Chapter 134: Acute Pain Pathophysiology

INTRODUCTION

Acute pain is defined as pain that is caused by noxious stimulation due to injury, a disease process, or the abnormal function of muscle or viscera. Nociceptive pain, which entails four physiological processes (transduction, transmission, modulation, and perception), is usually implicated in acute pain. Posttraumatic, postoperative, obstetric pain, and acute medical illnesses, such as myocardial infarction, pancreatitis, and nephrolithiasis are various forms of nociceptive pain. Acute pain is typically self-limited, but when it persists beyond the usual course of an acute disease or after a reasonable time of healing (1–6 months), it is classified as chronic pain.

CLASSIFICATION OF PAIN

Nociceptive pain can be classified into two types:

1. Visceral pain results from a disease process or abnormal function of an internal organ. There are four subtypes of visceral pain:

   1. True Localized Visceral Pain—Dull, diffuse, and usually midline. It may frequently cause abnormal sympathetic or parasympathetic activity, causing nausea, vomiting, changes in hemodynamics (heart rate, blood pressure), and diaphoresis.

   2. Localized Parietal Pain—Sharp, stabbing pain localized to the area around the involved organ.

   3. Referred Visceral Pain—A phenomenon involving pain that refers to cutaneous areas due to patterns of embryological development and migration of tissues, along with the convergence of visceral and somatic afferent input into the central nervous system (ie, neck/shoulder pain from a disease process that involves the peritoneum or pleura over the central diaphragm).

   4. Referred Parietal Pain—A phenomenon similar to referred visceral pain, except only involving parietal surfaces.

2. Somatic pain arises from nociceptive input from the skin, subcutaneous tissues, and mucous membranes, characterized as sharp, throbbing, or burning. Deep somatic pain refers to pain that arises from muscles, tendons, joints, or bones, resulting in a dull, aching pain that is not well localized. One feature of somatic pain is that both the intensity and duration of the stimulus affect the degree of localization. For example, somatic pain arising from a minor trauma to the knee may be well localized in the knee, but severe or sustained trauma may result in pain in the entire leg.

PAIN PATHWAYS

Pain is conducted along three-neuron pathways that transmit noxious stimuli from peripheral nerves to the central cerebral cortex. The primary afferent sensory neurons are located in the dorsal root ganglia. Each neuron has a single axon that bifurcates, one to the peripheral tissues and the other into the dorsal horn of the spinal cord, where the primary afferent neuron synapses with second-order neurons whose axons decussate and ascend via the contralateral spinothalamic tract to reach the thalamus. These second-order neurons synapse with third-order neurons in the thalamic nuclei, which send projections throughout the internal capsule and corona radiata to the postcentral gyrus of the cerebral cortex.

First-order neurons send the proximal end of their axons into the spinal cord via the dorsal spinal roots. These neurons originate in ...