Types of pain

Pain can be classified in a number of ways:

1) Pain can be classified in relation to its duration:

Acute pain is defined as short-term pain or pain with an easily identifiable cause. Acute pain is the body's warning of present damage to tissue or disease. It is often fast and sharp followed by aching pain. Acute pain is centralized in one area before becoming somewhat spread out. This type of pain responds well to medications.

Chronic pain is medically defined as pain that has lasted 6 months or longer. This constant or intermittent pain has often outlived its purpose, as it does not help the body to prevent injury. It is often more difficult to treat than acute pain. Expert care is generally necessary to treat any pain that has become chronic.

2) The experience of physiological pain can be grouped into four categories according to the source and related nociceptors (pain detecting nerves):

Cutaneous pain is caused by injury to the skin or superficial tissues. Cutaneous nociceptors terminate just below the skin, and due to the high concentration of nerve endings, produce a well-defined, localised pain of short duration. Example injuries that produce cutaneous pain include paper cuts, minor (first degree) burns and lacerations.

Somatic pain originates from ligaments, tendons, bones, blood vessels, and even nerves themselves, and are detected with somatic nociceptors. The scarcity of pain receptors in these areas produces a dull, poorly-localised pain of longer duration than cutaneous pain; examples include sprained ankle and broken bones.

Visceral pain originates from body organs. Visceral nociceptors are located within body organs and internal cavities. The even greater scarcity of nociceptors in these areas produces a pain usually more aching and of a longer duration than somatic pain. Visceral pain is extremely difficult to localise, and several injuries to visceral tissue exhibit "referred" pain, where the sensation is localised to an area completely unrelated to the site of injury. Myocardial ischaemia (the loss of blood flow to a part of the heart muscle tissue) is possibly the best known example of referred pain; the sensation can occur in the upper chest as a restricted feeling, or as an ache in the left shoulder, arm or even hand.

Neuropathic pain ("neuralgia") can occur as a result of injury or disease to the nerve tissue itself. This can disrupt the ability of the sensory nerves to transmit correct information to the thalamus, and hence the brain interprets painful stimuli even though there is no obvious or documented physiologic cause for the pain.

3)  Pain can be functionally classified into fast pain and slow pain:

· Fast pain - Fast pain is felt within 0.1 s of application of the pain stimulus. It can be described as sharp, acute, pricking pain and includes mechanical and thermal pain. It is mediated by type Aδ (A-delta) fibres at rates of between 6-30 m/s.

· Slow pain - Slow pain is an aching, throbbing, burning, chronic pain. Chemical pain is an example of slow pain. It is mediated by slower type C pain fibres at rates of between 0.5-2 m/s

There are 2 pathways for transmission of pain in the CNS. These are the neospinothalamic tract (for fast pain) and the paleospinothalamic tract (for slow pain).

Tags: pain receptors, Somatic pain, sensory nerves, intermittent pain, referred pain, body organs, Trauma (medicine)

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