Assessment of Pancreatic Exocrine Function
More than 90% of pancreatic exocrine function needs to be destroyed before symptomatic malabsorption results [see 4:V Diseases of the Pancreas].5 The most sensitive test of pancreatic exocrine function requires the passage of a double-lumen tube.6 Cholecystokinin (CCK) or secretin is administered intravenously, and gastric and duodenal secretions are collected separately. However, secretin became unavailable in the United States when the manufacturer discontinued production in 1999. If CCK is given, lipase or trypsin activity is determined using appropriate substrates. When secretin is administered, duodenal fluid volume and bicarbonate concentration are measured.
The noninvasive bentiromide test is based on the action of trypsin on bentiromide to yield p-aminobenzoic acid (PABA) and benzoyl-tyrosine [see Figure 1]. PABA is readily absorbed by the intestine and excreted into the urine. In healthy persons, when 500 mg of bentiromide is ingested, 57% or more of the PABA appears in the urine within 6 hours. In patients with chronic pancreatitis, the amount of PABA excreted is significantly less, averaging 42%. Using the 57% excretion as a cutoff, the sensitivity of the bentiromide test is 67% to 80% and the specificity is 95%.7 PABA may also be measured in the plasma 120 minutes after ingestion of bentiromide, which may enhance the sensitivity of the test.8 Plasma measurements are helpful in cases of impaired renal excretion, which may be seen in the elderly. PABA is identified col-orimetrically, and thus, other arylamines can interfere with its determination (e.g., acetaminophen, lidocaine, procainamide, sulfonamides, and thiazide diuretics).9 In cases in which intestinal absorption is impaired, such as with sprue, the absorption of released PABA may be reduced, leading to a falsely low urinary recovery. Unfortunately, the bentiromide test becomes positive only when the pancreatic gland is more than 90% destroyed. Nevertheless, in considering the workup of a patient with steator-rhea, the test may be useful because it takes an equal amount of glandular destruction to generate steatorrhea.
Although the vast majority of pancreatic proteases and lipases are stored in zymogen granules and are released from the apical portion of the pancreatic exocrine cell into the pancreatic duct, a small percentage leaks into the interstitium of the gland, is carried into the circulation, and can be measured by the serum trypsinogen assay. Because the activation peptide of trypsin is not yet released and any active trypsin is quickly bound by a1-antitrypsin, the free-circulating form of trypsin is trypsinogen. In patients who have chronic pancreatitis with exocrine insufficiency, the serum concentration of trypsinogen is lower than that in healthy persons (2 to 18 ng/ml, compared with 29 to 79 ng/ml in healthy persons).10 A low serum trypsinogen level appears to have a high degree of specificity for chronic pancreatitis but is only modestly sensitive.
Bile Acid Absorption Tests
Bile acids are synthesized from cholesterol in the liver and require conjugation by either glycine or taurine before they are excreted into the intestine via the common bile duct. The bile acid conjugates solubilize the products of triacylglycerol hydrolysis into complex micelles, which facilitate the rapid absorption of dietary lipid. Bile acids are not absorbed in the proximal intestine with dietary lipid but in the distal ileum. The bile acid pool recirculates six times a day. About 95% of bile acids are reabsorbed and recirculate in the enterohepatic circulation each day; approximately 0.5 g of bile acids appears in the stool daily, which equals the hepatic synthetic rate under steady-state conditions. If bile acids are not adequately absorbed, diarrhea results (choleretic enteropathy). In the complete absence of bile salts, fatty acids are less efficiently absorbed, with up to 25% to 50% of ingested lipid appearing in the stool. In patients with idiopathic diarrhea or with diarrhea after ileal resection (> 30 cm), the malabsorption of bile acids is an etiologic possibility. Also, children who have unexplained diarrhea may have a congenital defect of the sodium-dependent bile salt transporter in the terminal ileum.
To test for the presence of bile acid malabsorption, two methods are available, although not widely used. The first is the 14C-glycocholic acid breath test, and the second is the selenium-75-labeled homocholic acid-taurine (75SeHCAT) absorption test. In the former test [see Figure 2], a trace amount of 14C-glycocholic acid is given by mouth. Many bacteria are capable of hydrolyz-ing the amide bond and releasing the 14C-glycine; either it is absorbed and 14CO2 is produced in the liver or it is further metabolized in the intestinal lumen to 14CO2. In either event, the 14CO2 appears in the breath in measurable amounts. The percentage of the ingested dose excreted in the breath increases if the intestinal lumen contains more bacteria than normal or if an excess of bile acids is delivered to the colon (ileal dysfunction). A gastric anti-secretory drug may also increase the resident population of bacteria in the intestine to a level that results in an abnormal breath test.12 The usefulness of this test as an indicator of bile acid malabsorption is therefore limited. The 75SeHCAT test has more potential clinical usefulness because of its strong correlation with cholate excretion and the ease of measurement of 75Se retention by the whole-body gamma camera. Normal persons retain greater than 19% of an orally administered dose of 75Se after 7 days, whereas patients with significant ileal dysfunction or resection retain less than 12%.13
Figure 2 In the bile acid breath test, a small dose of 14C-glycocholic acid is ingested and its fate determined by measurement of 14CO2 excretion in breath. In a normal person, little of the 14C-glycocholic acid is metabolized for excretion in breath because it passes intact to the ileum for absorption and return to the enterohepatic circulation. If there is either intestinal bacterial overgrowth or ileal dysfunction, however, bacterial enzymes will deconjugate the bile acid (broken blue lines), releasing cholic acid and 14C-glycine. The radioactive glycine may be transported across the intestinal mucosa (upper broken gray line) and subsequently degraded to 14CO2 by tissue enzymes; alternatively, the C-glycine may be metabolized within the intestinal lumen to CO2, which then diffuses (lower broken gray line) into the circulation and is carried to the lungs. Consequently, 14CO2 excretion is 10 times greater in either intestinal bacterial overgrowth or ileal dysfunction than it is in the normal state.
Now that the human sodium-dependent bile acid transporter has been cloned, congenital defects are being discovered that lead to bile acid malabsorption resulting in diarrhea.11 Such defects may be the cause of primary bile acid malabsorption.
Diseases Producing Malabsorption
Gluten-sensitive enteropathy
Gluten-sensitive enteropathy (GSE) was once called celiac disease in children and idiopathic or nontropical sprue in adults. In 1960, it was recognized that the diseases were the same, caused by the major wheat protein gluten and, more specifically, its alcohol-soluble component, gliadin.
GSE is generally considered less common in the United States than in Western Europe. However, a recent large multicenter study indicated that the prevalence of GSE in the United States in symptomatic patients (1 in 56) and in not-at-risk persons (1 in 133) is similar to that reported in Europe.
Genetic and Etiologic Factors
GSE is associated with haplotypes HLA-DQ2 (DQA1*501, DQB1* 201) and HLA-DQ8 (DQA1*031, DQB1*302). In sets of monozygotic twins, 30% of the opposite twins from the incidence case do not have GSE. This condition leads to the belief that there is another, unknown (nongenetic) factor that is important in disease causation. A 33-amino-acid peptide part of gliadin has been shown to be poorly digested by proteases and to cause T cell activation in GSE patients.
Pathogenesis
GSE is associated with impaired cholecystokinin release. CCK cells are either reduced in number or so defective that the amount of CCK present in the duodenal mucosa is greatly re-duced.14 This CCK deficiency leads to a reduced amount of pancreatic lipase and bile acids delivered to the intestinal lumen in response to dietary lipid. The intestinal crypt cells are the major fluid-secreting cells of the intestine, via their cAMP-dependent Cl secretion with attendant water secretion. In GSE, the cryptal portion of the villous complex is greatly expanded, leading to increased water secretion. Because the villous tip cells, which normally absorb the water, are diseased and reduced in number, water and electrolyte absorption is not as effective as normal, and the intestine becomes secretory.14 Thus, the concentration of bile acids in the intestinal lumen is reduced below that expected simply from the impaired CCK release. The ability of bile acids to solubilize the products of lipolysis depends on the presence of bile salts at a concentration greater than their critical micellar concentration (CMC) of 1.4 mM.17 Normally, the intestine has a postprandial bile salt concentration of 10 mM.18 The brush borders at the surface of mature enterocytes are severely affected in GSE. Further, the villous structures are flattened. These two conditions lead to a severely reduced surface area that limits lipid absorption. The amount of reduction in surface area can be estimated by the D-xylose absorption test. The enterocytes that are at the surface of the intestine are not as mature as normal entero-cytes, because their turnover rate is greatly increased, which probably results in a reduced capacity to process absorbed lipid.
Diagnosis
Clinical manifestations Although GSE may start in childhood and respond to gluten withdrawal, children with the disease undergo a remission in their teenage years even if they ingest a diet containing gluten. As adults, these patients, 25% or more of whom were symptomatic in childhood, may present with a variety of complaints; usually, weight loss, fatigue, abdominal cramps, distention, bloating, and diarrhea (steatorrhea) are prominent, although there may be no loss of appetite. In some patients, the disease is insidious in onset and the symptoms are mild. It is only after these patients have been treated that they realize, in retrospect, how ill they were. In population studies in which the presence of disease was determined by intestinal biopsy, people with a biopsy consistent with GSE were often asymptomatic but sometimes of shorter stature than unaffected siblings. Because nothing specifically leads to the diagnosis, especially in the absence of clinically evident steatorrhea, the realization that the patient has GSE may be delayed. This problem is most likely to occur with patients who do not have stea-torrhea but do have osteoporosis, easy bruising as a result of vitamin K deficiency, or unexplained iron deficiency anemia. Iron deficiency anemia is common in adult patients with GSE,19 and one study suggested that fractures may be the only sign of mal-absorption in patients with undetected GSE.
Laboratory tests In a patient in whom the suspicion of GSE is high (e.g., a first-degree relative of a known GSE patient; a patient who has a history of a childhood disease that caused diarrhea, was evaluated by a specialist, and was treated with a special diet; or a patient with malabsorption who is not an alcoholic and does not have another obvious reason for malabsorbing fat), a positive tissue transglutaminase antibody test makes the diagnosis almost certain [see Figure 3].21 Alternatively, the diagnosis might rest on small bowel biopsy findings [see Figure 4]. Classic features include partial or complete villous atrophy, abnormal-appearing enterocytes at the villous tips, an increase in intraep-ithelial lymphocytes, a lamina propria infiltrate consisting predominantly of lymphocytes and macrophages, an increase in the size of the crypts both vertically and horizontally, and an increase in the number of mitotic figures.14 These features, although typical, are not pathognomonic. For the diagnosis to be definitive, the patient must respond to dietary therapy. Symptomatic improvement can be expected in 80% of patients within 1 month, but histologic improvement lags behind considerably. Another 10% of patients do not respond until after 2 months, and the remainder may take up to 2 years. Even under the strictest dietary control, the biopsy findings might not return to normal. Most often, the patient remains under good control because of the symptomatic improvement while on the diet, but many patients will eventually either test whether they are cured or be in a situation that forces them to commit a dietary indiscretion. This lapse inevitably results in recurrence of symptoms, further securing the diagnosis.
Another helpful test is the identification of an antiendomysial antibody. This antibody is present in up to 95% of cases and is rarely present in control subjects.22 Other tests of malabsorption, such as the D-xylose absorption test or stool fat studies, may be abnormal. Low clotting factors, the presence of anemia caused by folate or iron deficiency, or osteoporosis may also be present. None of these conditions are specific for GSE, however.
Treatment
The treatment of GSE is a strict gluten-free diet. Because a gluten-free diet is a lifelong commitment, is more expensive than a normal diet, and may carry social limitations, it should not be recommended until the diagnosis of GSE is firmly established. A gluten-free diet prohibits the intake of wheat, rye, and barley. Oats are thought to be safe but are usually avoided during the early stage, when the clinical response to the diet is being judged. Keeping the patient on the diet is sometimes difficult because many foods have hidden gluten content. Maintaining a gluten-free diet is important because intestinal lymphomas are more likely to develop in patients who do not follow such a diet.23
Support groups, such as those organized by the Celiac Society of America, can be helpful, especially when the disease is newly diagnosed. Information such as what to look for on package labels and interesting recipes can be very instrumental in helping the patient maintain the gluten-free diet. During the trial period, beer, ale, and whiskey, which may contain enough gluten to sensitize the patient, should not be consumed. After the dietary response is clear, these drinks may be tried, if desired, to determine whether the patient is sensitive. Other products that are not usually thought of as containing gluten, but often do, are ice cream, communion wafers, and even some drugs (as a filler). Despite the restrictions, many dietary options are open to the patient, including certain breakfast cereals, milk, cheese, eggs, meat, chicken, fish, chocolate, and products made from corn, rice, or potato flour.
Figure 3 Procedure for the diagnosis of gluten-sensitive enteropathy (GSE).
Figure 4 Biopsy specimen from the small intestine of a patient with untreated celiac sprue (a) demonstrates a flat surface with plasmocytic infiltration of the subepithelial region (magnification: 400x). In contrast, a biopsy sample taken from a patient with pancreatic exocrine insufficiency (b) is indistinguishable from a normal specimen and shows tall, scalloped villi and minimal subepithelial mononuclear infiltration (magnification: 100x).
If the patient does not respond, the most likely reason is that the patient is not accurately following the diet. In such cases, it is helpful to have a dietitian carefully go over the patient’s dietary history.14 Less often, the patient will have an intestinal stasis syndrome or pancreatic insufficiency. When these subsidiary problems are diagnosed and successfully treated, the patient usually shows a response to the diet. However, a minority of patients who do not respond to a gluten-free diet have what is called refractory sprue.24 Patients with refractory sprue may require treatment with corticosteroids and other immunosuppressants, including azathioprine.25
GSE-Associated Disorders
Dermatitis herpetiformis Many patients with dermatitis herpetiformis have GSE.21 The intensely pruritic, blistering lesions appear on the knees, elbows, shoulders, and buttocks [see 2:I Cutaneous Manifestations of Systemic Diseases]. Skin biopsies of dermatitis herpetiformis lesions have characteristic immuno-globulin A (IgA) deposits. On a gluten-exclusion diet, both the dermatologic and the intestinal lesions improve, indicating a linkage between the two. However, the skin lesions respond to dapsone treatment and the intestinal lesions do not, which indicates that there are differences between the two diseases as well.
Type 1 diabetes mellitus Patients with type 1 diabetes melli-tus are at increased risk of developing GSE. With the use of antiendomysial antibodies as a screen, gluten-sensitive enteropa-thy has been found in three of 47 diabetic patients (6%), a much higher incidence than would be expected by chance.26 Even when patients with type 1 diabetes mellitus show no apparent signs of malabsorption, they are at risk for developing celiac disease; screening for celiac disease in these patients therefore may be ad-visable.27 One study reported a 5.7% prevalence of celiac sprue in patients with type 1 diabetes and noted a finding of increased autoimmune diseases in these patients.28 The prevalence of autoimmune disorders in patients with celiac disease seems related to duration of gluten exposure, which provides further rationale for the early diagnosis and treatment of GSE.
Other spruelike disorders
Tropical Sprue
Tropical sprue is a malabsorptive illness that appears in certain areas of the world, especially the tropics, in both the indigenous populations and tourists. In two carefully studied populations, 5% to 13% of North Americans living in Puerto Rico for 6 months or longer experienced symptoms of tropical sprue. Expatriates from the United States who return from the tropics or other areas endemic for tropical sprue may experience symptoms of tropical sprue more than 10 years after their return.30 Peace Corps volunteers from the United States who spent time in Pakistan had demonstrable small bowel lesions and functional abnormalities that reverted to normal over several months after returning home.31 Indians and Pakistanis living in the United States may take a longer time (up to 4 years) to excrete normal amounts of D-xylose.32 Exactly what causes these changes in the small bowel is not clear, but the tropical sprue syndrome is thought to be caused by one or more species of coliform bacteria, such as Klebsiella species,33 which colonize the upper intestinal tract.
Diagnosis The symptoms of tropical sprue differ from those of GSE. Weight loss caused mostly by anorexia is very prominent, as is diarrhea. A sore tongue (70% of patients), pedal edema (25%), folate and vitamin B12 deficiency (75% to 100%), or an abnormal result on the Schilling test (96% to 100%) is much more common in tropical sprue than in nontropical sprue.33 The symptoms can be quite severe, sometimes leading to death in endemic areas. However, the prognosis, in general, is excellent for patients either remaining in the tropics or returning to the United States.
The diagnosis of tropical sprue is made by performing a small bowel biopsy in patients with a compatible clinical presentation and travel history. Villi are leaflike or blunt, and the lamina pro-pria are packed with inflammatory cells [see Figure 5]. Thin villous structures are seen in North Americans and Europeans [see Figure 4]. In considering this disease, it should be noted that intestinal biopsy results in residents of endemic areas or in tourists who do not stay in mainstream hotels in endemic areas would be classified as abnormal in persons living in the United States or Europe.
Treatment Treatment of tropical sprue should begin with folic acid (5 mg/day).33 This therapy is associated with rapid improvement in appetite, and it eliminates most of the clinical symptoms. In patients with a short duration of symptoms (< 4 months), folate given for 6 months to 1 year may suffice. For patients with a longer duration of symptoms (> 4 months), antibiotics, such as tetracycline (2 g/day for 1 year), should be added. Most patients returning to the United States gain weight quickly even if the results of absorption tests or intestinal biopsies are not normalized.
Collagenous Sprue
Collagenous sprue is a rare, devastating disease in which there is a layer of collagen underneath the enterocytes of the small bowel. The relation of collagenous sprue to collagenous colitis is unclear, but the basic histologic feature of subepithelial collagen deposition is the same. The origin of collagenous colitis is unknown, but it develops in approximately half the patients who have refractory celiac disease (those unresponsive to the gluten-exclusion diet).34 Although it is known that type 6 collagen is deposited in the more commonly diagnosed collagenous colitis, the type of collagen laid down in the small bowel in collagenous sprue is unknown. In collagenous colitis, the symptoms (primarily diarrhea) are usually modest, but in collagenous sprue, symptoms are more severe and include obvious malabsorption. This severity of symptoms is probably caused by the diffusion barrier presented by the collagen, which prevents nutrients from diffusing either into the portal capillaries or into the lymphatics.
Diagnosis The diagnosis of collagenous sprue is made by the classic histologic picture of villous atrophy and subepithelial collagen deposition. If the diagnosis is missed, however, and the patient is thought to have GSE on the basis of the flat villous structure, the patient will usually not be responsive to the gluten-free diet.
Treatment Therapy for collagenous sprue is uncertain. The most common problem is the osmotic diarrhea caused by the gross malabsorptive state induced by the disease process. In this event, the patient is treated as if he or she had the short bowel syndrome. Some patients respond to steroid therapy. A few respond to steroids and a gluten-exclusion diet, with the patient’s improved condition eventually making it possible to taper the steroid dosage.
Figure 5 In a biopsy specimen from the small intestine of a patient with tropical sprue, villi are broadened and shortened and the crypts are deepened; these changes yield a villus-to-crypt ratio of 1:1 (magnification: 100x).
