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Pets with
Cancer
Kevin A.
Hahn, D.V.M., Ph.D., D.A.C.V.I.M. (Oncology)
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PAIN MANAGEMENT
Management of pain comprises an integral part of successful treatment for
a wide variety of human diseases. This is particularly true for patients
with malignancies, where the physiologic and psychological effects of pain
may have much greater impact. While advances in the understanding and treatment
of pain in animals have lagged some years behind similar progress in the
human field, it is now widely and increasingly accepted that appropriate
therapy for pain must accompany primary treatment of many animal diseases
including cancer.
Incidence
Approximately 50-80% of human patients with advanced cancer experience
pain during the course of their disease. The majority of these patients
will not obtain satisfactory relief. This fact constitutes a major problem
in the medical field, because unrelieved pain can significantly diminish
the patient's quality of life. Among the factors that may contribute to inadequate
pain management are the overriding fears of addiction, health care professionals'
lack of knowledge about pain medication and new pharmacological interventions,
and the lack of confidence in the efficacy of behaviour techniques. Similar
factors are known to exist in the veterinary medical community and are not
limited in application to pets with cancer.
Cancer pain can be acute or chronic. Acute pain generally results from
tissue damage and is of limited duration. The physiological effects (e.g.
tachycardia) observed result from stimulation of the autonomic nervous system.
Once the cause of pain has been identified, it can be successfully treated
and is often completely eradicated. Chronic pain, on the other hand, is persistent,
usually greater than 3 months in duration. Because the pathology or cause
of the pain cannot be altered, the nervous system eventually adapts and ceases
to be hyperactive; the pain may then manifest itself as depression or anxiety.
Causes
The severity and prevalence of pain that cancer pets experience depends
on many factors, including the site and stage of the disease and the location
of metastases. Cancer related pain can result from the disease process or
cancer therapy. The most common causes pain from direct tumour involvement
are metastatic bone disease, nerve compression or infiltration, and hollow
viscous (e.g. bowel) involvement resulting in obstruction. All of the major
treatment modalities may also cause pain syndromes. Additionally, pets may
have pre-existing chronic pain that is not associated with either the disease
or its treatment.
Pain affects each pet differently, depending upon factors such as age,
perception, pain threshold, and past experiences with pain. Insomnia, fatigue,
and anxiety can lower the pain threshold, while rest, sleep and diversion
can raise it.
Pain Assessment
An accurate assessment of the pet's pain experience provides a basis for
an evaluation of various pain management techniques. A comprehensive assessment
includes information about the following dimensions of pain: location, intensity,
factors influencing it occurrence, observed behaviours during pain, psycho
social variables (i.e., attitudes, situational factors), effects of pain,
effects of therapy, and patterns of coping. A variety of pain assessment
tools have been developed for use in humans, ranging from simple self reports
about pain intensity to detailed descriptive information. In veterinary medicine,
assessment of chronic pain may be enhanced through the pet owner's use of
a pain diary, in which descriptions of the characteristics of the pain and
the effectiveness of management techniques can be recorded.
Pain Management
The goal of pain management is not only relief from pain, but also the
maintenance of the pet's normal quality of life. All methods of pain management
attempt to either control the cause of the pain or alter the pet's perception
of it.
Although pain management techniques are many and varied, therapeutic approaches
can be classified as either pharmacologic or non pharmacologic. Pharmacologic
pain control involves the use of analgesics, as well as other medications
that potentiate the analgesics' effects or modify the pet's mood or pain
perception. Nonpharmacologic approaches include behavioural techniques, radiation,
surgery, neurological and neuro surgical interventions, and tradition nursing
and psycho social interventions, the latter measures attempting to promote
comfort and evaluate the effectiveness of the therapy. Because of the complex
nature of cancer related pain, successful management usually involves a combination
of techniques.
Cancer pain management in the geriatric pet calls for special considerations.
Ageing pets are at an increased risk for drug reactions, because drug adsorption,
distribution, metabolism, and elimination change with age, disease status,
and medication interactions.
Pharmacologic Management
Veterinary care personnel must aggressively manage acute pain in the cancer
pet with medication to return the pet to a pain free state as soon as possible.
Once the pain is relieved, the pain medication is decreased to the lowest
dosage or mildest analgesic that will maintain the pain free state. When
the pain cycle is broken, pets can be sustained on minimal amounts of pain
information.
Chronic pain, however, requires very different medication management. For
example, a pet with chronic pain is usually started on a non narcotic analgesic
and moves to a narcotic as more effective pain control is needed.
The World Health Organization (1987) states that "analgesic drugs are the
mainstay of cancer pain management" and advocates a three step "analgesic
ladder" for decision making. This schema is appropriate for use in veterinary
medicine. Step one includes the use of a non-opiod drug with or without
an adjuvant drug (e.g. aspirin, carprofen + misoprostil). If pain persists
or increases, pain management moves to step two, a weak opioid plus a non-opioid,
with or without an adjuvant drug (e.g. acetaminophen, codeine +/- carbamazepine).
If pain persists or increases, pain management moves to step three, a strong
opioid, with or without a non-opioid, with or without an adjuvant drug (e.g.
morphine +/- acetaminophen +/- dexamethasone).
Non narcotic pain agents are best used for mild cancer pain. This
category includes aspirin, acetaminophen, and non-steroidal anti-inflammatory
drugs (NSAIDS). Non narcotic agents may also be used to potentiate the effect
of narcotic analgesics in pets with severe pain. However, these agents have
a ceiling effect: increasing the dosage beyond a certain point doesn't produce
additional pain relief. NSAIDS reduce the production of prostaglandin by
inhibiting cyclooxygenase (COX). Their analgesic properties are primarily
due to their anti-inflammatory effects. Common side effects of these compounds
include gastrointestinal and renal toxicity. The newer agents in this class
include carprofen and etodolac. These compounds have a favourable COX1:COX2,
which in theory will reduce the possible side effects associated with this
class. Piroxicam is also included in this class. Other NSAID’s that may be
useful in the small animal patient include ketoprofen and ketorolac.
Narcotic analgesics (opioids) are used for the treatment of moderate to
severe cancer pain. They are categorized as either narcotic agonist or narcotic
agonist-antagonist drugs. This is the largest and perhaps most valuable
class of analgesic agents. These agents interact with specific receptors
in the brain and spinal cord and are very efficacious. The side effects
associated with this class are usually minimal with respiratory depression,
bradycardia and hypertension being the most concerning. Using opiate antagonists
can alleviate life threatening side effects. The opioid agents most commonly
used include morphine, butorphanol, fentanyl and buprenorphine. Innovative
modes for administration include continuous rate infusion, epidural administration
and the transdermal fentanyl delivery system (patch).
One model explaining the actions and effects of the opioids, the Multiple
Opioid Receptor Theory, proposes that narcotic agonist drugs, such as morphine
and codeine, bind with specific opiate receptor sites. There are three kinds
of receptor sites, or portions of the nerve cell to which a drug can bind:
the mu receptor associated with analgesia and respiratory depression; the
kappa receptor with sedative effects; and the sigma receptor with psychomimetic
effects.
Although the Multiple Opioid Receptor Theory is still evolving and does
not yet completely explain narcotic analgesia, pure narcotic agonists such
as morphine and codeine are thought to occupy the mu receptor without antagonizing
activity at the other receptor sites. Narcotic agonists-antagonists occupy
the kappa receptor for pain relief which also antagonizing the effects of
pure agonists at the mu receptor. Three agonist-antagonists are butorphanol,
nalbuphine, and pentazocine.
Adjuvant analgesic drugs are also used to treat cancer pain. This group
includes amphetamines, anticonvulsant agents, phenothiazines, tricyclic antidepressants,
steroids, antihistamines, and levodopa. Although their exact mechanisms
of action for pain relief are not well understood, these drugs relieve pain
when used alone or in combination with other non narcotics or narcotics.
Pain medication may be given by the following routes: orally, rectally,
subcutaneously, intramuscularly, intravenously, intrathecally, and epidurally.
Conditions such as thrombocytopenia, neutropenia, and duration of a medication's
effect must be taken into account when selecting the route. The peak of a
drug's effect is largely dependent on the route of administration. Oral medications
usually peak in 2 hours, intramuscular drugs in 1 hour, and intravenous
drugs in 15-30 minutes. Duration of effect varies widely and should be carefully
considered.
The schedule for administering analgesics appears to be an important factor
in their effectiveness. Research shows that around-the-clock (ATC) rather
than as needed (PRN) administration of analgesics is more effective in the
control of chronic pain. In many cases, doses of analgesics may be decreased
with ATC scheduling because the pain intensity is consistently less.
Pets with cancer may be under medicated for their pain because veterinary
care personnel may believe that cancer pets usually develop a tolerance
to the effects of opioids rather than an addiction. Tolerance to narcotics
can occur at anytime and requires increasing doses to produce the same level
of analgesia. Pets also develop tolerance to the serious side effects of
narcotics (e.g. sedation, respiratory depression) at the same rate as tolerance
to analgesia, so they can accept larger doses of narcotics without overdosing.
Analgesics and Dosages Available for Use in Dogs and Cats.
Narcotic
Analgesics in Dogs
Morphine, 0.25
to 5.0 mg/kg IM or SQ every 4 hours
Oxymorphone,
0.2 mg/kg IM, SQ or IV every 6 hours
Butorphanol,
0.4 to 0.6 mg/kg PO, IM, SQ or IV every 4-8 hours
Nonsteroidal
Anti-inflammatory Drugs in Dogs
Aspirin, 10
to 25 mg/kg PO every 8 hours
Phenylbutazone,
10 to 25 mg/kg PO every 8-12 hours
Carprofen,
1mg/lb, PO every 24 hours
Piroxicam,
0.3 mg/kg PO every 24 hours
Other Analgesic Methods
Like insulin, pain medications can be administered on a continuous basis
into subcutaneous tissue through a small gauge butterfly needle taped in
place.
Narcotics can also be continuously infused epidurally or intrathecally
with the placement of an indwelling intrathecal catheter. This technique
is associated with fewer central nervous system effects than systemic administration
of narcotics. The major problem observed with intraspinal delivery of narcotics
is respiratory depression.
Fentanyl patches are labelled for managing chronic cancer pain in humans,
specifically people who cannot tolerate oral medications. The efficacy and
convenience of this delivery system have generated interest in the use of
this treatment for postoperative pain in both human and veterinary medicine.
Fentanyl is not approved for use as a single agent in dogs and cats. The
patch consists of a gel matrix containing fentanyl, a lipid soluble opiod
that diffuses through the skin and achieves blood concentrations high enough
to produce analgesia. A variety of sizes deliver 25, 50, 75, and 100 m g/hr,
with the delivery rate a function of the surface area of the patch. Patches
are designed to provide continual release for about 72 hours. In dogs, there
appears to be a 6 to 12 hour latency to achieve plasma concentrations associated
with analgesia and up to 24 hours for plasma concentrations to reach a plateau.
The principle advantage of fentanyl patches is provision of "hands off"
analgesia. One disadvantage is body temperature, fever or other causes of
elevated body temperature (laying on a heating pad) can increase drug delivery
substantially. To circumvent this, many apply the patch to the dorsal neck
and secured with a light wrap. For cats and small dogs, a 25 m g patch is
used. For medium-sized dogs (5 to 20 kg) a 50 m g patch is used. Larger
dogs (20 to 30 kg) may require a 75 m g patch and giant dogs (> 30 kg)
may require the 100 m g patch. Side effects may include euphoria (in cats)
and increased appetite. Deleterious side effects may include agitation,
dementia, heightened response to the environment and mild sedation or ataxia.
However, side effects, when used properly, are uncommon.
Nonpharmacologic Management
Nonpharmacologic pain management approaches include surgery, radiation,
neurological and neurosurgical interventions, behavioural techniques, and
nursing interventions.
Both radiation therapy and surgery may be used for cancer pets with enlarging
tumours or advanced disease to decrease the tumour mass and reduce painful
compression of adjacent structures. These procedures are conducted for palliative
purposes only.
Neurosurgical interventions are generally reserved for pets that cannot
obtain adequate relief with analgesics, palliative radiotherapy or surgery.
Most neurosurgical procedures involve interruption or destruction of the pain
pathway at some point along the route to the brain or in the brain itself.
The risks and benefits of these techniques must be discussed thoroughly
with pet owners because, in many cases, the pet will have residual motor
or sensory deficits.
Neurostimulation techniques are base on the "gate control" theory of pain.
Some research indicates the pathways in the spinal cord can accommodate
only a certain amount of stimulation before sensory overload occurs. With
neurostimulation, competitive non painful (e.g. vibratory) impulses are
used to block the transmission of painful impulses along nerve pathways.
Neurostimulation can be applied transcutaneously (as occurs with transcutaneous
electrical nerve stimulation, or TENS) or via surgically implanted electrodes
in the spinal cord.
Behavioural techniques such as relaxation, distraction, biofeedback,
imagery, and hypnosis are now widely used to manage cancer related pain
in people. In general, behavioural techniques are designed to alter the
response to pain by fostering a deep relaxation and a shifting of attention
to something other than the pain. These approaches, although controversial
and likely ineffective for use on most pets, should be used in combination
with, and not as a substitute for, appropriate medications. Care must be
taken not to misinterpret the efficacy of these techniques as an indication
that the pet is not really experiencing pain.
The above information is simply informational.
It's intent is not to replace the advice of a veterinarian nor to assist you
in making a diagnosis of your pet. Please consult with your own veterinarian
for confirmation of any diagnosis. Your pets life may depend on it.