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Controlling pain (Proceedings)

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Oct 01, 2008

Pain is defined by the International Association for the Study of Pain (IASP) as:

An unpleasant sensory or emotional experience associated with actual or potential tissue damage or described in terms of such damage. Controlling pain is a vital role for clinicians.

Types of Pain

Physiologic: High intensity fibers that act to protect by warning of contact w/ tissue damaging stimuli

Pathologic (clinical): Produced by peripheral tissue injury or damage to nervous system, and can be inflammatory (visceral/somatic) or neuropathic

Idiopathic: Pain that persists in absence of identifiable substrate

Definitions

Hyperalgesia is characterized by an increased response to a noxious stimulus, without a change in nociceptor threshold. This is illustrated by the slope of the line being greater than normal. Allodynia on the other hand is characterized by a decrease in the nociceptor threshold required to produce a response, (ie a non painful stimuli can induce a painful response).

Nociception is the process of detecting pain via pain receptors (nociceptors) and signaling noxious stimulus, it consists of transduction, transmission, modulation which leads to conscious perception of pain.

Transduction is the process that involves translation of noxious stimuli into electrical activity at sensory nerve endings. Nociceptors respond to thermal, mechanical, and chemical stimulation. Transmission is the process of sending the impulses throughout the sensory nervous system. Afferent signals from the periphery are relayed through the dorsal root ganglia to the dorsal horn of the spinal cord where sensory input can be modulated. Information then travels rostrally, finally reaching the cerebral cortex.

Modulation is the modification of nociceptive transmission. The body can regulate and modify incoming impulses at the dorsal horn.

Perception of pain can only occur in a conscious animal and is a result of the interaction of transduction, transmission, and modulation.

The concepts of peripheral and central sensitization explain a great deal about an animal's response to an injury and also pave the way for better pain management. Surgery produces inflammation and a change in sensitivity to noxious stimuli. As the local area of tissue injury becomes more sensitive, the threshold for subsequent stimuli decreases (termed hyperalgesia). The hypersensitivity is not localized just to the original site of injury; it spreads to other parts of the body, which is termed secondary hyperalgesia. When neurons in the dorsal horn are repeatedly stimulated, their rate of discharge dramatically increases with time, and this is central hypersensitization. The barrage of signals that arrive in the spinal cord causes changes in the dorsal horn neurons, which become "wound up." The neurons are hypersensitive even after the noxious stimulus stops. Specific receptors are involved in the process of "wind-up"; one is the N-methyl-D-aspartate (NMDA) receptor in the spinal cord. Ketamine is a noncompetitive NMDA antagonist, and this has resulted in numerous studies on ketamine's ability to prevent or treat pain, particularly when given prior to the painful insult.

Clinical Signs of Pain

Tachycardia, hypertension, dilated pupils, tachypnea, behavioral changes, vocalization, and abnormal posture/gait

Physiologic Consequences of Pain

Increased sympathetic tone, vasoconstriction, increased systemic vascular resistance, increased cardiac output, increased myocardial work, decreased gastro-intestinal tract tone, catabolic state, increased blood glucose, increased protein catabolism and lipolysis, and renal retention of H20 and Na with increased K excretion and decreased GFR. Coagulation abnormalities include increased blood viscosity, prolonged PT/PTT, fibrinolysis and platelet aggregation.

Treatment

Non-pharmacologic Treatment of Pain

Caring and supportive (environment clean and dry environment), visiting with family and familiar objects, and giving cats a place to hide. Direct drug therapy at the four steps in the nociceptive.

Options for therapy

Anesthetics, opiods, α2 agonists, benzodiazepines, phenothiazines, local anesthetics, NSAIDS, NMDA antagonists, TCA, anticonvulsants, corticosteroids.