Myoclonus

Causes of myoclonus

Metabolic dysfunction

Neurological disorders

Drug toxicity

Drug withdrawal

If part of pre-terminal restlessness

Myoclonus is random sudden brief involuntary jerking of a single muscle or group of muscles. Myoclonus occurs more frequently when asleep than awake. It can be a normal physiological occurrence and is not necessarily pathological e.g. muscle jerk experiences on falling asleep or “sleep-starts”. 

Myoclonus in the palliative care setting can range from mild twitching to severe jerking. It is frequently seen in the last days of life and can progress to full blown seizures if left untreated. Myoclonus can be focal (single muscle) or regional or multifocal (generalised). It can be unilateral or bilateral. 

Multifocal Myoclonus is a central pre-epileptiform phenomenon which can be seen in the terminal phase. It can occur after high doses of any opioid but especially with pethidine given in repeated doses parenterally. (e.g. over 250 mg/24 hrs) Multifocal Myoclonus is not to be ignored as it can lead to full blown seizures. 

Appropriate assessment to identify cause and severity of symptoms 

Causes of myoclonus 

Myoclonus can be primary (essential) or secondary to metabolic dysfunction, drug toxicity, chemical toxicity and neurological disorders. Myoclonus can be part of the terminal restlessness syndrome. Myoclonus is associated with high doses of any opioid but is a particular problem even in low doses with Pethidine. Pethidine is not recommended for pain management in Palliative Care or any chronic pain condition requiring ongoing analgesia. 

Metabolic dysfunction 

• Hypoglycaemia 
• Renal failure 
• Encephalopathy due to hepatic failure 
• Severe hyponatraemia 
• Hypocalcaemia 
• Hypoxia 

Neurological disorders 

• Brain damage 
• Viral infection of the CNS 
• Neurological degenerative disease e.g. MND 
• Cerebral oedema 

Drug toxicity 

• All opioid analgesics, especially Pethidine (Myoclonus is rarely seen in morphine use unless high doses are given repeatedly, especially intrathecally) 
• Dopamine antagonists e.g. haloperidol 
• Neuroleptics e.g chlorpromazine 
• Metoclopramide 
• Gabapentin (gabapentin induced myoclonus is relatively frequent but is usually mild and most patients choose to continue treatment. Gabapentin increases the risk of myoclonus in end stage renal disease) 

Drug withdrawal

• Benzodiazepines 
• Alcohol 
• Anticonvulsants 
• Barbiturates 

Explanation to patient and family 

Reassure the patient that the myoclonic jerks are not seizures and not life threatening. Discuss the patient’s concerns about the myoclonus. There may be specific implications for the patient e.g. effect on sleep, worsening of bone pain from painful bone metastasis. The cosmetic appearance in public may be very bothersome to the patient and family. 

Explain the possible cause/s of myoclonus in the patient. Explain that many of the treatments cause sedation and decide whether to treat the myoclonus or not. Most patients are satisfied with the explanation of its aetiology and elect for no treatment of this symptom. 

Multifocal myoclonus may be part of pre-terminal restlessness and can be very distressing for relatives to witness. Careful explanation and reassurance is required for family and carers. 

Correct reversible factors 

• As with muscle spasms and cramps, any reversible causes should be addressed and managed appropriately. Review drugs, reduce doses or switch to an alternative opioid.(Opioid rotation) Consider using an alternative analgesic e.g. NSAID. 
• Treat hypoxia and hypoglycemia. Restore electrolyte imbalance and manage encephalopathy if feasible. 
• Take a careful history to ascertain any possible drug/alcohol withdrawal. 
• Exclude renal failure. Worsening myoclonus may herald the onset of renal failure, and measures to improve renal output should be considered. 
• In a patient with terminal restlessness and deteriorating renal function the dose of opioid analgesic needs to be reviewed as it may cause or aggravate myoclonus. 

Non-pharmacological interventions 

• Where possible use non-pharmacological measures to control pain such as palliative radiation, acupuncture, physiotherapy, TENS, peripheral nerve block etc in order to reduce the doses of opioid required. 
• Give oxygen if needed 
• Rehydrate patient
  

First line therapy 

• Clonazepam 0.5 mg nocté is usually effective 
• Alternatively Lorazepam 1 mg stat, repeat as needed bd-tds 

If part of pre-terminal restlessness:

• Diazepam 5-10 mg pr hourly until settled, then 10 mg pr once daily – bd. or 
• Midazolam 5-10 mg S/C stat hourly until settled, then 10-30 mg/24 hrs via CSCI or 
• Clonazepam 0.5 mg S/C hourly until settled, then 1-2 mg/24 hrs via CSCI 

Adjust doses up or down as needed. 

Second line therapy 

• Flunitrazepam 
• Phenobarbitone 

Involvement of the interdisciplinary team 

• Involve the pharmacist in reviewing the many drugs that are used in Palliative Care which could cause or exacerbate myoclonus. 
• Physiotherapist 
• Refer to a specialist if needed e.g. if there is a neurological complication or radiation needed 
• Relaxation therapy, counselling.