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INTRODUCTION

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Pain in the lower abdomen and pelvis is a common complaint. But, pain is subjective and often ambiguous, and thus, difficult to diagnose and treat. To assist, clinicians ideally understand the mechanisms underlying human pain perception, which involves complex physical, biochemical, emotional, and social interactions. Providers are obligated to search for organic sources of pain, but equally important, avoid overtreatment of a condition that is minor or short lived.

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PAIN PATHOPHYSIOLOGY

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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 inevitable behavioral consequences. These are often as important as the pain itself. The mere threat of pain may elicit emotional responses even in the absence of actual injury.

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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 categorizations are helpful for diagnosis and treatment.

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Somatic or Visceral Pain

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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. 11-1).

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FIGURE 11-1

Dermatome maps. A dermatome is an area of skin supplied by a single spinal nerve. A. Body dermatomes. B. Perineal dermatomes. (Adapted with permission from Steege JF, Metzger DA, Levy BS (eds): Chronic Pelvic Pain: an Integrated Approach. Philadelphia: WB Saunders; 1998.)

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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 stretching, distention, ischemia, necrosis, or spasm of abdominal organs. The visceral afferent fibers that transfer these stimuli are sparse. Thus, the resulting diffuse sensory input leads to pain that is often described as a generalized, dull ache.

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Visceral pain often localizes to the midline because visceral innervation of abdominal organs is usually bilateral (Flasar, 2006). As another attribute, visceral afferents follow a segmental distribution, and 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 pain in the suprapubic or hypogastric area (Gallagher, 2004).

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Visceral afferent fibers are poorly myelinated, and action ...

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