4.8 PCA + Phantom Limb Pain

Question 1

You are called to see a patient who has had a below knee amputation 24 hours ago. Despite using a patient controlled analgesia (PCA) pump with intravenous morphine he is still in pain.

a) Why might his pain control have become inadequate? (6 marks)

Answer 1

● Low dose of morphine in the PCA.

● Comorbidities leading to the administration
of a reduced dose of analgesics —
it is difficult to balance effective analgesia and
increasing morbidity due to the side effects of polypharmacy.

● A combination of pains including stump pain
(surgical wound pain — predominant element)
and phantom pain.

● Extensive tissue trauma causing severe nociceptive pain.

● An associated neuropathic component of
pain secondary to neural tissue injury.

● Pre-existing pain in the limb.

● Infection or haematoma in the stump.

Question 2

b) How would you re-establish optimal pain control? (6 marks)

Answer 2

● Multimodal analgesia from the start of the procedure.

● Regional anaesthesia —
either by epidural analgesia or a perineural catheter:

• epidural analgesia (check coagulation status before). The
  appropriate timing of catheter insertion and removal with the
  anticoagulation protocols in patients having vascular insufficiency is important;
• perineural catheter for a local anaesthetic infusion around the
  nerve (sciatic nerve +/- femoral nerve) placed by the surgeon
  intraoperatively or by anaesthetists postoperatively. This is useful, especially in the first 72 hours postoperatively.

● Intravenous PCA,
e.g. morphine, fentanyl, oxycodone, etc, if regional
analgesia is inadequate or cannot be administered.

● Opioid-sparing drugs —
paracetamol, NSAIDs (only if appropriate).

● For neuropathic pain — gabapentinoids (e.g. pregabalin 25-75mg per day to start with) or tricyclic antidepressants.

● Ketamine infusions in severe pain (0.1-0.2mg/kg/hour).

Question 3

c) What features could indicate that this patient is suffering from post-amputation pain syndrome (phantom limb pain)? (3 marks)

Answer 3

● Pain is felt in the distal extremity of the amputated limb.

● Usually episodic pain that comes in short bouts lasting a few seconds to hours.

● Pain is described as burning, shooting or cramping in nature.

● Occurs in up to 80% of amputees.

Question 4

d) What further pharmacological options are available for managing post-amputation pain syndrome? (5 marks

Answer 4

Multimodal analgesia:
● Regional blockade — (preventive):

• continuous perineural blockade with local anaesthetic

(e.g. ropivacaine 0.2%)
solutions using elastomeric or similar pumps,

postoperatively for the first few days
in an attempt to reduce sensitization;

there is some evidence to
support this preventive approach;

• perineural clonidine (alpha-2 agonist) as an
  adjuvant to the local anaesthetic infusion.

-

● Antineuropathic analgesic agents are
the mainstay of management:

1 gabapentinoids —
the use of pregabalin in a suitable dose to start with,
e.g. 50mg BD followed by gradual uptitration as needed.

Gabapentin — dose 100mg TDS to start with;

2 tricyclic antidepressants,
e.g. amitriptyline or nortriptyline,
if there are no contraindications;

3 ketamine —
NMDA antagonist with antineuropathic,
antinociceptive and antihyperalgesic properties;

4 memantine —
NMDA antagonist with much less psychotropic
effects (compared with ketamine);

5 salmon calcitonin subcutaneous injection — a neuropeptide with novel analgesic action, which has shown early promising results.

● Simple analgesics,
including paracetamol,
NSAIDs (if no contraindications),
weak opioids — these can be tried and continued if found useful.

Question 5

c) List the types of post-amputation pain that patients may
present with

Answer 5

● Stump pain.

● Phantom limb pain.

● Mechanical pain
(due to altered weight and force distribution).

● Back pain, especially in the rehabilitation phase.