Myths and Facts about Cancer Pain

by Dr Gordon Coates

Series Context

This article is the first section of the book "How Cancer Pain is Treated" (a non-technical guide for patients and their carers). You will find links to the other articles in the series at the bottom of this page. Alternatively, you may download the whole book in various formats HERE. (All downloads are free.)

Disclaimers

This article, written by a senior medical practitioner with considerable experience in palliative medicine and hospice care, is offered purely for educational purposes. Nothing in it should be taken as therapeutic advice for any particular patient. Mention of any trade (brand) name should not be taken as an endorsement of the brand or its manufacturer.

Cautions

If you read the articles in this series carefully, and think about the information in them, in relation to a particular pain management problem affecting you or someone you love, you may sometimes be able to think of modifications to the current treatment which might be expected to improve the situation.

However, it is very dangerous to make changes to a patient's medication without first discussing them with the prescribing doctor. The doctor must always know exactly what the patient is taking, as virtually all medications can cause unwanted side effects and interact in various ways with other medications.

Importantly, this also applies to "natural", "alternative" or "complementary" therapies, many of which have significant interactions with prescribed medications. Therefore, even if you feel that the current pain management is not optimal, never make any changes without first discussing them with the doctor.

Introduction

Most patients with advanced cancer experience severe pain if they do not receive good pain management, but their pain can almost invariably be relieved if it is managed correctly.[1], [2] Patients whose cancer has been cured or is in remission may also need treatment for pain in some cases.

The principles of cancer pain management are well established[1], and indeed they have changed very little since I summarised them for a medical readership a quarter of a century ago.[2] However, there have been a number of useful practical developments, which have made treatment more convenient, reduced associated side effects, or, in some cases, provided better solutions to previously difficult problems.

This series of articles is intended for a general readership. Therefore, although many medications will be named in it, no medical or nursing knowledge will be assumed. It is my hope that a better understanding of what can be achieved, and how to achieve it, will be helpful to patients and their relatives when discussing treatment with their doctors or other health care professionals.

Cancer pain can be relieved by treating the cancer itself; by inhibiting the mechanisms by which cancers can give rise to pain stimuli; by interrupting the "pain pathway" which carries pain stimuli to the brain; or by inhibiting the perception of pain stimuli which reach the brain, either by the use of medications or by non-drug interventions. I will discuss all of these methods in this series of articles, but I will go into more detail about the optimal use of medications, as these are often the mainstay of treatment.

Myths about Cancer Pain

There are many myths and misconceptions about cancer pain, but the one which is most important to dispel in the present context is the mistaken idea that cancer, and especially advanced cancer, inevitably causes pain which cannot be relieved. Nothing could be further from the truth!

Although the majority of cancer patients need pain management as part of their overall treatment, excellent pain control, without severe side effects, can be achieved in most cases by using simple, cheap and readily available methods.

In the approximately fifteen percent of patients who do not respond to these methods, more complex interventions are necessary, often requiring the services of a Pain Clinic or Palliative Care Service. But even in the most difficult cases, there is no such thing as "pain for which nothing can be done".

Unfortunately, even today, many people do not know this. There are even some doctors and nurses who do not have a good understanding of pain management. They may have great expertise in some other field, but perhaps not in this one. So, if you are ever told that "nothing more can be done for your pain", or if something similar is said about a loved one who is suffering, you should certainly request referral to a Pain Clinic, a Palliative Care Service, or a Pain Management specialist.

Another problem which sometimes interferes with good pain management is simply failure to acknowledge the need for it. For various reasons, cancer patients or their doctors sometimes brush the issue aside as if it did not exist. This is, of course, a particular example of the denial which is very common as an initial response to any unwanted experience or situation.

Unfortunately, denial of its existence never solves any problem, and cancer pain is no exception to this rule. The first step in pain management is therefore to face and accept the fact that something needs to be done. That may sound obvious, but denial is a common error, so it is well worth watching out for.

Mistaken beliefs about strong "opioid"[3] analgesics are also responsible for inadequate pain control in many cases. Although the misuse of strong opioids as recreational drugs often leads to severe psychological addiction as well as rapidly escalating physiological tolerance, it has been known for many decades that neither tolerance nor addiction is a significant problem when strong opioids are used correctly in the management of cancer pain. Indeed, a strong opioid such as morphine is very often (though by no means always) an essential component of the overall management of cancer pain.[4]

Some patients also fear that the use of strong medications too early in the course of their illness might leave them with nothing to relieve severe pain later on. If anything, the opposite is true, as pain control is easier to maintain than to achieve, and easier to achieve before a pain has been too long established. This is probably at least partly because the ability of the central nervous system to perceive pain improves with practice.

Various other common attitudes, such as the ideas that pain should be accepted stoically, that "mind over matter" should be sufficient to control pain, or that doctors are so busy with "more important" matters that they should not be interrupted by complaints about pain, may also inhibit timely intervention. (It is true that doctors are usually very busy. However, the relief of suffering is one of their most important tasks.)

Yet another misconception about the management of cancer pain is the idea that a single method of pain control should be tried first, and then replaced with something else if the results are not satisfactory. The truth is that multiple methods of pain control are frequently necessary in the treatment of cancer pain.

Facts about Pain

Definition

Firstly, what is the definition of physical pain? I think most people would say it is "anything that hurts". A more specific definition is "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage".[5] As is the case with most brief definitions, a little further explanation may be useful.

Being an "experience", pain is essentially subjective. It therefore cannot be measured directly by an external observer. Having both "sensory" and "emotional" components, both of which can vary enormously, pain is inevitably a complex and highly individual phenomenon. Incidentally, the word "described" in the definition is not intended to restrict the existence of pain to patients who can describe it. On the contrary, it is very important to consider the possibility of pain in those who, for whatever reason, are unable to communicate.

General Classification

It is important to distinguish between different types of pain, because different types of pain respond to different treatments. In general terms, the two main types are "acute" and "chronic" pain. Some people also talk of an intermediate classification, called "subacute". These three types of pain, and some extra terms which are used in certain situations, are explained below.

Acute Pain

Acute pain comes on fairly quickly, usually as a result of an accident or a new or recurrent illness, and is usually fairly brief in duration. This is usually either because it motivates the patient to seek urgent treatment, or because the pain goes away by itself. Occasionally, it is because the cause of the pain is so serious that it soon results in the death of the patient.

Chronic Pain

Pain is described as chronic when it has been present for a long time. It may have developed so gradually, and been present for so long, that it simply seems endless. Because it is not a sign of a new problem, requiring a new diagnosis and new treatment, it may seem meaningless as well as endless. Alternatively, if a meaning is ascribed to it, it is quite often a frightening one.

Subacute Pain

Some doctors use the term "subacute pain" when referring to cancer pain, because patients with cancer sometimes have some features of acute pain and some features of chronic pain, both at the same time. Other patients with cancer may have no pain, or occasional episodes of acute pain, or chronic pain with or without the additions described under the next heading.

New, Incident and Breakthrough Pain

Of course, the first episode of a new pain might occur during the course of any chronic pain. Until the cause is diagnosed, it is best to simply refer to it as a new pain. When it is known to have a specific cause, such as a medical procedure or an accident, it is often referred to as "incident pain". On the other hand, if it represents a temporary failure of the relief of the existing chronic pain, it is called "breakthrough pain".

Physiological Classification

It is also useful to classify pain according to the physiological mechanism by which it is produced. From this perspective, most pain is either "nociceptive" or "neuropathic", as described below. I will also mention some less well understood mechanisms by which pain can probably be produced. In some cases, more than one mechanism may be involved, and in other cases the mechanism may be difficult or impossible to determine.

Nociceptive Pain

How do we become conscious of pain? The simplest, and also the commonest, way is as follows. Processes affecting the body in such a way as to cause actual or potential tissue damage are almost always recognised by specialised sense organs at the ends of peripheral nerve fibres. These sense organs then send electrochemical signals back along their nerve fibres, which join larger and then still larger nerves, pass through nerve plexuses, and ultimately connect with one or more spinal nerve roots. The spinal nerve roots pass between adjacent vertebrae to join the spinal cord, which then carries the signals up to the brain.

Because they have become specialised so that they respond specifically to potentially or actually "noxious" (harmful) processes, the sense organs described in the previous paragraph are called "nociceptors". For the same reason, the pain which is experienced when such a process causes signals to be sent from nociceptors to the brain is called "nociceptive pain". Much of the pain caused by cancer is in fact nociceptive pain.

Neuropathic Pain

The second major mechanism which can cause pain to be experienced occurs as a result of an injury or illness affecting the nervous system itself. Damage to various parts of the nervous system can sometimes result in electrochemical phenomena which are ultimately interpreted by the brain as pain. Because the nervous system is the origin of the phenomenon, rather than simply being the transmitter of the signals, this second type of pain is called "neuropathic pain".

Cancer is only one of many possible causes of neuropathic pain, other examples being the "post-herpetic neuralgia" which sometimes develops after shingles (herpes zoster), and the "phantom limb pain" which may develop after an amputation. However, cancer is a very important cause, as neuropathic pain due to cancer accounts for many of the most difficult pain management challenges encountered in medical practice.

In many cases, neuropathic pain appears to be a response by some part of the central nervous system (brain and spinal cord) to the absence of normal incoming signals. Because incoming signals are referred to as "afferent" (meaning towards the brain) this type of neuropathic pain is called "de-afferentation pain".

De-afferentation pain sometimes, though far from always, develops some time after an injury to, or destruction of, a peripheral nerve, a spinal nerve root, or a "tract" of nerve fibres carrying signals up the spinal cord to the brain. The injury or destruction might be due to a local tumour deposit, a complication of cancer therapy, or a toxic substance produced by cancer cells elsewhere in the body and released by them into the bloodstream. In a few cases, it might be caused by something else, which might not be related to the cancer at all.

It is also possible for neuropathic pain to be caused by direct stimulation of a nerve or spinal tract by a local tumour deposit, even though no permanent damage is done. This causes signals to flow up the nerve fibres as if they had been sensed by nociceptors at the nerve endings. This second type of neuropathic pain is thus produced by the presence of abnormal signals, rather than the absence of normal signals, and is therefore not quite the same as de-afferentation pain.

However it is caused, neuropathic pain can often be suspected from the description given by the person suffering from it. It is often described as "burning", "tingling", "shooting" or "like electric shocks", and it has often persisted after other pains have responded to analgesic medication. Also, the region in which the pain is felt usually corresponds anatomically to the area supplied by some part or parts of the nervous system.

Examination by the doctor may also reveal a neurological abnormality, such as the absence of a normal reflex, or an area of diminished or heightened sensation. It may also be possible to demonstrate "allodynia" (pain resulting from a stimulus which is not normally painful). However, the exact details of the chain of events responsible for a particular example of neuropathic pain are often difficult to establish.

Sympathetic Dependent Pain

Sympathetic dependent pain, which is also called sympathetic(ally) maintained pain, is probably a rare and poorly understood type of neuropathic pain which has its origin in the sympathetic division of the autonomic nervous system[6]. However, at the time of writing it is usually classified separately, as its characteristic features are very different from those described above under the heading Neuropathic Pain.

Sympathetic dependent pain often affects part or all of a limb, usually a lower limb, in a distribution which does not match that of any spinal nerve roots or peripheral nerves. It is often associated with abnormalities of skin colour, hair growth and sweating in the affected region. It responds poorly to analgesic medication, and is usually best treated by a specialised type of nerve block called a "sympathetic (plexus) block".

Vascular Pain

The term "vascular pain" should really be reserved for pain which arises from the walls of a blood vessel, but it is sometimes used loosely to mean ischaemic pain (see next heading). Blood vessels probably have specialised pain receptors in their walls which can produce pain stimuli when something damages the vessel or causes it to dilate or constrict, but vascular pain, like sympathetic dependent pain, is very incompletely understood at the time of writing.

Some patients with cancer experience pain which seems to arise from blood vessels damaged by tumour deposits. The region in which pain is felt usually corresponds anatomically to the area supplied by some part of the vascular system, rather than the nervous system. Medical examination or special investigations may sometimes provide independent evidence of vascular damage. The treatment of vascular pain is complex and difficult, and is usually best left to pain specialists.

Ischaemic Pain

Ischaemic pain is pain which results from an insufficient blood supply, and it may involve a number of mechanisms. When the blood supply of a tissue or organ is insufficient for its needs, the tissue or organ is said to be "ischaemic" and the condition is referred to as "ischaemia". Depending on the severity of the ischaemia, a variable amount of tissue irritation or damage occurs, thereby stimulating local nociceptors and causing the nociceptive type of pain previously described.

However, depending on the tissue involved and the degree and duration of ischaemia, damage may also occur to nerves within the tissue, giving rise to neuropathic pain as well. In addition, if blood vessels in the tissue suffer ischaemic damage, vascular pain may also occur. Ideally, ischaemic pain is treated by medical or surgical treatment to relieve the ischaemia itself. When this is not possible, treatments which target nociceptive, neuropathic and/or vascular pain may be necessary.

Pain Disorder

Sometimes, no known physiological mechanism can be discovered to account for a patient's pain, or else the severity of the pain is greater than can be accounted for by any known mechanism. In this situation, the patient is sometimes said to be suffering from a "pain disorder" if the physiological mechanism is completely unknown, or from an "organic pain disorder" if a known mechanism exists but is considered to be only partially responsible for the pain which the patient reports.

Pain disorders probably have various psychological factors as major or contributing causes in many cases, but there may also be unknown physiological factors awaiting discovery. Pain disorders are rarely the main reason for pain in patients with cancer, but various psychological factors can certainly contribute to the severity of cancer pain in some cases.

How Cancer Causes Pain

There are many types of cancer, but they all share the same essential feature: the cells of which a cancer consists have lost the ability to respond to the body's normal control over cell division. Not only that, but they have also gained the ability to survive the body's available defences (which would otherwise recognise them as abnormal, and then destroy them). As a result, the number of cancer cells increases, and the cancerous tissue therefore keeps growing larger and larger.

To make matters worse, any cancer cells which are washed into the blood capillaries or lymph vessels passing through the cancerous tissue may "metastasise" (spread) to other parts of the body, where they then continue to divide and thus create secondary deposits of the same type of cancerous tissue. Either the primary cancer, or one or more of these secondary deposits, may then cause pain by damaging adjacent body tissues.

An enlarging mass of cancer naturally causes pressure on nearby structures, and it can also lead to stretching of other tissues, such as the capsule of a solid organ. Either compression or stretching can stimulate the nociceptors in nearby nerve endings. In addition, compression of blood vessels can lead to damage to the tissues which were previously supplied with oxygen by those blood vessels.

Another possible cause of pain occurs when a normally hollow structure (such as the intestine, or a duct which carries secretions) is obstructed by a growth inside it, or by pressure from outside it. The muscles in the walls of the hollow structure then try very hard to overcome the obstruction, causing the recurring spasms of pain often referred to as colic.

As mentioned previously, a peripheral nerve, or a part of the central nervous system, could be damaged by a cancer deposit, which sometimes results in neuropathic pain. Some of the methods used to treat cancer or to relieve its effects also have complications which can result in pain. In addition, simply being unwell, with the associated lack of normal activity, can cause or exacerbate musculoskeletal pains (common types of pain from muscles, tendons, ligaments and joints).

Finally, if a cancer deposit is growing inside a bone, then in addition to causing pain in the ways mentioned above, it may also weaken the bone so much that a fracture can occur without any significant injury. This is called a "pathological fracture". The fracture itself will then be a cause of pain in its own right until it is treated to prevent movement of the broken pieces of bone against each other. If a blood vessel or some part of the nervous system is also damaged as a secondary effect of the fracture, this may, of course, lead to further painful results.

As you can see from the above examples, there are many different ways in which cancer can cause pain. In many cases, the mechanism responsible for a pain can be suspected from the description of the pain itself. Knowing the mechanism can be helpful when choosing the best method of treating the pain, so it is very helpful to the doctor to be given a good description.

For example, infiltration of an organ by cancer often causes a dull and poorly localised pain as a result of damage to its component tissues. However, if the cancer deposits increase the overall size of the organ sufficiently to stretch its capsule, that causes a sharper, and more localised type of pain.

Infiltration of bone causes a pain rather like toothache, which is quite well localised and is associated with local tenderness, especially on impact. (It may also include a neuropathic element, which, as mentioned previously, might cause burning, tingling or shooting pains, or might feel like electric shocks.)

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Declaration of Interest

None.

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Footnotes: (Click the number of a footnote to return to its reference in the text)

[1] World Health Organization. 1996. Cancer Pain Relief: with a Guide to Opioid Availability - 2nd ed. Geneva: WHO Publications.

At the time of writing, the above guidelines are available in 17 languages and a 1998 companion edition about cancer pain relief and palliative care in children is available in 11 languages. English, Spanish and French editions can be downloaded at http://www.whocancerpain.wisc.edu/?q=node/130 and information about editions in other languages is available at http://www.whocancerpain.wisc.edu/?q=node/26.

(The English version of the 1996 guidelines can be opened directly by any browser with a PDF plugin at http://whqlibdoc.who.int/publications/9241544821.pdf. However, the file size is 1.84MB, so quick access requires a broadband connection.)

[2] Coates, GT, "Management of cancer pain. A practical approach", Med J Aust 1985, vol. 142, pp. 30-35. (Available online, as a 225KB PDF file, HERE)

[3] Although "opiates" were originally extracts of the crude opium harvested from the common edible poppy, while "opioids" also included synthetic compounds with similar effects, the two terms have effectively become synonyms. I will use the latter term in this series of articles.

[4] Twycross, RG. Relief of pain. In: Saunders C, ed. The management of terminal disease. London: Edward Arnold, 1978: pp. 65-92.

[5] This definition was first proposed, with slightly different wording, by Dr Harold Merskey in 1964 (Merskey, H, 1964, An Investigation of Pain in Psychological Illness, DM Thesis, Oxford). With various qualifications, it has been used by the International Association for the Study of Pain (IASP) since 1979.

[6] The autonomic nervous system is a specialised part of the peripheral nervous system which automatically controls all those bodily functions that go on constantly without our conscious awareness. Its two main divisions are the sympathetic nervous system, which joins the central nervous system via certain thoracic and lumbar spinal nerve roots, and the parasympathetic nervous system, which joins the central nervous system via certain cranial nerves and sacral nerve roots.

Articles in the Cancer Pain Series:

1. Myths and Facts about Cancer Pain (this page)

2. How Cancer Pain can be Relieved

3. Medications used to Relieve Cancer Pain

4. Optimal Use of Opioid Analgesics

5. Appendices:

Appendix 1: Use of Opioids in Renal Failure

Appendix 2: Use of Opioids in Hepatic Failure

Appendix 3: Special Medication Delivery Systems

For more free articles and ebooks by the same author, on a wide range of topics, visit http://www.wanterfall.com