Page 367 Guide to Pain Management in Low-Resource Settings

Drug Profi les, Doses, and Side Eff ects 355

Coanalgesics

Equianalgesic doses of morphine Coanalgesics are drugs that were originally developed for
Intravenous (i.v.) 10 mg purposes other than analgesia, but were then found to be
Subcutaneous (s.c.) useful in certain pain states. Th eir use is common in neu-
Intramuscular (i.m.)
ropathic pain, where NSAIDs and antipyretics are ineff ec-
Oral 30 mg tive most of the time and opioids often fail to be eff ective.
Epidural 2–3 mg
Although a number of substances have shown
Intraspinal 0.1–0.3 mg to have “coanalgesic” properties (among others: capsa-
icin, mexiletine, amantadine, ketamine, and cannabis),
only antidepressants, anticonvulsants, and steroids are
Transdermal opioids
used regularly and are most likely to be available in low-
Two patches are now available for the delivery of resource settings. Th e use of coanalgesics necessitates
opioids—the fentanyl patch and the buprenorphine knowledge of how to balance benefi ts and risks and
patch. These drugs are strongly lipophilic, allowing avoid side serious side eff ects.
good passage through the skin into the circulation As with opioids the doses of most coanalgesics
and avoiding first-pass metabolism in the liver. Con- have to be titrated to the eff ect, meaning, that the dose
sider that analgesia and side- effect profile do not recommendations for their original indications cannot
change by using the transdermal route. Therefore, be transferred to the indication “pain”. As always when
only patients with swallowing problems or recurrent treating pain, use thorough patient education to gain
vomiting would benefit from this route of applica- good patient compliance and adjust and readjust doses
tion. If transdermal systems are used, remember that and drug selection to gain the best results for your pa-
they are indicated only in patients with stable opi- tients. Don´t forget to give a message of hope to your
oid requirements and that it takes around half to one patient but be honest with him and set realistic goals:
day for the patch to produce a steady state of opioid coanalgesics will not take away the pain, but will only be
delivery to the patient (and the same time for blood able to give some relief!
levels to decrease if the patch is taken off). In con-
clusion, the vast majority of patients in cancer and Anticonvulsants
palliative care may be treated well with opioids with- Th ey reduce neuronal excitability and suppress par-
out the use of transdermal systems (which are also oxysmal discharge of the neurons by stabilizing neu-
considerably more expensive!). ral membranes. Anticonvulsants work by interacting
with diff erent mechanisms, e.g., the voltage dependent
Adjuvant medications for opioid-related sodium channel or by the high voltage calcium chan-
side eff ects nels. Anticonvulsants of the sodium channel blocking

Nausea, vomiting, and constipation associated with type (carbamazepine, oxcarbazepine or lamotrigine)
opioids need a concomitant “adjuvant” medication. show best results in attack like shooting pain, e.g., in
Without one, your patients’ compliance will be low! patients, where the cancer has infi ltrated nerve plexus
For the fi rst week of opioid therapy, metoclopramide or in trigeminal neuralgia. Anticonvulsants of the cal-
10 to 30 mg q.i.d. should always accompany the opi- cium channel blocking type (gabapentin, pregabalin)
oid. As mentioned above, earlier tolerance to the nau- are indicated above all for continuous burning pain,
seating side eff ects of opioids will then develop. Seda- e.g., in patients with polyneuropathies or postherpetic
tion must to be explained to the patient, since there is neuralgia. Th e latter seem to have a synergistic eff ect
no eff ective adjuvant medication to counteract it. For on the calcium channels with opioids. Phenytoin can
constipation, a constant prophylactic laxative therapy be used as a “rescue” substance for severe and therapy
must be initiated immediately with the start of an opi- resistant neuropathic pain. All anticonvulsants should
oid. Milk sugar or bisacodyl are good choices. See the be titrated according to the rule “start low, go slow”.
chapter on constipation for further details on this ther- Recommended dose ranges for the most common an-
apeutic problem. ticonvulsants in pain management are: