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Opioid Induced Neurotoxicity

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Opioid Induced Neurotoxicity FEATURE Opioid Induced Neurotoxicity By JAN MAGNUSSON BSc (Pharm) and DR JAN MULDER BSc (Pharm) MD GOAL therapies intended to comfort and Allodynia is a condition where To educate the pharmacist on support individuals and their friends stimuli that are not normally painful opioid induced neurotoxicity (OIN) and families who are living with, or precipitate pain in the patient. For and its management. dying from, a progressive life threaten- example a bed sheet resting on the ing illness, or are bereaved.”(3) Comfort patients leg causes a significant INTRODUCTION care is also one of the terms used to amount of pain where normally it The pharmacist plays a key role in describe palliative care. When patients wouldn’t even be consciously noticed. monitoring the effect and adverse are enrolled on a palliative care Myoclonus is a sudden, brief, effects of medications that are taken by program it usually means that the shock-like, involuntary movement that patients. Many patients taking opioids goals of treatment are aimed at pallia- are caused by muscle contractions. The over extended periods of time may tion of symptoms and not cure. jerks can occur singly or repetitively, have chronic progressive pain second- Palliative care does not mean that “we small (hard to detect contractions) or ary to cancer. Many of their treatments do nothing”. Many patients will want gross contractions where the limb will be for palliation rather than cure. treatment of acute reversible condi- movement can be visibly detected. Opioid induced neurotoxicity is a term tions (eg. antibiotics, blood Myoclonus can involve a single region used to describe symptoms of cogni- transfusions) if they are still experienc- or multiple regions of the body. It tive impairment, severe sedation, ing good quality of life. This often normally occurs during periods of hallucinations, myoclonus, seizures depends where they are in the trajec- drowsiness and light sleep, but is not and hyperalgesia.(1) (2) These are some tory of their illness. limited to these situations.(6) of the signs and symptoms that we as Neuropathic pain is an area that is pharmacists should instruct the patient currently being evaluated for its rela- SIGNS AND SYMPTOMS and families to watch for. tion to N-methyl-D-aspartate (NMDA) Signs and symptoms of OIN are not always obvious. Confusion is a side DEFINITIONS effect that may be encountered for the Pain is a subjective condition. Pain Signs and symptoms of OIN are first few days-weeks when started on is what the patient tells you it is. Acute not always obvious. Confusion an opioid. This is normally mild and pain is sometimes easier to visualize self-limiting once tolerance to the on a patient’s face. There may be signs is a side effect … opioid occurs. Delirium in cancer is not of grimacing, the patient may groan always easily linked to one identifiable and cry out loud. The autonomic nerv- cause. Opioids may be the cause of the ous system may show signs of toxicity but other potential causes hyperactivity (sweating, pallor, tachy- receptors that may be involved in the should be ruled out. (See section on cardia, and hypertension) during an pain mechanism. This type of pain is investigations). Delirium may present acute episode of pain. Chronic pain is due to a peripheral neurological lesion clinically either with agitated, hyperac- harder to detect at first glance. The and may be described as: burning or tive symptoms or be hypoactive. patient doesn’t show any of the auto- radiating pain, sudden episodes of Sometimes both symptoms will coex- nomic signs. Long term pain that is not shooting pain, or pain with light touch. ist.(1) relieved will cause a patient to be tired, This type of pain is often less respon- Myoclonus may be an early indica- and depressed. The patients face looks sive to typical opioids. When the dose tor that the patient is starting to sad, quiet and sleepy.(3) Assessing pain of opioid starts to escalate in the treat- develop OIN. Mild myoclonus that by visualization alone can be deceiv- ment of pain one should consider that occurs during sleep may be normal, ing. The visual analogue scale (VAS) is this type of pain may be involved. but if the a useful tool that can provide a record Methadone can sometimes be useful in myoclonus of the patients pain history and may treating this type of pain, along with becomes noticeable Acute delirium help to show response to medication. anticonvulsants and steroids.(1) during waking Myoclonus The scale is traditionally 10cm long Treatment of neuropathic pain is hours treatment beyond the scope of this article. with numbers from 1-10. The patient options should be Seizures grades his pain at a particular time and Hyperalgesia is a term to describe a considered. If not the measurement is the patient’s pres- lowering of the pain threshold. It can treated myoclonus Hyperalgesia ent pain intensity. be a paradoxical pain where the pain can escalate into Palliative care has many different becomes overwhelming and ceases to seizures. Mild Hallucinations definitions depending on the reference. be relieved or is actually worsened by myoclonus may be In 1995 the Canadian Palliative Care further administration of the opioid.(4) observed and Table #1 Association defined it as “the combina- (5) The patient may tell you, “ I have treated with clon- Signs and tion of active and compassionate pain all over.” azepam. However, symptoms 6 COMMUNICATION MAY/JUNE 2001 when a patient is on high doses of adverse effects of narcotics. When Palliative care is best delivered by a opioids or has been on the same opioid choosing an opioid, those with shorter team of professionals who can help to for a long period of time, consider half-lives may be safer.(10) address the many issues that this rotating the opioid that the patient is Morphine is an opioid that has population of people deal with.(3) taking.(1) pharmacodynamic implications. It has Hyperalgesia, or paradoxical pain, two active metabolites; morphine-6- INVESTIGATIONS can be misdiagnosed as inadequate glucuronide (M6G) and We want to investigate looking for pain control. This may lead to increas- morphine-3-glucuronide(M3G). M6G possible underlying causes other than ing the dose of opioid. This pain is may contribute to both the analgesia opioid toxicity. One needs to check for actually caused by the opioid (a neuro- and the adverse effects observed while infection, dehydration, hypercalcemia, toxic side effect), and other treatments on morphine.(13) These active metabo- uremia or other metabolic changes. should be considered. (See section on lites need to be considered when a Metastatic disease to the brain or other treatment) patient is receiving morphine for pain organs can be ruled out by doing a Hallucinations, either visual or relief, especially if the patient is elderly bone scan, CT scan or MRI. In comfort tactile, can occur with opioids. Patients or has reduced renal function. Studies care the patient’s needs and goals of may be reluctant to relate these show in these populations that the care must come first. Expensive tests concerns to health care professionals ratios of M3G and M6G accumulate and blood work are appropriate at for fear they will be considered psychi- exponentially, making toxicity more times depending on the patient’s qual- atrically ill.(1) If the patients do not likely.(14) A case study article explains ity of life and their wishes for how verbalize their hallucinations, they that elevated concentrations of the much investigation is to be done. may be seen to pick at the air. Often metabolites may play a role in the MANAGEMENT the only signs may be a change in a development of hyperalgesia, allody- patient’s mood.(1) Family or nurses nia and myoclonus.(15) 1. Opioid Rotation who have more contact with the Meperidine is an analgesic that we Opioid induced neurotoxicity can patient will often be the source of must look at for pharmacodynamic be a result of an opioid/active metabo- information on changing mood. The considerations. It is metabolized to lites. If this is suspected then a rotation hallucinations can be managed by normeperidine which is an active to a different opioid will allow the changing or reducing opioids, or treat- metabolite. Normeperidine is a strong opioid/metabolites to be excreted and ing with medications like haloperidol. convulsant and a weak analgesic. The still maintain pain control with the problem arises because the half-life of new analgesic.(4) There is no ideal RISK FACTORS normeperidine is three-four times opioid to rotate to. The first line agents Risk factors will differ for each longer than that of meperidine. When include morphine, hydromorphone, patient. Some patients who have a normeperidine starts to accumulate fentanyl patch or oxycodone. terminal illnesses like cancer may be tremors, multi focal myoclonus and in Methadone is considered a second line on high doses of opioids or may have severe cases seizures will develop. This agent because it has an unpredictable been on them for a prolonged period opioid should not be used long term half-life and the equianalgesic dose is of time. There is no ceiling dose for due to these risks.(10) (12) not well documented.(1) On the positive strong opioids (i.e. morphine, hydro- Nephrotoxic medications will exac- side methadone has the benefit of low morphone etc) which allows one to erbate the renal dysfunction. NSAIDs, cost, and no active metabolites have escalate the dose as needed. Sometimes which are nephrotoxic, have been been identified yet. It is used especially this escalation may occur rapidly.(7)(8)(9) shown to be associated with increased when neuropathic pain is suspected.
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