Pain Classification

• region affected
• systems involved (visceral, nervous system, musculoskeletal, cutaneous-subcutaneous)
• etiology (trauma, infection, inflammation, degeneration, genetic-congenital causes, psychological causes)
• duration (acute, chronic)
• temporal characteristics (single episode, continuous, paroxysmal, intermittent)
• mechanism-based (somatic, visceral, nociceptive, neuropathic, etc)

History of Pain Research

• pain-gate theory of the early 1960's (Wall & Melzack)
• acceptance of the multidimensional nature of pain and recognition of the importance of behavioural factors in the expression of pain
• discovery of nerve endings that are specifically activated by tissue damage (nocicpetors) with unique ion channels
• findings of processes related to the release of inflammatory mediators which enhance the excitability of nociceptors
• documentation of nociceptor pathways to spinal cord neurones, brainstem and higher centers
• findings of segmental sensory modulation and descending facilitatory and inhibitory influences through opioids, chatecholamines, and other endogenous neurochemicals
• molecular cloning of neurochemicals and receptors related to pain
• documentation of neuroplasticity of the central nociceptive pathways and processes that can amplify pain
• improved and more diversified approach to diagnosis and management of pain through novel pharmacological techniques, psycho-behavioural methodology, virtual reality training and imaging techniques (fMRI, PET) which have determined the modification of sensory, cognitive, emotional and attentional functions
• practitioner acceptance/recognition of multimodal approaches for the treatment of a multifactorial & multidimensional problem - 'sensory discriminative', 'cognitive', 'affective/emotional' and 'motivational' dimension
• immune responses

Pain Theories

• hard wired lines from 'pain receptors' to specific regions in the CNS that process only pain-related signals - "the specificity theory"
• noxious stimulation activates several different types of receptors and that the summation of the signals in the CNS leads to pain - "the summation theory"
• a pattern of signals produced by noxious stimulation results in a distinct recognition of pain over other sensations - "the pattern theory"
• sensory stimulation interacting with nociceptive input - "the sensory interaction theory"

Pain Generating Mechanisms

• Spontaneous ectopic activity in damaged primary afferent nociceptors (PANs)
• Increased mechanical or thermal sensitivity of PANs
• Adrenergic mediated sensitization of PANs
• Neurogenic and non-neurogenic inflammation
• Loss of central inhibitory effects of myelinated primary afferents
• Reorganization of spinal cord connectivity
• Spontaneous activity in deafferented spinal pain transmission neurons (SPTNs)
• Prolonged excitation of SPTNs
• Ectopic impulse generation from the dorsal root ganglion (DRG)
• Sympathetic/DRG ectopic impulse excitation

Pain Modulating Mechanisms

• Descending sensorimotor cortical and pontine sympathetic modulation onto the superficial dorsal and ventral horn of the spinal cord
• Peripheral non-noxious input onto WDR neurons in the dorsal horn of the spinal cord
• Inhibition of peripheral nociceptors through peripheral sympathetic and DRG mechanisms
• Intra cortical, cerebellar and brainstem modulation of pain (PET images)

Hungary 1989