the brain interprets the signal as coming from the skin (or somatic structure) - because the brain has been "trained" by a lifetime of cutaneous stimuli to associate those particular projection neurons with the corresponding skin area
• The brain cannot determine the true source of the stimulus and mistakenly attributes the pain to the somatic structure

"Somatic and visceral neurons converge in the ipsilateral dorsal horn. When the visceral stimulus is prolonged, facilitation of activity from the somatic fiber endings occurs. They now stimulate the second-order neurons, and of course the brain cannot determine whether the stimulus came from the viscera or from the area of referral."

Embryological Basis of Referred Pain Patterns

• Pain is usually referred to a structure that developed from the same embryonic segment (dermatome) as the painful organ
• The heart and the arm share the same spinal segmental origin (C8-T1/T2) - explaining cardiac pain referred to the arm
• The testicle migrated with its nerve supply from the primitive urogenital ridge, from which the kidney and ureter also developed - explaining testicular pain from ureteric pathology

Clinical Importance of Referred Pain

1. It is often the only clinical sign of visceral disease
2. Misdiagnosis is common if the clinician does not recognize referral patterns
3. Referred hyperalgesia (increased sensitivity of referred area) can mislead the examining physician
4. Always perform a thorough examination of the organ actually causing the pain, not just the region where pain is felt

PART II: RADIATING PAIN

Definition

Key Distinction from Referred Pain

Characteristics of Radiating Pain

1. Sharp, stabbing, or shooting quality - often described as electric-shock-like
2. Follows a dermatomal distribution - corresponds to a specific spinal level
3. Associated neurological deficits - sensory loss, motor weakness, and reflex changes in the same dermatomal/myotomal distribution
4. Exacerbated by nerve-stretching maneuvers - e.g., straight leg raise (Lasegue's sign) for sciatica, neck flexion for cervical radiculopathy
5. Worsened by increased intraspinal pressure - coughing, sneezing, straining (Valsalva maneuver)
6. Dermatomal localization helps identify the level of nerve root compression

Common Examples of Radiating Pain

• Sciatica - L4, L5 or S1 nerve root compression → pain radiating from the back, through the buttock, down the posterior/lateral leg to the foot
• Cervical radiculopathy - C6 root compression → pain radiating from the neck to the thumb
• Carpal tunnel syndrome - median nerve compression → pain/paresthesia radiating to the first three fingers
• Angina pectoris - left arm radiation (some overlap with referred pain)
• Herpes zoster (shingles) - radiating burning pain along a dermatome before rash appears

"When a nerve root is irritated or injured, the projected pain is radicular. Radicular pain commonly has a sharp, stabbing quality or causes dysesthesia. It may be exacerbated by activities that stretch the affected nerve root."

PART III: GATE CONTROL THEORY OF PAIN

Historical Background

• Specificity Theory - specific pain receptors send signals directly to the brain
• Pattern Theory - pain results from overactivation of any receptor

Basic Premise

• Opened - allowing more pain signals to reach the brain
• Closed - preventing or reducing pain signal transmission

Nerve Fiber Types Involved

Components of the Gate Control Circuit

1. Primary afferent fibers (Aβ and C fibers) - bring signals from the periphery
2. Inhibitory interneuron (Substantia Gelatinosa cell, lamina II of dorsal horn) - acts as the "gate keeper"
3. Projection neuron (Transmission/T cell) - sends pain signals up the spinothalamic tract to the brain

Mechanism of the Gate - Step by Step

1. C fibers directly excite the projection neuron (+) → pain signal travels to brain
2. C fibers simultaneously inhibit the inhibitory interneuron (-)
3. Inhibitory interneuron is now suppressed, so it cannot inhibit the projection neuron
4. Result: projection neuron fires strongly → pain is perceived

1. Aβ fibers excite the inhibitory interneuron (+)
2. The activated inhibitory interneuron now inhibits the projection neuron (-)
3. The projection neuron is suppressed and cannot fire
4. Result: pain signals are blocked → pain is reduced or abolished

"The C fibers indirectly inhibit the interneurons, thus increasing the activity of the projection neurons (thereby 'opening the gate'), whereas the Aβ fibers excite the interneurons, thus suppressing the output of the projection neurons (and 'closing the gate')."

The Substantia Gelatinosa (SG) as the Gate

• Located in Lamina I and II of the dorsal horn (Rexed's laminae)
• The inhibitory interneurons in the substantia gelatinosa are central to the gating mechanism
• SG cells tonically inhibit T-cells (transmission neurons)
• Large fiber (Aβ) input activates SG cells → more inhibition → gate closed
• Small fiber (C fiber) input inhibits SG cells → less inhibition → gate open

Descending Control (Supraspinal Modulation)

• Periaqueductal gray (PAG) - stimulation produces profound analgesia
• Nucleus raphe magnus - releases serotonin in the dorsal horn → inhibits pain
• Rostral ventromedial medulla (RVM) - releases norepinephrine → inhibits pain
• Psychological factors (attention, emotion, expectation, memory) influence pain through descending pathways - explaining why soldiers wounded in battle may feel no pain initially

Clinical Applications of Gate Control Theory

Modifications and Limitations of the Original Theory

1. The original 1965 model has been refined - interactions between large and small fibers also occur at supraspinal relay centers (thalamus, cortex), not just the spinal cord
2. The exact wiring of the SG interneurons is more complex than originally proposed
3. Central sensitization (wind-up phenomenon) was not fully accounted for in the original theory - prolonged C fiber activity leads to NMDA receptor activation and enhanced pain sensitivity
4. Neuromatrix theory was later proposed by Melzack to account for phantom limb pain and chronic pain states not explained by peripheral input alone

SUMMARY TABLE