Central Sensitisation

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Tips for writing this page:

• Define central sensitation
• Describe the processes underlying central sensitization
• Explain how this is similar and different from peripheral sensitization.
  
• How it might impact upon a pain experience?

This page is coming together,but still needs a little work..can you help? For example..Could you add some videos any videos to support the text..perhaps around assessment findings? Are there any blogs that could be used to support and develop the content of the page? Perhaps fromy bodyinmind.org? Could you add a section about management of patients with suspected central sensitisation?

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Central Sensitisation

Nociception is described by IASP as the neural process of encoding noxious stimuli. Central sensitisation describes the circumstances in which there is an enhancement of the function of neurons involved in nociception resulting in

1. hypersensitivity to stimuli,
2. responsiveness to non-noxious stimuli
3. and an increased pain response evoked by stimuli outside the area of injury, an expanded receptive field.

 The International Association for the Study of Pain (IASP) describes central sensitization as

“Increased responsiveness of nociceptive neurons in the central nervous system to their
normal or subthreshold afferent input.”

 Neural plasticity plays a role in cellular changes with a demonstrable increase in both membrane excitability and synaptic efficacy. The effect of this process is the recruitment of additional, sub-threshold synaptic inputs to nociception resulting in a greater field of receptivity and an increased output of nociception.The effects of this process may persist beyond the duration of the initial noxious input resulting in pain hypersensitivity to normally innocuous stimuli.

It is also thought to  play a role in affecting pain facilitation and inhibition; inhibiting descending pathways and over-activation of the ascending, pain facilitatory pathways. Which, simplifed, means too many messages going in and not enough coming out.

 The use of the term "central sensitisation" varies sometimes referring solely to the neural plastic changes only and sometimes to the the complex and multiple processes and systems which contribute to changes in the elicitation and perception of pain. A discussion about the various descriptions and defintion s can be found on the Body in MInd website. The term was first used in a study of pain hypersensitivity in rats following repeated noxious stimuli to describe the use dependant neural plasticity demonstrated. These central changes due to peripheral noxious stimuli have been referred to as "activity-dependant central sensitisation".

 

Central vs Peripheral Sensitization

While descriptively central sensitization and Peripheral sensitisation may appear to be comparable processes, they represent quite distinct and processes and clinical features.

 Peripheral sensitisation is described by the IASP as

"Increased responsiveness and reduced threshold of nociceptive neurons in the periphery to
the stimulation of their receptive fields." 

 It is initiated when the peripheral terminus of nociceptors are exposed to noxious stimulus, for example inflammatory mediators in damaged tissue. On going stimulation results in a lowering of the activation threshold and thus and increase in responsiveness of nociceptors .

As a result of this  it generally requires on going  peripheral pathology for the sensitization to be maintained and is generally localised to the site of injury. It has a role in altered heat sensation, but not in mechanical sensitivity.

 In the process of central sensitisation novel inputs are recruited to the nociceptive pathways such as large, low-threshold mechanoreceptors classified as Aß fibres. This results in hypersensitivity in non-inflamed tissue and to touch.
 

Features of Central Sensitization

 A survey of expert clinicians in a Delphi-derived survey found the following characteristics to describe central sensitisation in the clinical setting .

Subjective features

• Disproportionate, non-mechanical, unpredictable pattern of pain provocation in response to multiple/non-specific aggravating/easing factors.
• Pain persisting beyond expected tissue healing/pathological recovery times.
• Pain disproportionate to the nature and extent of injury or pathology.
• Widespread, non-anatomical distribution of pain.
• History of failed interventions (medical/surgical/therapeutic).
• Strong association with maladaptive psychosocial factors (e.g. negative emotions, poor self-efficacy, maladaptive beliefs, and pain behaviours, altered family/work/social life, medical conflict).
• Unresponsive to NSAIDs and/or more responsive to anti-epileptic (e.g. Lyrica) /anti-depressants (e.g. Amitriptyline) medication.
• Reports of spontaneous (i.e. stimulus independent) pain and/or paroxysmal pain (i.e. sudden recurrences and intensification of pain).
• Pain in association with high levels of functional disability.
• More constant/unremitting pain.
• Night pain/disturbed sleep.
• Pain in association with other dysesthesias (e.g. burning, coldness, crawling).
• Hyperpathia or pain of high severity and irritability (i.e. easily provoked, taking a long time to settle).

Clinical features

• Disproportionate, inconsistent, non-mechanical/non-anatomical pattern of pain provocation in response to movement/mechanical testing.
• Positive findings of hyperalgesia (primary, secondary) and/or allodynia and/or hyperpathia within the distribution of pain.
• Diffuse/non-anatomical areas of pain/tenderness on palpation.
• Positive identification of various psychosocial factors (e.g. catastrophisation, fear-avoidance behaviour, distress).

Identification in the Clinical Setting

Nijs et al provided guidelines to aid the recognition of central sensitization on musculoskeletal patients.

In their paper they suggest that a patient's medical diagnosis can offer insight into the likelihood of the presence of central sensitization (fig 1) and this in conjunction with observable features (fig 2) can inform the therapist as to the presence of central sensitization.

fig 1. Table of Medical diagnoses likely to suggest presence of central sensitization reproduced from Nijs et al.

Medical Diagnosis	Central sensitization is a characteristic of this disorder	Central Sensitisation is present as a subgroup
Chronic lower back pain		X
Chronic Whiplash associated disorders	X
(Sub)acute whiplash associated disorders		X
Tempromandibular disorders		X
Myofascial pain syndrome		X
Osteoarthritis		X
Rheumatoid arthritis		X
Fibromyalgia	X
Chronic fatigue syndrome	X
Chronic headache		X
Irritable bowel syndrome	X

Fig 2: table reproduced from Nijs et al of symptoms related to central sensitization

Symptom	Characteristic of central sensitization	Might be related to Central sensitization
Hypersensitivity to bright light	X
Hypersensitivity to touch	X
Hypersensitivity to noise	X
Hypersensitivity to pesticides	X
Hypersensitivity to mechanical pressure	X
Hypersensitivity to medication	X
Hypersensitivity to temperature (high or low)	X
Fatigue		X
Sleep disturbances		X
Unrefreshing sleep		X
Concentration difficulties		X
Swollen feeling (e.g. in limbs)		X
Tingling		X
Numbness		X

Activity dependant Central sensitization

Latremoliere and Woolf describe the changes demonstrated in their group's 1983 study as "activity dependent central sensitization". This describes the mechanism of functional synaptic plasticity evoked by in the dorsal horn neurons by input from nociceptors . They found to invoke sensitization the noxious stimuli had to be sustained, intense and repeated.  The changes can be divided into two, time dependent phases; An early, short duration phase which is phosphorylation-dependant/transcription-independent and a longer lasting transcription-dependant phase.

Activation of the NMDA receptor is an essential step in initiating and maintaining the sensitization. Under normal circumstances this receptor channel is blocked by Mg²⁺ ions Sustained release by nociceptors of glutamate, substance P and CGRP leads to membrane de-polarization, forcing the Mg²⁺ from the NMDA receptor. This rapidly boosts synaptic efficacy and allows Ca²⁺ in to the neuron, activating intracellular pathways and maintaining central sensitization.

 A comprehensive description of the cellular processes can be found in Latremoliere and Woolf's 2009 review.