Page 648 - Small Animal Internal Medicine, 6th Edition

620 PART IV Hepatobiliary and Exocrine Pancreatic Disorders

CHAPTER 37
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The Exocrine Pancreas

GENERAL CONSIDERATIONS which is necessary for vitamin B 12 absorption in the ileum,
is secreted only by the pancreas in the cat. In the dog the
The pancreas is located in the cranial abdomen, with the left pancreas is the main source of IF, but a small amount is also
limb positioned between the transverse colon and the greater secreted by the gastric mucosa.
curvature of the stomach, and the right limb running next Diseases of the exocrine pancreas are relatively common
to the proximal duodenum. Any or all of these neighboring but often misdiagnosed in dogs and cats because of nonspe-
structures can be affected when there is pancreatic inflam- cific clinical signs, relative difficulty in accessing the organ
mation. The exocrine acini make up about 90% of pancreatic for diagnostic imaging and biopsies, and lack of sensitive and
tissue, and the endocrine islets interspersed among the acini specific clinicopathologic tests. Pancreatitis is the most
make up the remaining 10% (Fig. 37.1). The close anatomic common disease of the exocrine pancreas in cats and dogs;
association between the acini and islets allows subtle signal- EPI, although less common, is also recognized frequently.
ing between them to coordinate digestion and metabolism, Uncommon diseases of the pancreas include pancreatic
but it also means that there is a complex cause and effect abscess, pseudocyst, and neoplasia.
relationship between diabetes mellitus (DM) and pancreati- Recent advances in the understanding of the pathophysi-
tis. The major function of the exocrine pancreas is to secrete ology, prevalence, and potential causes of pancreatitis in
digestive enzymes, bicarbonate, and intrinsic factor (IF) into dogs and cats may provide clues about treatment in the
the proximal duodenum. Pancreatic enzymes are responsible future, although treatment of acute pancreatitis remains
for the initial digestion of larger food molecules and require largely nonspecific and supportive in all species.
an alkaline pH to function—hence the concurrent bicarbon- Important differences in the anatomy of the pancreas and
ate secretion by pancreatic duct cells. The pancreas secretes associated areas between the dog and cat are outlined in
several proteases, phospholipases, ribonucleases, and deoxy- Table 37.1.
ribonucleases as inactive precursors (zymogens), and
α-amylase and lipase as intact molecules. The pancreas is the
only significant source of lipase, and hence steatorrhea (fatty PANCREATITIS
feces) is a prominent sign of exocrine pancreatic insuffi-
ciency (EPI). Trypsin is central to the pathogenesis of pan- Pancreatitis may be acute or chronic. As with acute and
creatitis, as discussed later, and inappropriate early activation chronic hepatitis, the difference is histologic and not neces-
of the zymogen trypsinogen to trypsin within the pancreatic sarily clinical (Table 37.2; Fig. 37.2), and there is often clini-
acini is the final common pathway triggering pancreatic cal overlap between the two. Chronic disease may present
inflammation. In the normal animal, pancreatic secretion is initially as an acute-on-chronic episode; in postmortem
triggered by the thought of food and stomach filling, and studies of fatal acute pancreatitis in dogs and cats, up to
most potently by the presence of fat and protein in the small half of the cases were actually acute-on-chronic disease.
intestinal lumen. The vagus nerve, local enteric nervous Differentiation of acute disease from an acute flare-up of
system, and hormones secretin and cholecystokinin from the chronic disease is not important for initial management,
small intestine stimulate pancreatic secretion. Trypsinogen which is the same in all cases, but is important to allow
is activated in the small intestine by the brush border enzyme recognition of the potential long-term sequelae of chronic
enterokinase, which cleaves a peptide (the trypsin activation disease (see later). The causes of acute and chronic pancre-
peptide [TAP]) from trypsinogen. Activated trypsin then atitis may be different, but there may also be some overlap
activates the other zymogens within the intestinal lumen. IF, between them.

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