NERVE PAIN

Definition — Pain which occurs in an area of abnormal or absent sensation, due to lesions in the central or peripheral nervous system is called “nerve pain”. It is also called deafferentation pain (pain that does not depend on activity in pain receptors).

Discussion — Nerve pain is poorly understood and difficult to treat. Nerve pain often responds poorly to morphine and other opioid drugs.

The clinical picture includes:

• Continuous burning and aching pain

• Stabbing pain superimposed

• May be in distribution of nerve

• Numbness, tingling

• Hyperalgesia (slight pain like pinprick felt as severe)

• Allodynia (light touch feels painful)

• Pain-induced insomnia

The pain is severe and continuous and often disturbs sleep, but it can fluctuate in severity and can be reduced by diversional therapy or elevation of mood. There may be a positional element to the pain (presumably because of pressure on nerve roots).

It typically occurs in the arm when there is involvement of the brachial plexus (from Pancoast’s syndrome or axillary nodes) or in the leg when there is involvement of the lumbo-sacral plexus with pelvic tumors.

Brachial plexopathy occurs with tumors in the apex of the lung or direct infiltration of the plexuses due to metastases, usually from breast, lung or lymphoma. Pain typically precedes neurological signs by weeks or months. (see Pancoast’s Syndrome)

Involvement of the lower cord of the plexus causes:

• Pain (in shoulder or paraspinal region)

• Tingling and pain (in 4th and 5th fingers)

• Wasting (in small muscles of the hand)

• Sensory loss (in medial hand and forearm)

• Ptosis (in sympathetic fibers near lower cord of plexus)

• Cord compression (epidural invasion)

Tumor in the supraclavicular region involves the upper cord of the plexus causing burning pain in the index finger and thumb. A CT scan will usually demonstrate a well defined mass which may extend down to the apex of the lung, or across the infraclavicular region.

Lumbo-sacral plexopathy occurs with invasion of the sacrum due to pelvic tumors (usually colo-rectal or cervical) but also occurring in breast cancer, lymphoma and sarcomas. It usually suggests advanced disease. CT scan may demonstrate tumor but can appear normal.

The clinical picture (which can be bilateral) includes:

• Sacral pain (aching, pressure-like)

• Weakness in the leg

• Root pain in the leg (burning, stabbing)

• Numbness or tingling:

  • Anterior thigh (L1, 2, 3)

  • Lateral calf (L4)

  • Big toe (L5)

  • Back of the leg (sacral nerves)

Assessment — Diagnosis is often suspected from the description of the pain (burning, stabbing) and confirmed by evidence of reduced or altered sensation in the region of pain.

«  TENS is rarely helpful for nerve pain, and tends to exacerbate it.

Management options for nerve pain include:

1. Explanation

2. Trial of morphine

3. Tricyclics

4. Anti-convulsants

5. Membrane stabilizing drugs

6. Trial of high dose steroids

7. Diversional therapy

8. Special techniques (rarely)

1. It is helpful to explain the nature of the pain to the patient, emphasizing the following:

• It is nerve damage pain (not tissue or organ damage)

• It responds poorly to analgesics like aspirin and morphine

• Tricyclics should be started to treat pain

• First step is to get a good night’s sleep

2. A trial of morphine is important because some neuralgic pains respond partially, especially if there is a deep, aching component to the pain. There can also be an element of soft tissue pain superimposed on the neuralgic pain. The correct dose of morphine is the dose that reduces the pain as much as possible without causing drowsiness. (Drowsiness without pain relief means the pain is not morphine-responsive or poorly morphine-responsive.) Start with a low dose and promptly increase in steps. (see Morphine)

3. Tricyclics — Imipramine is the drug of choice, although other tricyclics are probably equally effective. The aim is to increase the dose as quickly as possible to 100mg to 150mg per day (it is unusual to need more than 100mg).

The speed of increase in dose depends on the severity of pain and the degree of supervision, but a useful guide is:

Elderly or frail patients need lower doses. Side-effects (especially a dry mouth) often limit the tolerated dose. Onset  of relief is unusual before Day 4 or 5, and unlikely with only 25mg.

4. Anti-convulsants can sometimes relieve the severe stabbing (lancinating) pains of neuralgia. They act by inhibiting trans-synaptic discharges and reducing neuronal excitability.

The useful drugs are:

• Clonazepam

• Carbamazepine

• Valproic acid

• Phenytoin

They should be tried in the above order. No one particular pain syndrome responds to a particular drug. These drugs have been studied in non-malignant nerve pain. Controlled studies of their use in nerve pain of malignancy are lacking.

Clonazepam has been the most effective drug in comparative studies (there are no double-blind studies) but it also causes the most drowsiness. About 40% of patients respond. The starting dose is 0.5mg at bedtime for 1 week, increasing gradually to a maximum of 3mg per day in divided doses.

Carbamazepine is effective for trigeminal neuralgia (80% respond within 24 hours), and for painful diabetic neuropathy (in one double-blind study 28 out of 30 patients responded). 20% of patients with nerve pain due to malignancy respond. The starting dose is 100mg 2 times a day, increasing gradually (to avoid drowsiness) until a response is achieved, up to a maximum of 400mg 3 times a day.

Valproic acid starting at 200mg 3 times a day or 500mg at bedtime (increasing to 600mg 4 times a day) is commonly chosen as first line treatment for stabbing neuropathic pain, because it causes less drowsiness than clonazepam or carbamazepine. About 20% of patients respond.

Phenytoin is known to be significantly more effective than placebo for painful diabetic neuropathy. The usual starting dose is 300mg at bedtime (increasing to 600mg per day). About 20% of patients respond.

Ideally a trial of each drug should last 4 to 6 weeks, but with severe pain and a short prognosis this period may have to be reduced to 1 to 2 weeks.

5. Membrane stabilizing drugs — Flecainide is a membrane stabilizing drug originally used to treat tachyarrythmias. It has been used to treat the pain of malignant infiltration of nerve following a double-blind study which demonstrated that mexiletine (a similar drug) produced significant improvement in painful diabetic neuropathy. The dose of flecainide is 100mg 2 times a day. In one series 6 out of 17 patients with nerve pain had complete analgesia for 1 to 26 weeks.

Clonidine 25 micrograms 3 times a day increasing to 100 micrograms 3 times a day has been successfully used to control nerve pain. Clonidine is an alpha-2 agonist which may work by stimulating the descending pathways. Epidural clonidine has been successfully used for pain control. (see Nerve Blocks, Pain Pathways, Spinal Opioids)

6. A trial of high dose steroids can reduce nerve pain, probably by relieving nerve compression. A 10-day trial of dexamethasone 8mg per day may be indicated especially if other drugs have failed. This can occasionally produce a dramatic response.

7. Diversional therapy — Pain is a somato-psychic experience. A person is more able to cope with a continuous pain if he feels cheerful, secure and preoccupied with an interest or a project. An imaginative occupational therapist or recreational therapist can sometimes transform the patient’s existence.

8. Special techniques — In intractable nerve pain special techniques may very occasionally be required, including nerve blocks, spinal opioids or cordotomy. (see Cordotomy, Nerve Blocks, Spinal Opioids)

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