Basics of Chronic Pain Management

INTRODUCTION

The International Association for Study of Pain defines pain as ‘An unpleasant sensory and/or emotional experience associated with, or resembling that associated with, actual or potential tissue damage.’ Even if the patient is experiencing pain in the absence of actual tissue damage, it should be accepted as pain. According to a global burden disease study, one in ten adults is diagnosed with chronic pain each year globally.^[1]

PAIN PATHWAYS

(1) Transduction, (2) Transmission, (3) Perception, and (4) Modulation. These are the sequence of events that are involved in the neural processing of noxious stimuli [Figure 1].^[2-6]

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Figure 1:

Afferent pain pathway.

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TYPES OF AFFERENT FIBERS THAT CARRY PAIN

• A-Delta fibre– myelinated and fast, with a conduction velocity of 5–15 m/s, transmits sharp, localised, and fast pain

• C-fibres – unmyelinated with slow conduction <2 m/s, carry dull, diffuse, aching, and delayed pain.

AUTONOMIC NERVOUS SYSTEM

Autonomic nervous system plays an important role in different types of pain. For example, pain signals from thoracic or abdominal viscera and intervertebral disc-or vertebral body are carried by different afferent sympathetic fibres.^[1,2-4,7]

SOMATIC SYMPATHETIC COUPLING AND MECHANISM

Somatic and sympathetic nerves are well insulated with no crosstalks in between them. However, with pathological conditions such as peripheral nerve injury/neurolysis and complex regional pain syndrome (CRPS), there is a cross-connection between somatic and sympathetic signals. Sympathetic efferent fibres release catecholamines, which instead of acting on sympathetic receptors may stimulate the nociceptors and generate action which is carried by somatic nerves^[2]

SYMPATHETIC BLOCKADE AND ITS SIGNIFICANCE IN PAIN MANAGEMENT

Sympathetic blockade results in interruption of transmission by both the efferent and afferent fibres. It does not result in sensory or motor loss as seen in the somatic blockade.^[2]

Examples of sympathetic blockade at various levels:

1. Stellate ganglion and lumbar sympathetic blockade for CRPS of the upper and lower limbs

2. Splanchnic and celiac plexus block for the upper abdominal malignancy and chronic benign pain

3. Superior hypogastric plexus block: Cancer and non-cancer chronic pelvic pain.^[8]

TYPES OF PAIN

ASSESSMENT AND DIAGNOSIS OF PAIN

PAIN ASSESSMENT TOOLS^[2,8,18]

1. Visual Analogue Scale (VAS): This is the most commonly used scale. In this, the patient is asked to place a marker on a 100 mm continuous line between no pain and the worst imaginable pain on every visit [Figure 2]

2. Numerical Rating Scale (NRS): The patient directly assigns a number between 0 (no pain) and 10 (the worst pain imaginable) [Figure 2]

3. Verbal Categorical Scale: The patient describes the severity of pain ranging from no pain to ‘mild,’ ‘moderate’ and ‘severe’ [Figure 2].

   • Mild pain: VAS or NRS of 0–3

   • Moderate pain: VAS or NRS of 4–7

   • Severe pain: VAS or NRS of 8–10 or even if the pain is moderate but the patient says I cannot bear it, it can be counted as severe pain.

4. The faces pain rating scale is used mainly for children and mentally impaired patients. The scale depicts six faces of facial features, each with a numeric value of 0–5, ranging from a smiling and happy face to a sad and teary face.

5. Comprehensive Pain assessment Questionnaires:

   • McGill pain questionnaires

   • Brief pain inventory

   • Oswestry pain questionnaire

   • Patient health questionnaire-9 scoring is to rule out depression.

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Figure 2:

(a) Visual analogue scale (VAS) and (b) Combined pain assessment tool showing numerical rating scale, VAS, Verbal categorical scale, and Wong – Baker fascial pain rating scale.

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CLINICAL EXAMINATION

General physical examination

Pain diagram

It is essential to draw a pain diagram for assessment of pain as it provides [Figure 3]^[8]

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Figure 3:

Pain diagram.

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• Visual confirmation of patient’s pain

• It can be stored as a medical record

• It helps to note the change in the area of pain especially if the patient’s pain keeps on changing with every visit.

NEUROLOGICAL EXAMINATION

It consists of an examination of all cranial nerves, spinal nerves, and autonomic nervous systems.

SPINE EXAMINATION

It includes sensory and motor examination of spinal nerves.

Spine symmetry and kyphosis or scoliosis should be noted.

Spinal flexibility – Pain on certain movements of the spine could indicate possible diagnosis in spine pain patients like pain in the neck or back on forward bending will indicate discogenic pain. On the contrary; pain on backward bending will point toward facet arthritis or muscular spasm [Table 1].

PHARMACOTHERAPY IN PAIN MANAGEMENT

The most common method of pain management is the use of drugs. Nociceptive pain responds well to simple analgesics such as anti-inflammatory agents, but neuropathic pain responds better to coanalgesics such as anticonvulsants and anti-depressants.^[2,8]

Drugs should have

• Rapid onset, long duration, and effective pain relief

• Minimal side effects on long-term use

• Easily self-administered.

WHO STEP LADDER

The World Health Organisation 1986 devised a step-wise approach popularly known as the ‘WHO step ladder’ approach for the management of chronic pain and cancer pain patients. In 2016, with the advent of newer modalities and interventions, this original analgesic ladder was upgraded to the current modified four-stepladder approach [Figure 4].^[2]

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Figure 4:

Modified four-step ladder.

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Drugs used in pain management can be categorised under two broad headings: Analgesics and Coanalgesics.

• Analgesics are anti-inflammatory, non-selective COX – inhibitors like NSAIDs, and selective COX-2 inhibitors. Acetaminophen/Paracetamol and Opioids

• Coanalgesics or adjuvant group includes anticonvulsants, antidepressants, local anaesthetics, steroids, muscle relaxants, botulinum toxins, and NMDA receptor antagonists. Others such as Alpha 2, agonists, calcium channel blockers, vitamins, and nutritional supplements.

ANALGESICS

OPIOIDS

Opioids can be classified according to their receptor activity or duration of action (long acting and short acting) or their analgesic and ceiling effects (weak and strong opioids). Opioid receptors are G protein-coupled receptors. mu is the major receptor responsible for analgesia. Pure agonist like morphine is preferred in moderate-to-severe chronic pain like cancer pain. Opioids with agonist-antagonist have a ceiling effect for analgesia. Weak opioids are tramadol, tapentadol, and codeine. Strong opioids are morphine, fentanyl, and buprenorphine. Long-acting opioids are good to treat baseline pain whereas short-acting opioids are useful in breakthrough pain. If a trial of opioids is considered, then: [Table 2]^[8]

• The patient should be explained the benefits and risks of opioids use

• The WHO analgesic ladder to be followed

• Strong opioids like morphine should never be used as SOS drug

• Side effects should be explained and majors to be taken to correct them if possible

• For elders ‘start low and go slow’, instructions for follow-up and emergency should be provided.

The oral route is the preferred root of administration. Other routes are subcutaneous, IM, IV, transdermal patches, and intrathecal pumps.

Common side effects are nausea, vomiting, itching, drowsiness, euphoria, constipation, urinary retention, respiratory depression, tolerance, dependence, sexual dysfunction, etc.

COMMONLY USED ANTICONVULSANTS IN PAIN PRACTICE

These are mainly either sodium or calcium channel blockers which raise the threshold for nerve depolarisation and thus suppress abnormal neuronal discharge [Table 3].

COMMONLY USED ANTIDEPRESSANTS PAIN PRACTICE

These drugs have a direct analgesic effect at doses much lower than that required for antidepressant action with the added advantage of sedation, diminished anxiety, muscle relaxation, and a restored sleep cycle. These drugs act on descending inhibitory pain pathways. Depending on the mechanism of action antidepressants are-Tricyclic antidepressants, selective serotonin reuptake inhibitors, serotonin norepinephrine re-uptake inhibitors [Table 4].

COMMONLY USED MUSCLE RELAXANTS IN PAIN PRACTICE

Muscle relaxants are used in addition to rest, physical therapy, and other measures to relieve discomfort. They are typically prescribed for short-term use to treat acute, painful musculoskeletal conditions [Table 5].

COMMONLY PERFORMED INTERVENTIONS FOR CHRONIC PAIN MANAGEMENT

SPINAL CORD STIMULATOR

Spinal cord stimulation (SCS), also known as neurostimulation, utilises an implant and electrodes to deliver mild electrical pulses to the nerves around the spinal column usually the dorsal column of spinal cord, to stimulate the nerves and block or lessen pain signals that are being sent to the brain [Figure 5].^[21,22]

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Figure 5:

Spinal cord stimulator.

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The major benefit of SCS is that it provides targeted pain relief up to 60% with improved mobility, allowing the patients to return to their daily activities. It minimises the need for invasive surgeries to correct spinal conditions and majority of the times remains the last resort for treating chronic back pain.

CANCER PAIN MANAGEMENT: SPECIAL POINTS

Cancer pain affects more than 9 million people worldwide annually. An estimated one-third (24–60%) and two-thirds (62–86%) of cancer patients suffer from pain, with more than one-third having moderate-to-severe pain. Pharmacotherapy constitutes the mainstay of treatment and approach for pain management as per the WHO analgesic ladder. Indications for interventions are unacceptable side effects and well localised pain syndromes in which pain may get relieved by nerve blocks or a comprehensive trial of pharmacologic therapy fails to provide pain relief. Interventional techniques for pain management include [Figure 6]^[23-27]

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Figure 6:

Cancer pain management continuum: A flexible approach.

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• Regional Infusions

• Neuraxial analgesia (Intrathecal pump)

• Neurodestructive procedures

• Neuromodulation

• Percutaneous vertebroplasty/kyphoplasty

• Radiofrequency ablation.

CONCLUSION

Pain treatment goals should include improved functioning and pain reduction. Along with pharmacological treatment, non-pharmacological therapies including integrative medicine therapies such as yoga, physiotherapy, and acupressure which widen the horizon of pain medicine should be routinely considered.

Knowledge of interventional nerve blocks and recent advances such as regenerative therapies and spinal cord stimulators has revolutionised the management of pain in CNCP and cancer patients. Thus, adequate control of chronic pain can be achieved in nearly all patients in a way that adequately balances benefits and potential harms.