This is a summary of Chapter IV of David Butler’s “The Sensitive Nervous System.”
Central sensitization is a phenomenon that occurs in the dorsal horn, which can be best described via 4 different states:
1) Normal: Inputs = outputs; innocuous sensations are perceived as such.
2) Suppressed: Inputs that would hurt do not; think an athlete who injures himself but finishes the game.
3) Increased sensitivity: Pain system has lower activation threshold, leading to pain spreading and pain with light touch and gentle movement. This change occurs because A beta fibers begin taking over C fiber locations in the dorsal horn.
4) Maintained afferent barrage, CNS influences, and morphological changes: Long lasting changes in the dorsal horn from a persistent driver, such as…
- A fiber phenotype changes.
- Persistent DRG discharge.
- Persistent inflammation.
- Supraspinal influences
- Gene transcription change in dorsal horn neurons.
- Inflamed dorsal horn or DRG
- Maladaptive beliefs, fears, and attitudes.
- Dorsal horn sprouting; A Beta fibers take over C fiber space.
- Persistent glutamate activity.
The CNS has an endogenous pain control system which activates during injury threat, noxious cutaneous input, or expectations and learning. Such an example of this is when you go to a healthcare practitioner’s office and no longer hurt. Another example of when this system is activated is during aggressive manual therapy. Think about how good your body may feel after sustained pressure or even a needle to a trigger point.
Central Sensitization Patterns
- Symptoms not in neat anatomical/dermatomal boundaries.
- Original pain spreads.
- Multiple areas: Either linked or get one pain, then the other.
- Contralateral side may be painful, though not like the other side (mirror pain).
- Clinicians end up chasing pain.
- Sudden, unexpected stabs.
- Patients call the pain “It.” For example, “It has a mind of its own.”
- Ongoing pain perception past normal healing times.
- Summation via repetitive activities (sitting at a computer).
- Distorted stimulus/response relationship. May get pain 10 seconds or days after stimulus is applied.
- Unpredictable response to treatment/input, but ends up being predictable (x may only work 2 days).
- Every movement hurts, but not big ROM loss (Symptom instability).
- “It hurts when I think about it.”
- Can be cyclical.
- Change in other systems.
- Links to traumatic life events.
- Miracle cures can work.
- Likely in most syndromes (fibromyalgia, complex regional pain syndrome).
1) Educate, educate, and educate. We must let our patients know that pain does not equal damage.
2) Do not focus on finding an anatomical pain source which can make matters worse.
3) False positive are frequent on testing when tissues may be healthy (Eg, positive straight leg raise may just be adding slightly noxious input that results in an increased afferent barrage).
4) The physical exam is best considered a sensitivity test.
Increased levels of norepinephrine (physical stress), epinephrine (mental stress), and cortisol (shut down nonessential systems to maintain homeostasis) are upregulated and contribute to, but do not cause, pain. High sympathetic states can lead to problems that include tissue degeneration, mood swings, slow tissue healing, and increased infection susceptibility in people with chronic pain. Tissue change is particularly evident because the sympathetic nervous system innervates and interacts with muscles, joints, skin, connective tissue, inflammatory chemicals, AIGS, and the DRG. The parasympathetic nervous system is also important to mention to our patients in terms of healing effects of sleep and relaxation.
Motor Response to Pain and Stress
Can include the following:
- Muscle spasm.
- Change in facial expression/tone of voice.
- Muscle imbalances (a coping strategy)
- Loss of or decreased quality of ROM.
- Loss of movement selection variety.
Cytokines are the major player here, as they predominantly function to combat infection. However, these components also contribute to inflammation and pain.
Immunity changes can occur over several mechanisms and is interrelated to all other body systems.
- CNS can activate the immune system to respond to any stressor.
- Sympathetic nervous system modulates the immune response.