What are the pathways and physiology of pain?

From the Guidelines

Pain pathways and physiology involve complex neural mechanisms that transmit and process nociceptive signals, and understanding these pathways is crucial for developing effective pain management strategies. Pain begins with nociceptors, specialized sensory receptors that detect potentially harmful stimuli like extreme temperatures, mechanical pressure, or chemical irritants, as described in the 2017 HIVMA of IDSA clinical practice guideline for the management of chronic pain in patients living with HIV 1. These receptors convert these stimuli into electrical signals through a process called transduction. The signals then travel along primary afferent neurons, primarily A-delta fibers (fast, sharp pain) and C fibers (slow, burning pain), to the dorsal horn of the spinal cord. In the dorsal horn, these signals synapse with second-order neurons that cross to the contralateral side and ascend via the spinothalamic tract to the thalamus. From the thalamus, signals are relayed to various brain regions including the somatosensory cortex (for pain localization), the limbic system (for emotional aspects), and the prefrontal cortex (for cognitive processing).

Pain modulation occurs through descending pathways from the brain, involving neurotransmitters like endorphins, serotonin, and norepinephrine, which can inhibit pain signals at the spinal level, as noted in the EULAR recommendations for the health professional's approach to pain management in inflammatory arthritis and osteoarthritis 2. This endogenous pain control system explains phenomena like placebo effects and why pain perception varies between individuals. Key factors that influence pain perception include:

• Biological factors, such as inflammation and joint damage
• Psychological factors, such as emotions, cognitions, and behavior
• Social factors, such as work, support facilities, and economic status
• Lifestyle factors, such as sleep quality, obesity, and other health risks

Effective pain management requires a patient-centered approach that takes into account these multiple factors, as emphasized in the EULAR recommendations 2. This approach should involve a comprehensive assessment of the patient's pain, including its severity, location, and impact on daily life, as well as an evaluation of the underlying causes of the pain, such as inflammation, joint damage, or other underlying conditions. By understanding the complex neural mechanisms involved in pain transmission and modulation, and by taking a patient-centered approach to pain management, healthcare providers can develop effective treatment strategies that address the unique needs of each patient.

From the Research

Pathways of Pain

• Pain is a complex phenomenon that involves biological, psychological, and social factors 3
• The experience of pain is activated by various stimuli, including biological, physical, and psychological factors 4
• Nociceptive and neuropathic pain are two different types of pain that require distinct treatment approaches 3

Physiology of Pain

• The physiology of pain is not fully understood, but it is known to involve the activation of pain pathways in the body 4
• The assessment of pain is crucial in understanding the underlying causes and developing effective management strategies 5, 4
• A multidimensional tool is recommended for assessing pain, as it takes into account various features such as pain qualities, duration, and impact on functional capabilities 5

Management of Acute Pain

• Acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) are first-line treatment options for most patients with acute mild to moderate pain 6, 7
• Topical NSAIDs are recommended for non-low back, musculoskeletal injuries 6
• Opioids may be necessary for severe or refractory acute pain, but their use should be minimized due to the risk of addiction and other adverse effects 6, 7
• Combination therapy using a small amount of opioid together with a nonopioid pain reliever has been shown to be effective in reducing opioid consumption 7

Assessment of Pain

• Pain assessment is often simplistic and brief, but it should be comprehensive and take into account various features such as pain qualities, duration, and impact on functional capabilities 5
• A "universal precautions" approach, including abuse risk assessment and abuse deterrence strategies, should be implemented for patients receiving opioids 5
• The use of a multidimensional tool is recommended for assessing pain, as it provides a more comprehensive understanding of the patient's experience 5