Central Sensitisation Syndrome

Conditions

An increase in the excitement of neurons in the central nervous system so that normal inputs from the neurons start to produce an abnormal response

What is Central sensitisation syndrome?

When we feel pain from a bump, knock or scald the body’s pain pathways send messages from the peripheral nervous system (nerves outside the brain and spinal cord) to the brain through the spinal cord to warn us that something harmful is happening. This allows us to protect the body from any further damage. When this happens, pain is seen as a protective function of the body. However, sometimes the pain pathways of the body can become too sensitive and this causes people to feel pain that has no benefit.

Normally when we feel pain it is conducted through the body by a system of neurons (a cell that conducts nerve impulses) which are designed only to deal with pain: the nociceptive system. This is the system responsible for causing the ‘ouch’ pain felt when we touch a hot surface or stub a toe. When the nociceptive system detects a harmful stimulus (something causing or regarded as causing a response) it relays the pain message along a peripheral nerve to the spinal cord and the brain. The speed by which these messages travel will differ according to the type and severity of the pain. Dull aching pain is relayed through fibres that travel at slow speed. Severe sharp pain is transmitted through fibres which travel at a greater speed and this pain is felt almost instantaneously.

There are other types of pain which can occur as a response to tissue damage (for example, following an injury or operation). These include inflammatory pain felt in response to inflammation and neuropathic pain which is caused by damage to the nervous system or changes to the function of the nervous system (functional pain). These types of pain can be spontaneous (that is, pain which occurs without any obvious peripheral stimulus or tissue damage) or it can be caused by hypersensitivity to peripheral stimuli (such as following an injury or an operation).

Central sensitisation is an increase in the excitement of neurons in the central nervous system so that normal inputs from the neurons start to produce an abnormal response. This increase in stimulation is usually triggered by activity in the nociceptive system such as following an injury. This changes the strength of the synaptic connections (a synapse is a junction that allows a neuron to pass an electrical or chemical signal to another cell) between the nociceptor and the neurons of the spinal cord.

Central sensitisation is responsible for ‘tactile allodynia.’ This means that pain may now be felt from a stimulus that would not usually cause pain such as clothing or sheets rubbing against the skin. The response to pain may also be amplified/increased and this is referred to as ‘hyperalgesia’. This means that an action such as stubbing a toe produces an exaggerated and long-lasting pain. Hypersensitivity can also spread beyond an area of tissue damage so that adjoining undamaged tissue is sore.

Central sensitisation may occur following an operation which results in pain on movement or touch, in migraine attacks where brushing hair can be painful and in some people with nerve damage where even a light breeze on the skin causes an intense burning pain.

The process of Central sensitisation syndrome

Central sensitisation has two phases: an immediate but short-lived phase and a second stage of slower onset which is longer lasting.

The first phase is a result of changes in connections within the spinal cord following a signal from the nociceptive system (such as following an injury or operation) which in turn causes excitability (increased stimulation) of the neurons. This can lead to increased firing, increased responsiveness and decreased threshold of the neurons.

A later stage of central sensitisation is triggered by an increase in protein production. The effect of these changes is that normally innocuous (harmless) inputs begin to activate the neurons and pain responsiveness is dramatically changed. This stage appears to be central to the increased sensitivity to pain that defines persistent and chronic pain syndromes.

Conditions considered part of the Central sensitisation spectrum

It is now thought that there are conditions that form part of the Central sensitisation syndrome and are due to altered function of the nervous system. These include: