The International Association for the Study of Pain (IASP) published a definition of pain (2020):
“An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage”
It is important to distinguish between two categories of pain:
- Acute pain – new onset of pain
- Chronic pain – pain present for 3 months or more
When managing pain, see local guidelines and seek advice from seniors and pain or palliative care specialists when in doubt. This section aims to help students prepare for exams and should not be used as a reference for managing pain in patients.
Basic Pain Physiology
There are two aspects to the experience of pain:
- Sensory – the sensory signal transmitted from the pain receptor (“it is a sharp sensation, likely a needle”)
- Affective – the unpleasant emotional reaction to the pain (“it is excruciating, I can’t bear it”)
Pain is supposed to indicate underlying or potential damage to tissues, but it can occur without tissue damage. The physiology of pain is very complex. There is still a lot that is not fully understood about the experience of pain.
Pain is subjective, meaning that when someone indicates they are in pain, we need to accept their experience, even when there is no apparent underlying cause.
Pain threshold refers to the point at which sensory input is reported as painful. For example, different temperatures can be applied to the skin to measure the pain at which the heat is interpreted as pain. A higher temperature indicates a higher sensory threshold for pain. Allodynia refers to when pain is experienced with sensory inputs that do not normally cause pain (e.g., light touch).
Pain tolerance is different to pain threshold. It is more difficult to define and generally refers to a person’s response to pain. One person may experience pain but think little of it and carry on with their activities as usual. Another person may experience a similar pain and worry that it indicates a serious underlying illness, take time away from work, and seek medical investigations and treatment. Pain tolerance varies massively between individuals and is influenced by many biological, psychological and social factors.
At the most basic level, pain receptors (nociceptors) at the ends of nerves detect damage or potential damage to tissues. Nerve signals are transmitted along the afferent nerves to the spinal cord. Afferent sensory nerves that transmit pain signals are part of the peripheral nervous system and are called primary afferent nociceptors.
Two groups of nerve fibres transmit pain:
- C fibres (unmyelinated and small diameter) – transmit signals slowly and produce dull and diffuse pain sensations
- A-delta fibres (myelinated and larger diameter) – transmit signals fast and produce sharp and localised pain sensations
The signal then travels in the central nervous system, up the spinal cord (mainly in the spinothalamic tract and spinoreticular tract) to the brain where it is interpreted as pain, mainly in the thalamus and cortex.
The main sensory inputs that generate a pain signal are:
- Mechanical (e.g., pressure)
- Heat
- Chemical (e.g., prostaglandins)
However, when directly measuring activity in the peripheral afferent sensory nerves:
- Pain can be experienced without activity in the primary afferent nociceptors
- Activity in the primary afferent nociceptors can be detected without the patient experiencing any pain
Referred pain refers to pain that experienced in a location away from the site of tissue damage. For example, patients with a heart attack may have pain in their left arm. There are several possible explanations for referred pain, including:
- Nerves may share the innervation of multiple parts of the body (e.g., the heart and left arm)
- Pain in one area amplifies the sensitivity in the spinal cord to signals coming from other areas
- Activation of the sympathetic nervous system in response to pain results in pain in other areas
Neuropathic pain is caused by abnormal functioning or damage of the sensory nerves, resulting in pain signals being transmitted to the brain. Typical features suggestive of neuropathic pain are:
- Burning
- Tingling
- Pins and needles
- Electric shocks
- Loss of sensation to touch of the affected area
Measuring Pain
There are no reliable ways to objectively measure the pain someone is experiencing. As it is a subjective experience, pain is measured by asking the patient about their perception of pain.
The two ways commonly used to measure pain are the visual analogue scale (VAS) and numerical rating scale (NRS).
The visual analogue scale (VAS) involves asking the patient to rate their pain along a horizontal line, where the left end indicates no pain and the right end indicates the worst pain imaginable. The distance along that line can be measured to get a numerical value to represent the pain (e.g., 75mm along a 100mm line).
The numerical rating scale (NRS) involves asking the patient to rate their pain on a numerical scale of 0 – 10, with:
- 0 being no pain at all
- 10 being the worst pain imaginable
Pain can also be rated on a graphical rating scale, with a series of faces going from happy to very unhappy. This may be helpful in children or patients with a learning disability.
Analgesic Ladder
The World Health Organisation (WHO) analgesic ladder was originally to help manage cancer-related pain. It is also often used for acute and chronic painful conditions. The idea is that patients with mild pain start on the first step, and when pain is more severe or does not respond to the lower steps, higher steps on the ladder are used until the pain is adequately managed.
There are three steps to the analgesic ladder:
- Step 1: Non-opioid medications such as paracetamol and NSAIDs
- Step 2: Weak opioids such as codeine and tramadol (tramadol has multiple mechanisms of action, including being an SNRI and agonist of opioid receptors)
- Step 3: Strong opioids such as morphine, oxycodone, fentanyl and buprenorphine
Other medications may be combined with the analgesic ladder for additional effect (called adjuvants) or used separately to manage neuropathic pain. These are:
- Amitriptyline – a tricyclic antidepressant
- Duloxetine – an SNRI antidepressant
- Gabapentin – an anticonvulsant
- Pregabalin – an anticonvulsant
- Capsaicin cream (topical) – from chilli peppers
Side-Effects
Medical overuse headache is a common side-effect of the long-term use of analgesic medication.
The key side effects of NSAIDs are:
- Gastritis with dyspepsia (indigestion)
- Stomach ulcers
- Exacerbation of asthma
- Hypertension
- Renal impairment
- Coronary artery disease, heart failure and strokes (rarely)
NSAIDs may be inappropriate or contraindicated in patients with:
- Asthma
- Renal impairment
- Heart disease
- Uncontrolled hypertension
- Stomach ulcers
Proton pump inhibitors (e.g., omeprazole or lansoprazole) are often co-prescribed with NSAIDs to reduce the risk of gastrointestinal side effects (e.g., acid reflux, gastritis and stomach ulcers).
The key side effects of opioids are:
- Constipation
- Skin itching (pruritus)
- Nausea
- Altered mental state (sedation, cognitive impairment or confusion)
- Respiratory depression (usually only with larger doses in opioid-naive patients)
Naloxone is used to reverse the effects of opioids in life-threatening overdose (usually due to respiratory depression).
Opioids in Palliative Care
Using opioids to control pain in palliative patients is a specific scenario where the doses are titrated and optimised over time. This involves using a combination of:
- Background opioids (e.g., 12-hourly modified-release oral morphine)
- Rescue doses for breakthrough pain (e.g., immediate-release oral morphine solution)
The rescue dose is usually 1/6 of the background 24-hour dose. For example, if the patient is getting 30mg in 24 hours of modified-release morphine (15mg every 12 hours), each rescue dose will be 5mg, given every 2-4 hours as required.
If the patient requires regular rescue doses for breakthrough pain, the dose of the background opioid can be increased. The rescue doses will also need increasing so that they remain 1/6 of the background 24-hour dose.
TOM TIP: Remember that each rescue dose is 1/6 of the 24-hour background dose. This is a very common exam question and something that seniors will commonly ask to test your knowledge. The question may be something like, “this patient is on 30mg of modified-release morphine every 12 hours; what would be the correct breakthrough dose?” In this scenario, 10mg is the correct answer, as the patient is getting 60mg background morphine every 24 hours (30mg twice a day).
Opioid Conversion
The information here is from the BNF, which gives approximate conversions between different opiates. It is helpful to remember the equivalent doses to 10mg of oral morphine. The conversations are not exact, and patients can respond differently to different opioids. Always check the BNF or other official reference material for accurate conversion figures (the information here may not be up to date or accurate and is only intended for study purposes):
|
Opioid |
Route |
Equivalent Dose |
|
Morphine |
Oral |
10mg |
|
Codeine |
Oral |
100mg |
|
Tramadol |
Oral |
100mg |
|
Oxycodone |
Oral |
6.6mg |
|
Morphine |
IV / IM / SC |
5mg |
|
Diamorphine |
IV / IM / SC |
3mg |
It is also possible to use opioid patches for background analgesia:
- Buprenorphine patches (5 mcg/hour patches are roughly equivalent to 12 mg/24 hours of oral morphine)
- Fentanyl patches (12 mcg/hour patches are roughly equivalent to 30mg/24 hours of oral morphine)
Post-Operative Analgesia
Adequate analgesia in the post-operative period is vital to encourage the patient to:
- Mobilise
- Ventilate their lungs fully (reducing the risk of chest infections and atelectasis)
- Have an adequate oral intake
Analgesia is usually started in theatre by the anaesthetist, with regular paracetamol, NSAIDs and opiates if required (e.g., regular modified-release oxycodone with immediate-release oxycodone as required for breakthrough pain). The surgeon may put a local anaesthetic into the wound to help with the initial pain after the procedure. Analgesia should be reduced and stopped as symptoms improve. There is more detail on analgesia in the anaesthetics section.
Patient Controlled Analgesia
Patient-controlled analgesia (PCA) involves an intravenous infusion of a strong opiate (e.g., morphine, oxycodone or fentanyl) attached to a patient-controlled pump. A PCA involves the patient pressing a button as pain develops to administer a bolus of opiate medication. The button will stop responding for a set time after administering a bolus to prevent over-use. Only the patient should press the button (not a nurse or doctor).
Patient-controlled analgesia requires careful monitoring. There needs to be input from an anaesthetist and facilities in place if adverse events occur. This includes access to naloxone for respiratory depression, antiemetics for nausea, and atropine for bradycardia. The anaesthetist may prescribe background opiates (e.g., patches) in addition to a PCA. Other “as required” opiates need to be avoided whilst a PCA is in use. The machine is locked to prevent tampering.
Chronic Pain
Chronic pain can be diagnosed when pain has been present or reoccurs in one or more areas over more than 3 months.
Some studies suggest up to 50% of the adults in the UK are affected by chronic pain. Common areas of chronic pain include:
- Headaches
- Lower back pain
- Neck pain
- Joint pain (e.g., knees or hips)
The NICE guidelines on chronic pain (April 2021) separates chronic pain into:
- Chronic primary pain – where no underlying condition can adequately explain the pain
- Chronic secondary pain – where an underlying condition can explain the pain
There is a long list of causes of chronic secondary pain that could take up the entire page. A few examples are:
- Osteoarthritis
- Lasting pain after a traumatic injury (e.g., bone fracture)
- Migraines
- Irritable bowel syndrome
- Endometriosis
- Cancer
- Neuropathic pain (e.g., due to diabetes, nerve impingement, multiple sclerosis or post-herpetic neuralgia)
- Complex regional pain syndrome
Biological, psychological and social factors contribute to the persistence of the pain. The physical processes that can lead to chronic pain include:
- Sensitisation of the primary afferent nociceptors by frequent stimulation
- Increased activity of the sympathetic nervous system
- Increased muscle contraction in response to pain
Chronic pain is a complex condition that can be challenging to manage. Analgesia is often inadequate and can lead to side effects and dependence.
Good communication and building a relationship with the patient is an important part of managing chronic pain. In chronic primary pain, an underlying physical cause of the pain may never be found. Chronic pain may not improve and may get worse with time. It often fluctuates, with flare-ups where the pain gets worse. A big part of management is maintaining and improving the quality of life despite the pain.
Patients require a holistic, person-centred approach to assessing and managing their condition. This involves:
- Exploring the impact on their life
- Discussing what they already do to manage their pain
- Their ideas, concerns and expectations about the pain
Options for managing chronic pain detailed in the NICE guidelines (2021) are:
- Supervised group exercise programs
- Acceptance and commitment therapy (ACT)
- Cognitive behavioural therapy (CBT)
- Acupuncture
- Antidepressants (e.g., amitriptyline, duloxetine or an SSRI)
It is worth noting that the NICE guidelines (2021) advise that for chronic primary pain (where no underlying condition can adequately explain the pain), patients should not be started on:
- Paracetamol
- NSAIDs
- Opiates
- Pregabalin
- Gabapentin
In chronic secondary pain, analgesia may be helpful depending on the underlying cause. For example, in patients with pain caused by osteoarthritis, the use of analgesia involves a stepwise approach to control symptoms:
- Oral paracetamol and topical NSAIDs
- Add oral NSAIDs (consider co-prescribing a proton pump inhibitor, such as omeprazole, to protect the stomach)
- Consider opiates such as codeine
TOM TIP: Chronic pain is incredibly common. It is worth noting these recent guidelines that clearly state to avoid basically all forms of analgesia (other than antidepressants) in patients with chronic primary pain. These guidelines may come up in exams, potentially asking you the most appropriate medication for a patient with chronic primary pain (antidepressants). This is different to chronic secondary pain, where there is an underlying condition that explains the pain.
Neuropathic Pain
The DN4 questionnaire can be used to assess the characteristics of the pain and the likelihood of neuropathic pain. Patients are scored out of 10. A score of 4 or more indicates neuropathic pain.
There are four first-line treatments for neuropathic pain:
- Amitriptyline – a tricyclic antidepressant
- Duloxetine – an SNRI antidepressant
- Gabapentin – an anticonvulsant
- Pregabalin – an anticonvulsant
NICE recommend using one of these four medications to control neuropathic pain. If it does not help, it can be slowly withdrawn, and an alternative can be tried. All four can be tried in turn. Only one neuropathic medication should be used at a time.
Other options for managing neuropathic pain are:
- Tramadol ONLY as a rescue for short term control of flares
- Capsaicin cream (chilli pepper cream) for localised areas of pain
- Physiotherapy to maintain strength
- Psychological input to help with understanding and coping
Trigeminal neuralgia is a type of neuropathic pain. However, NICE recommend carbamazepine as the first-line medication for trigeminal neuralgia, and if that does not work to refer to a specialist.
Last updated August 2021