Imaging Evaluation of Acute Abdominal Pain



CHAPTER 8 Imaging Evaluation of Acute Abdominal Pain




ACUTE ABDOMINAL PAIN: A DIAGNOSTIC DILEMMA


Acute abdominal pain is a common clinical syndrome and a diagnostic dilemma for clinicians. For centuries, physicians relied on obtaining a history of illness that elicited characteristic clinical symptoms. This history, combined with careful elicitation of physical examination findings, allowed physicians to discriminate among many potential disease processes. Unfortunately, historical, physical examination, and laboratory data are often nonspecific and inconclusive, spurring a shift in the diagnostic approach to patients with abdominal pain toward imaging. As with physical examination, image interpretation is facilitated by familiarity with the relevant anatomic structures and common diagnoses to be considered in patients presenting with specific pain syndromes (Table 8-1).


Table 8-1 Causes of Acute Abdominal Pain Based on Location of Symptoms

































Location of Pain Relevant Structures and Common Diagnoses to Consider
Right upper quadrant







Left upper quadrant





Midepigastric





Right lower quadrant




Left lower quadrant


Generalized


Flank and back




Pelvis and groin








Scrotal or inguinal



Keep in mind that significant overlap exists between the diseases listed in Table 8-1 and their clinical presentations. A confusing influence in the diagnosis of abdominal pain is related to the overlapping innervation of the abdominal organs. For example, there exists considerable overlapping innervation of the stomach (T5-7), biliary tract (T6-8), and pancreas (T5-9). Therefore, a gastric process can be difficult to differentiate from biliary colic or pancreatitis based on location of pain alone. Overlapping innervation of structures such as the colon (T10-L1), small bowel (T8-10), and kidney (T10-L1) can obscure the precise origin of pain elsewhere in the abdomen as well. Also, remember that abdominal organs do not always reside in their expected locations. For example, the cecum and appendix have a notorious propensity to extend to locations beyond the right lower quadrant, and patients with situs anomalies or bowel malrotation are not uncommon. Finally, disease processes often present remote from their organ of origin. Inflammation and tumor can spread via the subperitoneal space within the ligaments and mesenteries of the abdomen and pelvis or via the peritoneal space. Knowledge of these pathways of disease spread as discussed in Chapter 6 can greatly facilitate image interpretation and explain apparent discrepancies between the clinical presentation and the actual diagnosis.




The role of laboratory data in narrowing a differential diagnosis should not be underestimated. At a minimum, a radiologist interpreting an imaging study of a patient with acute abdominal pain should be familiar with the patient’s blood cell counts, liver function tests, amylase and lipase levels, and urinalysis if they are available.


Not all imaging modalities are equally capable of demonstrating the structures and disease processes listed in Table 8-1. For example, computed tomography (CT) would be an inappropriate first test to perform in a patient presenting with scrotal pain, whereas transabdominal ultrasound would be a poor choice for excluding peptic ulcer disease. Therefore, when imaging is deemed necessary for further evaluation of a patient’s pain, the first decision to make involves the choice of imaging modality. A carefully optimized protocol must be selected and combined with appropriate patient preparation to maximize the effectiveness of the chosen modality.



COMPUTED TOMOGRAPHY: THE WORKHORSE OF THE EMERGENCY DEPARTMENT



Why Computed Tomography Rules the Emergency Department


Despite the wide variety of available imaging options, CT has been adopted as the modality of choice for the evaluation of patients presenting with acute abdominal pain not clearly localized to the right upper quadrant or pelvis (in females of child-bearing age). The use of plain radiographs in the acute setting has come under criticism, mainly because of low sensitivity of conventional radiography for most common causes of abdominal pain. Some studies recommend use of unenhanced CT of the abdomen and pelvis rather than abdominal radiography, arguing that plain films rarely provide a specific diagnosis. CT is much more likely than plain radiographs to show a specific cause of abdominal pain and can be useful in distinguishing between those processes that require urgent surgical intervention and those suitable for conservative management. Several studies have shown that routine evaluation of the abdomen and pelvis with CT in the emergency setting decreases both the number of hospital admissions and the length of stay in the emergency department.


Although ultrasound is routinely performed for the evaluation of right upper quadrant and pelvic pain, CT has many advantages over sonography for the evaluation of patients with other types of pain. CT can be performed in a matter of seconds on a modern scanner, allowing surgical triage to be accomplished with utmost efficiency. CT is less limited by patient body habitus, and displays all organ systems and many disease processes with near equal clarity. Finally, CT image quality is far less operator dependent than sonography.


One final advantage of CT is its exquisite ability to demonstrate the full extent of a disease process that spans multiple abdominal organ systems or abdominal compartments. No other modality has the ability of CT to fully document the spread of diseases via the peritoneal cavity or abdominal mesenteries and ligaments. This avoids the potential diagnostic pitfall created by diseases presenting remote from their organs of origin.



Computed Tomography Protocol Considerations for Acute Abdominal Pain


Because of the overlapping innervation of abdominal and pelvic organs and the possibility of a disease process presenting remote from the organ of origin, the entire abdomen and pelvis should be included on a CT examination performed for evaluation of acute abdominal pain. Although CT performed without the addition of enteric and intravenous contrast media can provide valuable diagnostic information, the appropriate use of these agents often improves sensitivity and defines disease to better advantage. For this reason, most survey examinations of the abdomen and pelvis for acute pain are performed using both intravenous and enteric (typically oral) contrast. The portal venous phase of enhancement is preferred in most situations. The CT examination can be modified to accommodate specific patient conditions, such as renal insufficiency, contrast sensitivity, or compromised hemodynamic status.


Traditional positive oral contrast (high attenuation) is useful for distinguishing bowel from other structures and for demonstrating perforation of the gastrointestinal tract. Negative oral contrast (low attenuation) accompanied by injection of intravenous contrast media can be useful for demonstrating inflammation and other enhancing lesions of the bowel wall (Fig. 8-1). When urgency dictates, examinations can be performed without enteric contrast media.




Helpful Computed Tomographic Findings in the Acute Abdomen


Because of the all-inclusive survey provided by CT and the overlapping clinical presentations that can cloud the origins of abdominal pain, radiologists must be facile with interpreting CT scans of the abdomen and pelvis. This skill requires a systematic approach and knowledge of findings that aid in disease localization. A number of signs have been proved effective in localizing the source of acute abdominal pain (Box 8-1).




Focal Fat Stranding or Fascial Thickening


Fat stranding adjacent to a bowel loop or solid organ is often a key CT finding indicating the origin of pain. Stranding is seen as linear or hazy increased attenuation within a region of fat, usually caused by fluid or inflammatory cells infiltrating the tissues (Fig. 8-2) and often accompanied by thickening of nearby fascial planes. Stranding is most useful when it is localized because generalized stranding is nonspecific and can be seen with any systemic cause of edema (e.g., congestive heart failure or hypoproteinemia). Focal fat stranding can signal a focus of acute disease and should always prompt focused inspection of the adjacent structures.



Disproportionate fat stranding refers to focal stranding adjacent to bowel associated with minimal or absent bowel wall thickening. Its presence suggests entities that typically result in a greater degree of inflammation in the adjacent mesentery than in the bowel wall itself. The presence or absence of disproportionate fat stranding is one method used to differentiate colonic diverticulitis from colon cancer, because diverticulitis tends to result in a greater degree of pericolic inflammation relative to the degree of bowel wall thickening (Fig. 8-3). Disease processes associated with disproportionate fat stranding are listed in Table 8-2.



Table 8-2 Disease Processes Associated with Disproportionate Fat Stranding























Diagnosis Clinical Discriminators Imaging Discriminators
Appendicitis Right lower quadrant pain, nausea, fever, leukocytosis Periappendiceal inflammation
Diverticulitis Left lower quadrant pain, fever, leukocytosis Pericolonic inflammation, adjacent diverticulum
Epiploic appendagitis Nausea and fever are rare Fatty pericolonic mass with dense rim and central high-attenuation focus
Omental infarction

Triangular or ovoid fatty/heterogeneous lesion deep to right anterior abdominal wall

Mesenteric fat stranding can result from a broad variety of inflammatory and neoplastic disease processes; therefore, it is important to consider this finding in the context of clinical presentation and other imaging findings (Fig. 8-4). The most common causes of mesenteric fat stranding are listed in Box 8-2.




Sclerosing mesenteritis is an uncommon primary inflammatory disorder of the mesentery. An elusive disease that can be difficult to diagnose and to treat, sclerosing mesenteritis has been known by many names, including mesenteric panniculitis, retractile mesenteritis, fibrosing mesenteritis, mesenteric lipodystrophy, and mesenteric Weber–Christian disease. The most common CT finding is fat stranding in the mesentery, a nonspecific finding sometimes referred to as a “misty” mesentery (Fig. 8-5). Additional findings of sclerosing mesenteritis may include: (1) a “fat halo,” or ring of low attenuation surrounding mesenteric vessels within a region of mesenteric fat stranding; (2) a dense pseudocapsule surrounding the area of involved fat; and (3) small soft-tissue nodules within the affected region of mesentery.



The most common malignant neoplasms to involve the mesenteries are lymphoma and carcinoid tumor, although other tumors, such as adenocarcinoma, can also extend into the mesenteries or incite a local desmoplastic reaction. Neoplastic disease of the mesentery rarely presents as acute abdominal pain.


Perinephric stranding is commonly seen in healthy individuals, but when asymmetric, should prompt a search for a cause. Some causes to consider include obstructive urinary calculus, pyelonephritis, inflammatory nephritides, renal infarction, and renal vein thrombosis.




Segmental Bowel Wall Thickening


Distinguishing between normal and abnormal bowel wall thickness is one of the greatest challenges in abdominal imaging. Even the combination of a diligent technologist and compliant patient rarely results in uniform bowel distention throughout the gastrointestinal tract. Because some bowel loops are likely to be incompletely distended during the examination, it is common to mistakenly describe a collapsed bowel segment as “thickened.” Of course, if one becomes accustomed to ignoring apparently thickened loops of bowel, critical diagnoses can be missed.


If a bowel loop appears thickened, it is useful to look for a segment of that loop that contains air. Because air rises to the most nondependent portion of the lumen and produces some degree of luminal distension, the presence of air underlying apparent bowel wall thickening suggests that the loop in question is truly abnormal.


Be cautious about calling wall thickening in the jejunum on CT examinations. The normal jejunum contains many more redundant folds than ileum. The CT correlate of redundant folds in the jejunum is a thicker appearance of the bowel wall (Fig. 8-7). As stated earlier, looking for areas of nondependent air between folds may help distinguish normal from thickened jejunum.



Whereas thickening of the entire cecum can be seen with colitis or colon cancer, focal thickening of the cecal apex near the base of the appendix is often seen with acute appendicitis. Sometimes called the cecal arrow sign, wall thickening at the junction of the cecum and appendix causes the lumen of the cecum to point toward the appendix, supporting the diagnosis of acute appendicitis (Fig. 8-8).



Table 8-3 lists the disease processes most commonly associated with segmental thickening of the bowel wall on CT, as well as some parameters that can be used to refine the differential diagnosis.


Table 8-3 Diseases Associated with Segmental Bowel Wall Thickening
























Diagnosis Imaging Discriminators
Appendicitis



Crohn disease


Colonic diverticulitis


Gastritis

Malignancy



Meckel’s diverticulitis Blind ending loop from ileum, not from cecum


Extraluminal Gas



PNEUMOPERITONEUM


The interpretation of CT examinations performed for acute abdominal pain should always include a directed search for peritoneal gas. Gas usually rises to the most nondependent recesses of the peritoneal cavity; therefore, a focused survey along the anterior peritoneal surface can increase sensitivity. During the search, it is helpful to use window and level settings that allow differentiation between fat and gas. The peritoneal space extends into the fissures for the ligamentum teres and ligamentum venosum, so include those locations in the search pattern. When gas is present in these fissures, duodenal or distal stomach perforation is often the source (Fig. 8-9). The most common causes of pneumoperitoneum are listed in Table 8-4.



Table 8-4 Causes of Pneumoperitoneum









Surgical Causes (90%)* Nonsurgical Causes (10%)











* Requirement for urgent, often surgical, therapy.


Notably, many causes of pneumoperitoneum do not require emergent therapy. The most common among these is postoperative pneumoperitoneum. Gas in the peritoneal cavity can be a normal finding up to 10 days after surgery. This can be confounding in a postoperative patient with acute abdominal pain or signs of infection. Postoperative peritoneal gas is typically seen in trace amounts after the first few days and should decrease in quantity on successive examinations. An increase in gas over time raises concern for complications.




Iatrogenic pneumoperitoneum can also occur when air is inadvertently injected into a peritoneal dialysis catheter. As with postoperative pneumoperitoneum, the presence of a dialysis catheter does not prove that gas in the peritoneal space is an insignificant finding in the patient with acute abdominal pain. Pneumoperitoneum is also occasionally seen when a percutaneous gastrostomy tube is present. Retrograde passage of air through the female genital tract has been reported in association with sexual activity and pelvic examination, as well as with activities such as horseback riding and water skiing.




INTRAMURAL GAS COLLECTIONS


Pneumatosis intestinalis describes gas in the wall of the alimentary tract. Because the gas is confined between the mucosal and serosal layers, pneumatosis does not move freely through the abdomen and is usually confined to one region of the intestinal tract. On plain film, pneumatosis appears as a combination of curvilinear and mottled gas, and can easily be confused with fecal material (Fig. 8-12). Although the curvilinear shape of the gas collections are better appreciated by CT, differentiation from fecal material can still be challenging. Because the mucosal and serosal layers can be thin when distended, pneumatosis can be difficult to detect when located adjacent to gas within the bowel lumen. However, when fluid is present adjacent to intramural gas, the gas is much more conspicuous.



Most of the many causes of intestinal pneumatosis are listed in Table 8-6. Pneumatosis is often seen in association with acute gastrointestinal disease processes such as bowel ischemia, bowel obstruction, and severe bowel infections. When seen in the presence of bowel ischemia, it is associated with reported mortality rates of 50% to 75%. However, pneumatosis is also associated with chronic disease processes such as obstructive pulmonary disease, Whipple disease, and scleroderma. Iatrogenic pneumatosis can result from endoscopy with or without biopsy, barium enema, or bowel surgery, particularly if a bowel anastomosis is performed. Occasionally, idiopathic pneumatosis is seen as an unsuspected finding in patients without acute abdominal symptoms. When pneumatosis is present, always look at the mesenteric vessels and liver for evidence of portal venous gas (Fig. 8-13).


Table 8-6 Causes of Intestinal Pneumatosis












Causes that Usually Causes that Usually
Require Urgent Therapy Do Not Require Therapy















As with the bowel, gas can also collect within the walls of other structures such as the gallbladder (emphysematous cholecystitis) and the urinary bladder (emphysematous cystitis). These entities are almost always infectious in nature and particularly common in individuals with diabetes. Gas within the vagina (vaginitis emphysematosa) is an uncommon but clinically benign entity characterized by gas-filled mucosal cysts. Although vaginitis emphysematosa can be associated with organisms such as Trichomonas vaginalis, it does not represent a life-threatening gas-producing infection.



OTHER GAS COLLECTIONS


A contained collection of extraluminal gas near bowel is a common sign of perforation with or without abscess formation. Abscesses contain infected material with variable proportions of fluid, debris, and gas (Fig. 8-14). Common causes of abscess include bowel perforation, superinfected postoperative fluid, hematogenous dissemination of bacteria, pyelonephritis, or anastomotic leak. Because the bowel also contains differing proportions of gas, fluid, and semisolid material, it is sometimes mistaken for an abscess. A key feature that distinguishes an abscess from bowel is discontinuity or eccentric location with respect to the alimentary tract. When an abscess is identified near a bowel loop without evidence of diverticula or inflammatory bowel disease, be certain to look for a foreign body such as a fish bone.



Gelatin bioabsorbable sponge (Gelfoam) and oxidized cellulose (Surgicel) used for intraoperative hemostasis can result in an appearance similar to abscess on CT. Clues to the presence of these materials on CT include a history of recent surgery, linear organization of small gas bubbles (best seen on bone windows), stable appearance of the gas on sequential examinations, and unusual shape of the collection. Because most postoperative abscesses contain at least some fluid, a predominantly mottled gas appearance and the absence of an air–fluid level favor hemostatic material (Fig. 8-15). This general rule does not work in reverse, however; because the material may be surrounded by hematoma, the presence of fluid does not necessarily indicate abscess. Note that this material can occasionally become superinfected, but because it is semisolid and typically in an area of recent hemorrhage, catheter drainage is not usually performed.



Hospitalized patients often have gas in the urinary bladder from bladder catheterization. However, gas within the urinary tract can also be a sign of urinary tract infection (UTI) or enterovesical fistula. Biliary gas is commonly present in patients who have had prior sphincterotomy, biliary stent placement, or choledochoenteric anastomosis, but it can signify acute cholangitis in an acutely ill patient with right upper quadrant pain.



Focal Fluid Collections


Under normal circumstances, fluid in the abdominal pelvic cavity is contained within the hollow viscera of the gastrointestinal, biliary, and urinary tracts. Women of reproductive age often have small amounts of “physiologic” fluid in the pelvis as a result of the rupture of follicular cysts. However, larger amounts of peritoneal fluid in women and more than trace amounts of peritoneal fluid in the male patient must be viewed with suspicion.


Fluid collecting adjacent to thickened or inflamed bowel often indicates bowel perforation with or without abscess formation. For example, although acute appendicitis and diverticulitis without perforation can result in inflammation of the adjacent fat, a nonlinear fluid collection near a thickened appendix or inflamed diverticulum increases the likelihood of perforation (Fig. 8-16).



A similar principle can be applied to perirenal collections. Urinary obstruction often results in stranding of the perinephric fat, appearing as thin linear and curvilinear areas of fluid attenuation within otherwise low-density fat. If the areas of fluid attenuation become wider or rounded in the setting of urinary obstruction, forniceal rupture should be suspected (Fig. 8-17).



The presence of fluid in the lesser sac presents a limited differential diagnosis, particularly if there is little or no fluid in the rest of the peritoneal cavity. The most likely causes of fluid in the lesser sac include processes that affect the pancreas, stomach or gastroesophageal junction (Fig. 8-18), duodenum, or spleen.



As mentioned earlier, the pelvis is the most common site for physiologic fluid to accumulate. In addition, peritoneal fluid from inflammatory or malignant disease processes originating anywhere within the peritoneal space often accumulates first within the pelvis because of its dependent location. Primary pelvic processes that result in focal pelvic fluid collections include pelvic inflammatory disease (PID), endometriosis, ovarian cancer, bladder injury, and pelvic surgery.


Extraperitoneal fluid is commonly identified in patients with acute abdominal pain. Knowledge of the compartments and fascial planes of the extraperitoneum provides a tool for identifying the causative factors of extraperitoneal fluid collections.


The anterior pararenal space contains the pancreas, the ascending and descending colon, and the extraperitoneal portion of the duodenum. Therefore, fluid in the anterior pararenal space is likely to originate from these structures. Although the anterior pararenal space is continuous across the midline, fluid collections from the duodenum and colon are often confined to the side of origin. Bilateral anterior pararenal fluid collections usually are of pancreatic origin (Fig. 8-19).





The perirenal space contains the kidneys and adrenal glands, and is divided into right and left because the anterior renal fascia fuses with the fascia surrounding the abdominal aorta and inferior vena cava. Despite this compartmentalization, fluid under pressure within the perirenal space can occasionally cross to the contralateral side (Fig. 8-20). The perirenal space extends caudally to the upper pelvis, where it communicates with the anterior pararenal space. Despite these potential avenues of communication, the majority of perirenal fluid collections remain confined to the perirenal space.



The posterior pararenal space contains no organs and is primarily filled with adipose tissue. Infection within this space can extend from the psoas compartment or result from penetrating trauma. Interestingly, idiopathic extraperitoneal hemorrhage is most frequent in the posterior pararenal space, left greater than right.


An additional potential space exists between the two layers of the posterior renal fascia, sometimes called the retrorenal plane. Fluid collections in this potential space most often arise from pancreatitis (Fig. 8-21) and are in continuity with the anterior pararenal space (see Fig. 6-33).

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Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Imaging Evaluation of Acute Abdominal Pain
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