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PAIN PATHOPHYSIOLOGY

Pain in the lower abdomen and pelvis is a common complaint. Diagnosis can be challenging due to the wide differential diagnosis that spans multiple organ systems. Clinicians ideally should understand the mechanisms underlying human pain perception, which involves complex physical, biochemical, emotional, and social interactions.

Pain is a protective mechanism meant to warn of an immediate threat and to prompt withdrawal from noxious stimuli. Pain is usually followed by an emotional response and behavioral consequences. The mere threat of discomfort may elicit emotional responses even without actual injury.

When categorized, pain may be considered somatic or visceral depending on the type of afferent nerve fibers involved. Additionally, pain is described by the physiologic steps that produce it and can be defined as inflammatory or neuropathic (Kehlet, 2006). Both categories are helpful for diagnosis and treatment.

Somatic or Visceral Pain

Somatic pain stems from nerve afferents of the somatic nervous system, which innervates the parietal peritoneum, skin, muscles, and subcutaneous tissues. Somatic pain is typically sharp and localized. It is found on either the right or left within dermatomes that correspond to the innervation of involved tissues (Fig. 12-1).

FIGURE 12-1

Dermatome maps. A dermatome is an area of skin supplied by a single spinal nerve. A. Body dermatomes. B. Perineal dermatomes.

In contrast, visceral pain stems from afferent fibers of the autonomic nervous system, which transmits information from the viscera and visceral peritoneum. Noxious stimuli typically include stretch, ischemia, or spasm of abdominal organs. The visceral afferent fibers that transfer these stimuli are sparse. Thus, the resulting diffuse sensory input leads to a generalized, dull ache.

Visceral pain often localizes to the midline because visceral innervation of abdominal organs is usually bilateral (Flasar, 2006). Also, visceral pain is typically localized by the brain’s sensory cortex to an approximate spinal cord level that is determined by the embryologic origin of the involved organ. For example, pathology in midgut organs, such as the small bowel, appendix, and caecum, causes perceived periumbilical pain. In contrast, disease in hindgut organs, such as the colon and intraperitoneal portions of the genitourinary tract, causes midline suprapubic or hypogastric pain (Gallagher, 2004).

Visceral afferent fibers are poorly myelinated, and action potentials can easily spread from them to adjacent somatic nerves. As a result, visceral pain may at times be referred to dermatomes that correspond to these adjacent somatic nerve fibers (Giamberardino, 2003). In addition, both peripheral somatic and visceral nerves often synapse in the spinal cord at the same dorsal horn neurons. These neurons, in turn, relay sensory information to the brain. The cortex recognizes the signal as coming from the same dermatome regardless of its visceral or somatic nerve origin. ...

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