Lecture Slides: Neurophysiology and Classification of Pain

Peripheral sensitization Central sensitisation Xie et al 2021 Pain Research to explore pain processing & modulation Yanfei Xie Referred pain • Pain that is perceived at a site different from its point of origin but innervated by the same spinal segment à makes it difficult for the brain to c...

Peripheral sensitization Central sensitisation Xie et al 2021 Pain Research to explore pain processing & modulation Yanfei Xie Referred pain • Pain that is perceived at a site different from its point of origin but innervated by the same spinal segment à makes it difficult for the brain to correctly identify the original source of pain • Visceral and somatic afferent neurons converge on the same dorsal horn project neurons Pain classification nociplastic nociceptive neuropathic Psychosocial factors Classification of pain Acute pain Chronic (or persistent) pain • Pain < 3 months • Pain > 3 months • A normal and time-limited response to inflammation or tissue damage • Pain plays an important role in protection from further injury • Pain that persists beyond the expected timeframe of healing • Some disorders have no tissue injury eg fibromyalgia • Pain serves no useful function Mechanism- based classification of pain • Several mechanism-based classification systems have been proposed • Pain mechanisms refer to the factors that contribute to the development, maintenance or enhancement Mechanism- based classification of pain Why classify based on pain mechanisms? • It allows treatments to target altered mechanisms • It can help individualise care to the patient, rather than based on their diagnosis • Two people with the same diagnosis could have different underlying mechanisms contributing to their pain Mechanism- based classification of pain National strategic action plan for pain management 2019 identified 3 main categories: nociplastic nociceptive neuropathic Mechanism- based classification of pain nociplastic nociceptive neuropathic A person may have multiple pain mechanisms occurring simultaneously Nociceptive pain What is it? • Is primarily due to nociceptor activation by injury, inflammation or mechanical irritants • Noxious signals are relayed to the spinal cord and up to the cortex resulting in the perception of pain Nociceptive pain How can we recognize it? • By its history • There is usually a history of injury or damage • Pain typically reduces in line with the expected healing time Nociceptive pain How can we recognize it? • By its quality • Often intermittent, sharp and localized to area of injury/damage • Can be constant, aching or throbbing Which nerve fibres might be involved? Nociceptive pain How can we recognize it? • By its behaviour • Pain often has an on/off pattern e.g. aggravated by specific movements, postures or activities • It responds well to simple pain relief e.g. panadol or NSAIDs Nociceptive pain • Examples of conditions • Ankle sprain • Rheumatoid arthritis • Osteoarthritis Nociplastic pain What is it? • Is essentially pain that relates to increased sensitisation within the nervous system rather than actual or threatened tissue damage or nerve injury • Nociceptive signals are modulated within spinal cord, cortex and brainstem to enhance facilitation or diminish descending inhibition Nociplastic pain How can we recognize it? • By its history • Often chronic • There may be an injury/condition, but pain has continued beyond the period of expected healing Nociplastic pain How can we recognize it? • By its nature/quality • Diffuse or on-anatomical areas of pain • Widespread pain and tenderness Nociplastic pain How can we recognize it? • By its behaviour (or lack of pattern) • Pain is disproportionate to the nature & extent of injury/pathology • Pain is often unpredictable • Pain might be associated/aggravated by things other than movement e.g. stress, mood, emotions Nociplastic pain • Examples of conditions • Fibromyalgia • Non-specific low back pain • Lateral elbow tendinopathy Neuropathic pain What is it? • Is where there is a lesion or disease within the somatosensory system • This could be due to direct injury to the nerve e.g. carpal tunnel syndrome; or due to metabolic disease e.g. diabetes Neuropathic pain How can we recognize it? • By its history • History of nerve damage/pathology Neuropathic pain How can we recognize it? • By its distribution • Symptoms are in the distribution of the affected nerve(s) • Nerve root distribution e.g. sciatica • Single peripheral nerve e.g. carpal tunnel • Symmetrical polyneuropathy e.g. diabetic neuropathy Neuropathic pain How can we recognize it? • By its nature/quality • Pain may be shooting, burning • Pain often associated with numbness, tingling, crawling (dysaesthesia), sensitivity to touch • Pain may be spontaneous e.g. electric shock-like Neuropathic pain How can we recognize it? • By its behaviour • Symptoms aggravated by movements that move/tension/compress the nerve • Poor response to simple analgesics • Large impact on quality of life Neuropathic pain • Examples of conditions • Diabetic peripheral neuropathy • Chemotherapy induced peripheral neuropathy • Post herpetic neuralgia • Phantom limb pain • Complex regional pain syndrome • Spinal cord injury Mechanism- based classification of pain nociplastic nociceptive neuropathic Psychosocial factors The 3 biological pain processes can be influenced by (and directly influence) psychosocial factors Psychosocial factors • Unhelpful thoughts, beliefs and emotions may lead to: • Kinesiophobia (fear of movement or reinjury) • Hypervigilance (more sensitive to surroundings or sensations) • Catastrophisation (the belief that their pain is the worst imaginable, that relief is impossible, and that this is the worst situation of their lives) • Fear avoidance Psychosocial factors • Fear avoidance model helps to link these factors Psychosocial factors • Pain has profound effects on emotional states and attentional control nociplastic nociceptive neuropathic Psychosocial factors How might you classify the following? Fibromyalgia • Pain is chronic and widespread • Pain is associated with fatigue, sleep, concentration & memory problems • Anxiety and depression may contribute • No diagnostic laboratory or imaging findings or other medical/neurological disorder that explains symptoms (ie no actual or threatened tissue damage or nerve injury) Fibromyalgia • n i Pain is chronic and widespread pa • ic t s a with fatigue, sleep, Pain is associated l p i c o concentration & memory problems N • Anxiety and depression may contribute • No diagnostic laboratory or imaging findings or other medical/neurological disorder that explains symptoms (ie no actual or threatened tissue damage or nerve injury) Diabetic sensory peripheral neuropathy • Systemic hyperglycemia (high blood sugar) results in damage of multiple nerves (polyneuropathy) • The longest nerves are affected first à glove and stocking distribution • Different sensory fibres can be affected causing different symptoms between patients e.g. shooting pain, numbness, sensitivity to touch, feeling like things are crawling on their legs Diabetic sensory peripheral neuropathy • Systemic hyperglycemia (high blood sugar) results in damage of multiple nerves (polyneuropathy) • n à glove and The longest nerves are affected aifirst stocking distribution • a p ro t p c hi u fibres can be affected causing Different sensory e N different symptoms between patients e.g. shooting pain, numbness, sensitivity to touch, feeling like things are crawling on their legs Post herpetic neuralgia • Nerve damage which occurs from reactivated varicella zoster virus, the same virus that causes chickenpox • Reactivated virus travels from DRG to skin causing vesicular eruption • Terminal sprouting and inhibitory interneuron loss • Nerve pain confined to area of skin innervated by that peripheral nerve (dermatome) • Severe, burning pain, electric-like pain, allodynia, abnormal itch Post herpetic neuralgia • Nerve damage which occurs from reactivated varicella zoster virus, the same virus that causes chickenpox • Reactivated virus travels from DRG to n skin causing vesicular eruption • • a p ic i th a pinhibitory interneuron loss Terminal sprouting and o r u e N to area of skin innervated by that Nerve pain confined peripheral nerve (dermatome) • Severe, burning pain, electric-like pain, allodynia, abnormal itch Complex regional pain syndrome (CRPS) • Rare, chronic neuro-inflammatory disorder with confirmed or uncomfirmed lesion of somatosensory system • Previously termed reflex sympathetic dystrophy • Can occur following trauma • Typically affects one or more limbs • Pain in association with allodynia, changes in temperature, colour, skin, hair, nails Complex regional pain syndrome (CRPS) • Rare, chronic neuro-inflammatory disorder with confirmed or uncomfirmed lesion of somatosensory system in a p • Previously termed reflex sympathetic dystrophy ic • Can occur following ro trauma • Typically affects one or more limbs • Pain in association with allodynia, changes in temperature, colour, skin, hair, nails u e N th a p Questions ? References • Davidson’s Principles and Practice of Medicine 23rd Edition Ralston (Ed) Elsevier Edinburgh • Smart et al 2011 The Discriminative Validity of “Nociceptive,” “Peripheral Neuropathic,” and “Central Sensitization” as Mechanismsbased Classifications of Musculoskeletal Pain. Clin J Pain • MusculoskeletalFramework.net