CANCER PAIN AND ITS
MANAGEMENT
Pain
Pain, as defined by the International Association for the Study of Pain, is
"an unpleasant sensory and emotional experience associated with actual or
potential tissue damage, or described in terms of such damage".
Alternatively, pain "is what the patient says it is, and exists when he or
she says it does".(1)
Defining what is meant by pain is notoriously difficult. The definition
given above may seem simplistic, but consider the issues carefully and realise
how true it is. Pain is always subjective.
It is probably best to regard pain as an unpleasant response to a given
sensation or group of sensations, rather than a sensation in itself. Everyone
has suffered their own pains at one time or another, so you will know from
direct experience that pain may be provoked by a wide variety of stimuli,
including intense heat, intense cold, direct pressure on nerves, and chemical
or mechanical tissue injury.
From experience, however, you will know also that pain is highly
unpredictable. Its severity is strongly influenced by emotional and
environmental factors, as well as by straightforward variation between
individuals. The same stomach ache, for example, may seem agonising when you
are worried that it may be a sign of a duodenal ulcer, yet seem only a mild
discomfort when you know it is only indigestion. For patients suffering the
severe, unremitting, pain that is frequently associated with cancer, this
emotional aspect to pain may be especially important.
Classification of pain
Pain can be classified on the basis of intensity, duration, and origin.
Intensity
The intensity of pain is very subjective, and can be modified by
psychological factors which have a bearing on an individual's reaction to the
pain. The intensity of pain may be described as:
These classifications are loosely defined, what one person judges to be
moderate pain may be severe to another. Therefore, the intensity of pain is
usually assigned to a range (e.g. mild to moderate, or moderate to severe). The
doctor seeks to diagnose the cause of the pain by asking for a description from
the patient (e.g. "sharp", "knife-like", "dull").
Duration
The duration of pain may be described as acute or chronic.
Acute pain is sudden and generally does not recur. It is the body's early
warning system of harmful stimuli and may abate relatively quickly. It is
classified on the basis of duration not intensity, although it may be severe
when it does occur.
Chronic pain is classified as discomfort lasting longer than six months. Its
onset may be insidious. Chronic pain alters a patient's capacity to function
normally, as well as affecting personality. An example of this type of pain is
that resulting from tumour infiltration of nervous tissue.
Origin
The origin of pain may be described as cutaneous or visceral.
Cutaneous pain is also termed superficial, because it originates in skin,
skeletal muscle, joints, ligaments, and other structures near the surface of
the body. It is often associated with tissue injury and inflammation, as in
arthritis, for example.
Visceral or deep pain has its origin in internal organs and tissues of the
body that are innervated by the autonomic (involuntary) nervous system. Stomach
cramps, period pain, and pain originating from bone metastases, are common
examples.
When the pain of a visceral organ is transferred to another area supplied by
the same sensory nerves that innervate the injured area, it is called referred
pain. For example, diaphragmatic pain is often referred to the tip of the
shoulder. A working knowledge of the main patterns of referred pain is
therefore useful in diagnosis of the origin of pain.
Transmission of pain impulses
Although the descriptions of pain are subjective, the physiological
processes underlying its perception are common to us all. Pain is detected by
free nerve endings located in the skin and certain internal tissues. These are
activated by strong stimuli such as intense pressure, intense heat or cold, or
strongly acidic or alkaline chemicals. It is suggested that such stimuli exert
their effect mainly through the tissue damage they cause. This damage triggers
the release of certain chemicals (e.g. histamine), which act directly on the
nerve endings.
It is thought that there are two different routes via which pain-inducing
stimuli travel to the spinal cord and brain. These produce two different types
of pain experience.
To understand this better, imagine what happens when you hurt yourself by
standing on a drawing pin. Immediately, a sharp, pricking sensation is felt
which causes you to lift the foot very quickly. Almost as soon as you remove
your foot from the pin, the pricking stops but then you start to feel a dull,
aching pain that may continue for some time afterwards.
The sharp, pricking pain is caused by activation of the free nerve endings
of a group of nerve fibres located in the superficial tissues. These fibres are
capable of transmitting impulses very quickly to the sensory cortex of your
brain, which allows you to localise the source of the pain very accurately and
respond quickly with an appropriate action.
The dull, aching pain is caused by activation of the free nerve endings of a
second type of nerve fibres, which are essentially nociceptive, located in the
deeper tissues. Impulses are transmitted around ten times more slowly along
this type of fibre to the mid-brain. This sensation is much less localised than
the initial pinprick. It is long-lasting and discourages you from using the
injured foot until it is healed.
Somatic (also called nociceptive) and visceral pain involve direct
activation of nociceptors and are often complications of tumour infiltration of
tissues, or injury of tissues as a consequence of cancer therapy.
The physiology of neuropathic pain is incompletely understood, but appears
to differ from that of somatic or visceral pain. It may be a complication of
injury to the central or peripheral nervous systems.
Modification of pain
The perception of pain in any individual is more complex than the
physiological processes alone can explain. Activation of nociceptive pathways
may be the trigger, but endogenous modification agents, in addition to
psychological and emotional processes, are also involved.
The way in which stimuli arising through local tissue damage are transferred
via nociceptors along nerve pathways running from the tissues to the CNS (i.e.
ascending pathways) has already been described. In addition, however, there are
pain-suppression pathways running from the CNS to the tissues
(descending-pathways) that sometimes help to partially alleviate pain
naturally.
These descending pathways use various chemical substances as their
neurotransmitters, including serotonin and noradrenaline. Serotonin is also a
pain mediator, capable of generating or aggravating certain kinds of pain and
inflammation. Other substances that can augment or influence the endogenous
pain-relieving pathways include bradykinin, histamine, acetylcholine, and
prostaglandins. Drugs that affect the pharmacological actions of these
substances may have analgesic activity. In addition, endogenous opioids are
involved in the modification of pain perception.
Opioids, such as morphine, bind to special receptors at various points in
both the ascending and descending nerve pathways, thereby intercepting the
transmission of painful stimuli and enhancing the body's natural painkilling
mechanisms, perhaps by stimulating the release of serotonin or noradrenaline.
Cancer-related pain
The onset and progression of severe pain is recognised as the single aspect
of cancer that patients and their families fear most. Worrying about how to
cope with the pain can be even more distressing than facing up to the knowledge
(or fear) that death is not far away.
This concern is well-founded, since pain is indeed a major problem in
cancer. It has been estimated that 70% of patients with far-advanced cancer
have pain, as do 50% of those still receiving anti-cancer treatment.(2) Furthermore, about 50% of cancer pain is of moderate to
severe intensity, while a further 30% is very severe or excruciating. In most
cancer patients, the pain becomes steadily worse as the disease progresses, and
increasingly strong painkillers become necessary. The majority of patients
eventually have pain severe enough to need a strong opioid, such as morphine,
to control it.(3) It is not surprising that there is good
evidence that patients with cancer who are in pain are more emotionally
distressed by their illness than are patients who are pain-free.
Pain in cancer patients can originate from the effects of curative
treatments, such as radiotherapy, chemotherapy, and surgery, or it can also be
a direct result of the tumour invading or compressing soft tissues and vital
organs. Pain may also be unrelated to the cancer, such as migraine headaches or
chronic low back pain. How well the patient tolerates the pain is dependent
upon their physical and psychological condition.
Bouts of pain that occur against a background of underlying, but controlled,
chronic pain are often described as breakthrough pain. Occasional bouts of
breakthrough pain occur in many (perhaps most) patients being treated for
cancer pain. Patients are routinely provided with additional, preferably
fast-acting, analgesia that they can use as required when breakthrough pain
occurs. If breakthrough pain starts to occur frequently, it is usually a sign
that the disease is progressing and that the underlying pain is worsening. To
combat this, the patient's regular analgesic dose needs increasing, or the drug
should be substituted.
Total pain
Recognition of the multi-factorial nature of cancer pain makes it easier to
understand why some patients continue to experience intolerable pain even when
given increasing amounts of analgesia. Different people respond differently to
pain. They have different pain thresholds and different levels of pain
tolerance. The concept of total pain has been developed to encompass all
relevant aspects of the nature of cancer pain, and to take into account the
needs of the patient and family. Psycho-social factors influence the physical
components of the pain such that they become major determinants of the severity
of that pain.
Physical (somatic) aspects
Most cancer patients experience pain which is predominantly neuropathic but
which is usually complicated by somatic and/or visceral pain. For example, a
patient with lung cancer is likely to experience nociceptive pain at the chest
wall due to tumour infiltration, accompanied by neuropathic pain, perhaps in
the arm, as a result of infiltration of nerves. The cancer and its treatment
may change body appearance and normal functions, with resulting disability and
sleep disturbances.
Psychological influences
The psychological status of patients and their families is central to the
perception and control of pain. Cancer patients' emotional response to
diagnosis and treatment can include denial, anger, hopelessness, fear, and
acceptance. (4)
Anxiety and depression are closely intertwined with pain. Patients in pain
who are unable to get relief from it may be unable to sleep or rest adequately,
and the pain itself may cause them additional anxiety (e.g. they may worry that
it is a sign that their condition is worsening). Thus, pain may cause anxiety
or depression, yet the anxiety or depression may also heighten the perception
of pain.
The psychological pain of cancer is intense, affecting not just the patient,
but the family and professional carers as well. Cancer pain carers are very
aware of the psychological pain involved and try to address it as sensitively
as possible. Financial worries and other practical issues, such as dependent
relatives, also add to the psychological burden and can exacerbate somatic
pain.
Causes
Pain in cancer has many different causes, only about two-thirds of which are
related directly to the cancer itself.(2) It is important
that these various causes are differentiated and correctly diagnosed, because
their treatment may differ considerably. It is also important that the likely
cause of the pain is explained adequately to the patient, to let them know
whether a new or worsening pain is related to a worsening of the cancer. Often,
the patient is polysymptomatic, with more than one pain presenting a problem.
Pain assessment and development of treatment plans
A major thrust of the WHO's Cancer Pain Relief programme is the premise that
nothing would have a greater impact on the quality of life of patients with
pain and cancer than implementation of the existing knowledge on pain and
symptom management.(8)
Therefore the major aim of pain assessment is to diagnose its cause, as far
as possible, in order that the most appropriate treatment can be given. Two
provisos of this approach are as follows:
- invasive investigations become contra-indicated with advancing terminal
illness
- investigations should not be too time-consuming, bearing in mind the time
remaining to the patient
In the clinical assessment of pain, certain general principles are followed,
central to which is the belief in the patient's complaint of pain. Doctors and
nurses sometimes need great skill in interpreting the intensity of a patient's
pain. In this, they need to be aware of their own prejudices and attitudes to
pain. Patients who do not request pain relief or mention their pain, for
example, may have pain at least as intense as those patients who do seek help.
Careful observation of the patient and/or talking to the family or other carers
is as important as asking the patient directly about pain.
Cultural factors and the patient's own attitude to pain may also be
important. Some cultures place great value on stoicism. Patients may not want
to admit to their pain, since they worry that "giving in" to it will
be seen as a sign of weakness, or that they will lose self-respect by it. This
attitude sometimes leads to difficulties in patient compliance with analgesic
medication.
Once the diagnostic criteria have been met, a treatment plan can be
formulated for discussion with the patient and family or carers. The patient's
personality and functional ability should be taken account of in developing or
adapting such treatment plans. The physician should have a clear idea of the
patient's goals for therapy and place symptoms of pain and psychological
distress in highest regard. Continual reassessment of treatment efficacy should
be carried out, with alterations being suggested and implemented as
appropriate.
To help in the clinical evaluation of pain, several tools have been
developed which validate as far as possible the patient's and carers'
perception of the nature and severity of the pain.
Realistic aims
With careful assessment, more than 80% of cancer pain can be controlled
using analgesics and adjuvant drugs.(2) It should be
possible to achieve some measure of pain relief within 24 to 48 hours in all
patients, but satisfactory relief may take as long two to three weeks of
inpatient treatment.(2)
It is worth noting that adequate control of pain does not necessarily mean
that pain is absent altogether. It may just mean that the pain has less
significance in the patient's life, enabling it to be to coped with better. It
is also worth remembering that, in some cases, the side-effects of the
analgesia prove more difficult to control than the pain itself.
It has been recommended that there should be different target levels for the
patient in pain (see below). Even for patients whose mobility remains limited
by pain, the encouragement and sense of achievement given by improved sleep and
relief at rest may give a new hope and quality to the lifespan remaining.
Targets(2)
For the patient in pain, the initial target is a painless night's sleep.
Remember that a patient with chronic pain may not have slept properly for weeks
or months. Achieving this first target is an important boost to both the
patient's and the doctor's morale.
The next target is relief at rest during the day, either in bed or in a
chair. This should be possible eventually. The optimal target is freedom from
pain even on movement, but this may not be possible in every patient.
Pain management is often difficult but, with appropriate and adequate
treatment, the great majority of patients can achieve a reasonable level of
comfort. It is estimated that, by following the treatment guidelines set out by
the WHO (see p. 22)(8) , doctors can achieve effective
relief in 80 to 90% of their cancer pain patients. Currently, in the UK, it is
estimated that this target is reached in less than 50%.(6)
Analgesics
An analgesic is defined as an agent that produces relief from pain without
causing loss of consciousness. There is a wide range of such agents available
to the physician, varying in their ability to control different seventies of
pain. A broad classification of analgesics divides them into non-opioids and
opioids, on the basis of whether or not they exert opioid-like effects, by
interacting with opioid receptors in the brain.(7)
Non-opioid analgesics are often referred to as "simple analgesics"
because of their lack of central nervous system activity. Examples of these are
paracetamol and aspirin. Their predominantly peripheral action limits the
occurrence of side-effects. Opioid analgesics are referred to as weak or
strong, depending on their potency. Codeine preparations are regarded as weak
opioids, whereas morphine is an example of a strong opioid. An exhaustive list
of examples of opioid drugs is given on pages 28, 30 and 31.(8) Opioid analgesics are associated with a greater number of
side-effects than non-opioids, including nausea and vomiting.
A fourth group, the adjuvant analgesics, include drugs whose primary
indication is other than analgesia but which, when used in pain, may either
enhance the analgesic effects of the opioids or have intrinsic analgesic
activity in certain situations.(7)
The fundamental concept that underlies the appropriate and successful
management of cancer pain by the use of opioid and non-opioid analgesics is
individualisation of therapy. This entails selection of the right analgesic,
administered in the right dose, and at the right time, so as to maximise pain
relief and minimise adverse effects.(7)
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