Patho-Physiology of Pain and Theory of Pain Production and Modulation, Slides of Pathophysiology

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PATHOPHYSIOLOGY OF PAIN

Prof. J. Han áč ek, MD, PhD

● Alteration in sensory function  dysfunctions of the

general or special senses

- Dysfunctions of the general senses  chronic pain, abnormal

temperature regulation, tactile dysfunction

Definitions of pain

- Pain is a complex unpleasant phenomenon composed of sensory experiences that include time, space, intensity, emotion, cognition, and motivation - Pain is an unpleasant or emotional experience originating in real or potential damaged tissue - Pain is an unpleasant phenomenon that is uniquely experienced by each individual; it cannot be adequately defined, identified, or measured by an observer

Pain categories

1. Somatogenic pain is pain with cause (usually known) localised in body tissue: a/ nociceptive pain – somatic, visceral b/ neuropatic pain

2. Psychogenic pain is pain for which there is no known physical cause

but processing of sensitive information in CNS is disturbed In this type of pain the psychological evaluation yields evidence that the pain itself is predominantly sustained by psychological factors

Acute and chronic pain Acute pain is a protective mechanism that alerts the individual to a condition or experience that is immediately harmful to the body Onset - usually sudden

Relief - after the chemical mediators that stimulate the nociceptors,

are removed

- This type of pain mobilises the individual to prompt action to relief it - Stimulation of autonomic nervous system can be observed during this

type of pain (mydriasis, tachycardia, tachypnoe, sweating, vasoconstriction)

Responses to acute pain

• increased heart rate - diaphoresis
• increased respiratory rate -  blood sugar
• elevated blood pressure -  gastric acid secretion
• pallor or flushing, -  gastric motility dilated pupils -  blood flow to the viscera, kidney and skin
• nausea occasionally occurs

Psychological response to chronic pain

Intermittent pain produces a physiologic response similar to acute pain

Persistent pain allows for adaptation (functions of the body are

normal but the pain is not reliefed)

Chronic pain produces significant behavioural and psychological changes

The main changes are:

• depression
• an attempt to keep pain - related behaviour to a minimum
• sleeping disorders
• preoccupation with the pain
• tendency to deny pain

Pain threshold and pain tolerance

The pain threshold is the point at which a stimulus is perceived as pain It does not vary significantly among healthy people or in the same person over time

Perceptual dominance- intense pain at one location of the body

may cause an increase in the pain threshold in another location

- The pain tolerance is expressed as duration of time or the

intensity of pain that an individual will endure before initiation overt pain responses.

It is influenced by - persons cultural prescriptions

• expectations
• role behaviours
• physical and mental health

Age and perception of pain

Children and the elderly may experience or express pain

differently than adults

Infants in the first 1 to 2 days of life are less sensitive to pain

(or they simply lack the ability to verbalise the pain experience).

A full behavioural response to pain is apparent at 3 to 12 month

of life

Older children, between the ages of 15 and 18 years,

tend to have a lower pain threshold than do adults

Pain threshold tends to increase with ageing

This change is probably caused by peripheral neuropathies and

changes in the thickness of the skin

Neuroanatomy of pain

The portions of the nervous system responsible for the

sensation and perception of pain may be divided into three

areas:

1. afferent pathways

2. CNS

3. efferent pathways

The afferent portion is composed of: a) nociceptors (nerve endings of nociceptive nerve cells) b) afferent nerve fibres c) spinal cord network

The brain first perceives the sensation of pain

- The thalamus, sensitive cortex :

perceiving describing of pain localizing

- Parts of thalamus, brainstem and reticular formation: - identify dull longer-lasting, and diffuse pain - The reticular formation and limbic system:

• control the emotional and affective response to pain Because the cortex, thalamus and brainstem are interconnected with the hypothalamus and autonomic nervous system, perception of pain is associated with an autonomic response

The role of afferent and efferent pathways in

processing of pain information

Nociceptive pain

Nociceptors: Endings of small unmyelinated and lightly myelinated

afferent neurons

Stimulators: Chemical, mechanical and thermal noxae

Mild stimulation  positive, pleasurable sensation (e.g. tickling) Strong stimulation  pain

Location: In muscles, tendons, epidermis, subcutanous tissue,

joints, visceral organs

• they are not evenly distributed in the body (in skin more then in internal structures)

Nociceptive pain:

• mechanisms involved in development

Transduction is the process by which afferent nerve endings participate in translating noxious stimuli (e.g., a pinprick) into nociceptive impulses

Efferent analgesic system

Its role: - inhibition of afferent pain signals

Mechanisms:

• pain afferents on their way up to CNS send branches to periaqueductal gray (PAG) - gray matter surrounding the cerebral aqueduct in the midbrain, and stimulates the neurons there  activation of efferent (descendent) anti-nociceptive pathways
• from there the impulses are transmitted through the spinal cord to the dorsal horn
• there thay inhibit or block transmission of nociceptive signals at the level of dorsal horn

Descendent antinociceptive systém

Enk – enkefalinergic PAG – paraaqueductal gray EAA – excitatory amino acids RVM – rostral ventro-medial medulla