Week 17: GI part 3/3: Small Intestines on Down

CT scan of the abdomen has rapidly become the imaging modality of choice in diagnosing small bowel obstruction (SBO) as it is both highly sensitive and highly specific. Patients with SBO present with crampy abdominal pain with nausea, vomiting, constipation and abdominal distension. The pain can be generalized and often will come in waves. The rapidity of onset of symptoms depends on how proximal the obstruction is. More proximal obstructions will have a more rapid onset and peak of symptoms. CT scan of the abdomen and pelvis is the diagnostic modality of choice as it has a high sensitivity and specificity and can give additional information including location of obstruction, cause of obstruction and alternative diagnoses.

The acute onset of abdominal pain with minimal tenderness on examination in an elderly patient is concerning for mesenteric ischemia. Patients with ischemia may develop rectal bleeding either with frank blood or hemoccult positive stool. As ischemic time increases, the lactic acid may become elevated. CT angiography is the gold standard for diagnosis. This is a disease of the elderly with a median age of 70. The differential diagnosis of acute severe abdominal pain also includes rupture or leak of an abdominal aortic aneurysm and in this case a 2 cm aorta (normal) was identified. Mesenteric ischemia most commonly involves the small bowel and right colon. Due to the presence of a larger supply of collateral flow, the left colon is rarely involved. The mesenteric vessels include the celiac trunk, superior mesenteric artery and inferior mesenteric artery. The intestines require a constant supply of blood and typically receive 20% of the cardiac output. As a result, injury occurs rapidly in the setting of acute ischemia. More than 50% of cases of mesenteric ischemia result from arterial embolization, most commonly from a cardiac origin (atrial, ventricular or valvular on the left side). Patients with atrial fibrillation are at high risk of developing mesenteric ischemia. The superior mesenteric artery (SMA) is at highest risk because of its large caliber and small angle of takeoff from the aorta. Once the embolus lodges in the SMA the gut becomes ischemic. Jejunum is at highest risk because it receives little collateral flow from the celiac trunk and inferior mesenteric artery.

The presentation of multiple individuals with diarrhea following a shared meal is highly suspicious for foodborne illness. Foodborne pathogens cause illness through a variety of mechanisms. Pathogens like Staph aureus, Bacillus cereus, and Clostridium botulinum (bolulism) exert their toxic effects via a preformed toxin. Toxins present in the food prior to ingestion cause rapid onset of symptoms, typically within 1-6 hours. Bacillus cereus is associated with watery diarrhea and lower abdominal cramping. Foods associated with Bacillus cereus include meats, gravies, stews, and vanilla sauces. Another mechanism by which pathogens cause symptoms is via bacterial production of toxin after ingestion, such as seen withVibrio, Shigella, and Shiga-toxin-producing E. Coli. Some toxins alter fluid and electrolyte movements, causing large volumes of fluid that overwhelms the colon’s ability to absorb them, leading to diarrhea. Other toxins directly disrupt cell membranes leading to cell death. Many pathogens, including enteric viruses, Salmonella,Campylobacter and Shigella directly invade the intestinal epithelium, leading to fever, abdominal cramping, and diarrhea which is sometimes bloody. For acute nonbloody diarrhea without systemic signs of illness, serum and stools studies are not typically required, and patients can be managed supportively.

Epiploic appendagitis is caused by inflammation of the pouches of serosal fat that line the large intestine called epiploic appendages. This is a rare cause of abdominal pain, but its presentation mimics appendicitis and diverticulitis and therefore is sometimes found on CT scan performed in the evaluation of lower abdominal pain. More than half of cases occur in the rectosigmoid colon. Epiploic appendagitis is an inflammatory condition likely caused by autoinfarction of the appendages from torsion. Rarely, colonic bacteria may accumulate in the area causing abscess formation. Most patients present with pain localized to the region of appendagitis. Pain is classically dull and non-radiating. Patients may develop fever and show leukocytosis on laboratory analysis. The condition is most often self-limited and treatment is with pain medication like nonsteroidal anti-inflammatories (NSAIDs).

Sensitivity or the true positive rate measures the proportion of actual positives that are correctly identified as such. It is determined by dividing the number of true positives of the test by the number of true positives + false negatives. Tests with a high sensitivity are good for ruling out disease as the test has very few false negatives. A test with high sensitivity is advantageous as a screening tool as it misses very few people with the disease. The onset of abdominal pain before vomiting has been found to be as high as 100% sensitive in diagnosing acute appendicitis.

The X-ray demonstrates massive dilation of the colon through the cecum consistent with a large bowel obstruction. However, CT scan does not show any obstructing lesion and therefore this is a pseudo-obstruction, also known as Ogilvie’s syndrome. The exact mechanism of the development of pseudo-obstruction is unknown but suspected to result from malfunction of the autonomic control of the bowel. Patients often have multiple other co-morbid conditions and risk factors include nursing home residence, anticholinergic medication, severe electrolyte disturbance, narcotic exposure, or a history of spine or retroperitoneal trauma. This is a diagnosis of exclusion. Patients may first be treated with a rectal tube and sigmoidoscopy and managed conservatively in the hospital. Neostigmine may be used as a pharmacologic intervention as its an acetylcholinesterase inhibitor.

Ischemic colitis is a syndrome caused by inadequate blood flow through the mesenteric vessels, resulting in ischemia and eventual gangrene of the bowel wall. The most important finding is pain that is disproportionate to physical examination findings. The pain is diffuse, and the patient may report frank blood in the stool. These patients typically have a history of atherosclerotic disease at other sites, such as coronary artery disease or cerebrovascular disease. Other risk factors include advanced age and tobacco use. Classic angiography is the diagnostic imaging of choice, however it is invasive and therefore in current clinical practice, computed tomography angiography is ordered much more frequently than classic angiography. Bowel wall edema is the most common finding on imaging. All cases of ischemic colitis with signs of peritonitis or possible bowel infarction, generally warrant immediate surgical intervention for the resection of the ischemic or necrotic bowel. If the ischemia is caused by vasospasm then surgery is not indicated and medical management with anticoagulants and intra-arterial vasodilators is appropriate.

Acute appendicitis is the most common surgical emergency in pregnant women. The rate of acute appendicitis in pregnant patients is the same as seen in nonpregnant patients. Unfortunately, delays in diagnosis lead to an increased rate of perforation, increased morbidity and mortality. Physiologic changes during pregnancy make diagnosis difficult. As the uterus grows, the abdominal organs are more distant from the abdominal wall making it difficult to detect tenderness and peritoneal signs. Additionally, the location of the appendix later in pregnancy is variable as it can rest anywhere between the right lower quadrant to deep within the right upper quadrant. Ultrasound, CT scan and MRI are all viable diagnostic tests depending on availability. However, on occasion, patients will require exploratory laparoscopy.

Octreotide is a useful addition to endoscopic sclerotherapy and decreases rebleeding occurrences. Patients with documented esophageal varices and acute upper GI bleeding should be treated with an intravenous infusion of octreotide at 50 µg/hr for a minimum of 24 hours while being observed in the intensive care unit.

Hives and bronchospasm in this patient point to excessive histamine release. In the setting of diarrhea, histamine release would most likely result from a parasitic infection. This is not common; however, it should be considered in patients presenting with diarrhea and histamine-induced skin changes. Of the choices listed, Giardia lamblia is the only parasite.