Types of pain

Pain can be subdivided into various types depending upon its location and duration.

Superficial pain

Superficial pain is pain resulting from damage to the skin or mucous membranes where there are densely packed nerve endings. The greater the number of nerve endings the more possible it is to localise the site of the pain, so patients often point to the exact site with a finger (rather than vaguely indicating the site with their open hand as they would with, say, back pain).

Various kinds of stimulation may damage (traumatise) the body surface:

• thermal eg. fire burn
• electrical eg. electrical burn
• mechanical eg. cut or abrasion
• chemical eg. insect sting, acid burn.

Superficial pain is usually described with words like: pricking, stinging, burning, sharp.

As with all kinds of pain, although the damage to the body surface stimulates local nerve endings, the pain is still felt and interpreted by the brain and brain dead people such as accident victims have no sense of pain.

Deep pain

Deep pain is pain that originates within the viscera such as the organs of the chest and abdomen. In these tissues, pain receptors are widely separated and so the site from which the pain originates may be difficult to localise. Patients often indicate the location with an open hand rather than pointing. Deep pain impulses travel to the brain like those of superficial pain. The pain is often described as 'gnawing' or 'aching'.

Referred pain

Referred pain is pain felt at a site different to the site of the stimulus eg.

• fallopian tube pain referred to shoulder
• appendix pain referred to mid abdomen
• angina (heart) pain referred to arm or neck.

Referred pain of angina

The reasons why pain is referred are not fully understood. However pain is usually referred from visceral organs wherever there are few, widely scattered nociceptors so the brain may become 'confused' about the site of origin. It should be noted that referred pain is real pain (not psychological) and responds to appropriate analgesics or other treatment.

Organic and functional pain

• Organic pain is the term for pain resulting from an identifiable cause such as an injury or disease

Organic pain is sometimes referred to as nociceptive pain because it originates from stimulation of nociceptors in damaged tissue.

• Functional pain is pain that cannot be traced to an identifiable cause, for example post-herpetic pain

A number of the functional pain disorders are now classified as psychiatric illnesses, such as somatoform pain disorder in which the patient may feel severe pain in some part of his body without any identifiable cause. This kind of pain may respond better to psychotropic drugs such as antidepressants rather than analgesics. (In these disorders the pain is very real to the patient and sympathetic treatment is essential. This is not the same as malingering, in which the patient 'invents' the pain in order to avoid work).

Acute and chronic pain

Acute and chronic pain are differentiated mainly by their duration but they are different and require different treatment.

Characteristics of acute pain are:

• sudden onset with forseeable end
• capable of resolution
• curable underlying cause.

Acute pain is treated with simple analgesics, non-steroidal anti-inflammatory drugs (NSAIDs) and sometimes opiates.

Chronic pain can be subdivided into cancer (malignant) and non-cancer (non-malignant) pain.

Non-malignant pain such as sciatica, back pain, post-herpetic neuralgia and arthritis is not life threatening but characterised by its poor likelihood of resolution. The patient may be taught coping strategies and powerful analgesics may be required, but opiates, such as morphine, are not widely used because of the side effect potential.

Malignant pain is pain due to cancer, and in terminal illness, the patient may also be anxious, despairing and frightened as well as in pain.

All of these symptoms may require treatment and drugs used may include:

• antidepressants
• anxiolytics
• NSAIDs
• steroids
• anticonvulsants
• opiates.

In terminal illness, a holistic approach (treating the 'whole person') is required and the patient may need constant nursing in a hospital or hospice. There is often a sense of hopelessness because cancer pain cannot be seen as a useful protective mechanism in the same way as acute pain, and the patient is aware that he is not going to 'get better'. Opiates are widely used in the treatment of cancer pain because in life threatening illness, the risk of addiction is unimportant.