Pain: An introduction

• What is pain?
• What causes the feeling of pain?
  • Referred pain
• Pain regulation
  • Sensitisation
  • Gating
• Types of pain
  • Nociceptive pain
  • Neuropathic pain
• Types of pain
  • Acute pain
  • Chronic pain
• Pain models

What is pain?

Pain is “an unpleasant sensory and emotional experience, associated with actual or potential tissue damage, or described in terms of such damage.” There are two very important points to notice in this definition:

1. Pain is not just a simple physical sensation – it is also an emotional experience and should be treated as such; and
2. Pain isn’t always associated with actual damage to the body’s tissues. It may be caused by potential tissue damage, or even just feel like tissue damage even though none has actually occurred. This is referred to as neuropathic pain.

What causes the feeling of pain?

Every tissue in your body is supplied by special nerve receptors called ‘nociceptors‘. These are nerves which are specially designed to detect painful (or ‘noxious’) stimuli, for example extreme heat, mechanical damage like a pinch, or irritating chemicals. When the nociceptors detect a painful stimulus, the nerve will fire off an impulse which travels back along the nerve fibre to your spinal cord. From there, the pain message is conveyed up to the brain via a spinal neuron (nerve), travelling up through a part of the brain called the thalamus before ending in many different areas of the brain’s cortex.

The parts of the brain that the pain signals are sent to are important because they affect the way we perceive pain. For example, some of the nerve fibres end in parts of the frontal lobe of the brain, which normally handles behaviour and decision-making. The pain fibres that end in the frontal lobe cause us to feel pain as an unpleasant emotional sensation, sometimes even producing fear. In contrast, the pain fibres that end in an area called the somatosensory cortex provide what we would think of as the purely ‘sensory’ aspects of pain, like its location and quality.

Referred pain

You may hear people talking about ‘referred pain‘. This means pain that originally comes from one organ or part of the body, but is felt in a different place. The commonest example is the pain of a heart attack, which may be felt in the left shoulder and down the left arm.

Referred pain occurs because of the way nerves meet up and interlink in your spinal cord. Deep structures such as the heart are supplied by different nerves from your skin. In the spinal cord, though, both of the deep and cutaneous (skin) nerves might meet up with just a single spinal nerve. This means that if the heart sends a pain signal up to the brain via that one spinal nerve, the brain can’t tell whether the pain comes from the heart or from the area of skin. Because most pain signals come from the skin, and we only rarely feel pain from deep organs, the brain will interpret the signal as having come from the skin – and that is where the pain will be felt.

Pain regulation

Sensitisation

Sensitisation refers to a situation where part of the pain pathway becomes over-sensitive. Sensitisation may occur after intense, repeated or prolonged stimulation of damaged tissues. When sensitisation does occur, pain fibres are more likely to be triggered in response to stimuli which would not normally be painful. This may contribute to the development of chronic pain (see below).

Gating

‘Gating‘ is a normal regulatory mechanism which affects the way we perceive pain. The first gating mechanism is found in the spinal cord. Here, normal (non-pain) sensory fibres are linked to pain fibres in such a way that one can suppress the other. For example, after stubbing your toe (a painful stimulus), gently rubbing the skin over the toe (non-pain sensation) can reduce the feeling of pain. The non-pain sensation takes over and inhibits the transmission of the pain sensation. This mechanism is the basis of some types of pain treatment, including spinal cord stimulation.

The second gating mechanism is found in the brain. Here, emotional and psychological factors can result in the release of neurotransmitters such as endorphins. These can suppress nociceptive (pain) signals, reducing the feeling of pain.

Types of pain: Cause

Pain can be divided into a number of different types, depending on the source of the pain.

Nociceptive pain

Nociceptive pain is pain caused by damage to tissues. Tissue damage activates the nociceptive nerve fibres (see above) and pain signals are sent up through the spinal cord to the brain.

Nociceptive pain can be further divided into somatic pain and visceral pain, depending on the type of tissue that is damaged. The viscera are the organs of the body, so visceral pain means pain originating in an organ or hollow structure, such as the stomach or the gall bladder. Compared to the skin, these organs have a very low density of nociceptive fibres, meaning that pain from the viscera is very hard to localise, or may even be felt as referred pain. Visceral pain may be described as gnawing or aching, and may be associated with feelings of nausea.

Somatic pain refers to pain from structures other than the internal organs. This includes the bones, ligaments and tendons. This pain tends to be better localised and sharper than visceral pain.

Neuropathic pain

Neuropathic pain is a special type of pain produced by damage to part of the pain pathway. It is not associated with true tissue damage; instead, the pain pathway is ‘misfiring’, causing the experience of pain without actual injury. Neuropathic pain may occur after a disease such as herpes zoster infection (shingles), where damaged nociceptive nerve fibres in the skin send pain signals up to the brain even when no tissue damage is actually occurring.

Neuropathic pain is often severe, and is classically described as tingling, burning or having an electric-shock type quality. It may be associated with allodynia (severe pain on very light touch). Neuropathic pain can be highly resistant to standard pain treatments such as opiates, and antidepressant and anticonvulsant medications may be needed to help control pain.

Types of pain: Duration

Pain can also be divided into acute pain or chronic pain.

Acute pain

Acute pain is a short-term feeling of pain felt in response to an easily identifiable cause. It might be caused by surgery, some kind of trauma, or an acute illness. The feeling of pain is acting like a sort of warning system to let you know that an injury has occurred. Acute pain lasts for less than three months.

Chronic pain

Chronic pain may begin as acute pain, but lasts longer than would normally be expected for the sort of injury that has occurred. Chronic pain may also occur when pain comes back for an unknown reason. In chronic pain syndromes, the pain may feel a lot worse than seems to fit with the injury or damage that can be seen. The link between the tissue damage and the pain is lost. Chronic pain may develop from acute pain for a number of reasons. Factors which may contribute to the development of a chronic pain syndrome include:

• Presence of a chronic, painful disease such as cancer, rheumatoid arthritis, migraine headaches, fibromyalgia or diabetic neuropathy;
• Damage to the sensory nerves, or painful reflex muscle contraction; or
• Psychological conditions, such as depression or anxiety.

Chronic pain can be very difficult to treat, and is often frustrating for both the patient and medical professional. The optimal treatment approach should involve looking not only for the source of the pain and other physical factors, but also consideration of environmental and psychological factors which may be contributing to the pain syndrome.

Pain models

The biomedical model of pain is an older model which considers the mind and body as separate entities. This model aims to treat a single cause of pain, with little focus on emotional, psychological and other factors. It is unlikely to result in good outcomes if used to manage chronic pain.

Newer understandings of pain acknowledge that the experience of pain involves complex interactions with every part of life. This is known as the holistic model of pain. A few of the elements involved are listed below:

• Physical: People with chronic pain often no longer have a single physical ’cause’ for their pain. This makes treatment with medications difficult. Instead, other physical factors such as nutrition and exercise can often be more helpful.
• Emotional: Anxiety, anger, guilt, blame and depression can all affect the way we experience pain. Feelings of peace and happiness can cause the release of endorphins which inhibit the feeling of pain.
• Thoughts: Our thoughts and the way we think can affect messages travelling from the brain to physical structures via nerves, hormones and immune system controls.
• Social: Relationships and social supports can have significant positive or negative influences on the perception of pain.
• Environmental: Environments at home, or in the workplace and the broader community can also affect pain perception.
• Spiritual: Addressing issues such as meaning, acceptance, hope and connection can have a positive influence on pain.

A major aim of the holistic model is to involve and empower the person with pain to help themselves.

 

References

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