Investigations
It is possible to obtain information about: (1) pancreatic damage by measuring levels of pancreatic enzymes in body fluids; (2) pancreatic function by measuring bicarbonate and enzymes produced in the pancreatic juice; and (3) morphological abnormality of the parenchyma and duct system by ultrasonography, computerised tomography (CT), magnetic resonance imaging (MRI) and endoscopic retrograde cholangiopancreatography (ERCP) (Table 55.3).
Estimation
of pancreatic enzymes in body fluids
When the pancreas is damaged, enzymes such as amylase, lipase, trypsin,
elastase and chymotrypsin are released into the serum. Measurement of amylase is
the most widely used test of pancreatic damage, and none of the other tests
shows better specificity or sensitivity. The serum amylase rises within a few
hours of pancreatic damage and declines over the next 4—8 days. A markedly
elevated serum level is highly suspicious of acute pancreatitis. Urinary amylase
and amylase—creatinine clearance ratios add little to diagnostic accuracy. If
confirmation of the diagnosis is required, CT of the pancreas is of greater
value.
Pancreatic
function tests
Pancreatic secretion in response to a standardised stimulus can provide
an assessment of the functional capacity of the gland. The tests can be divided
into those where the stimulus to secretion is indirect — produced by the
ingestion of a test meal — and those where secretion is directly stimulated by
injection of a hormone. Previously, duodenal intubation with the introduction of
a triple lumen tube was performed so that the gastric and duodenal juices could
be aspirated, and a nonabsorbable marker such as polyethylene glycol used to
assess the completeness of the aspiration. A meal stimulus can be used such as
in the Lundh test, or secretin and CCK can be administered intravenously to
produce both bicarbonate and enzyme stimulation. These tests have been largely
abandoned because chronic pancreatitis cannot be diagnosed until over 70 per cent of
the gland has been destroyed or there is obstruction to the duct. The more
practical test is the nitroblue tetrazolium—para-aminobenzoic acid (NBT PABA)
test in which the recovery in the urine of PABA is measured after oral
administration. The p-aminobenzoic acid is liberated by pancreatic chymotrypsin
and excreted in the urine after absorption and liver conjugation. In the
pancreolauryl test (PLT), the substrate is represented by fluorescein esterified
with two molecules of lauric acid. This complex is hydrolysed by a specific
pancreatic aryl-esterase and the fluorescein, liberated into the intestinal
lumen, absorbed by the intestinal mucosa and then excreted by the kidneys. This
test is cheap, is easy to perform and has a high reliability in recognising
severe pancreatic exocrine insufficiency. Careful interpretation of the test
results is required in postgastrectomy patients, in patients with extensive
intestinal inflammatory disease, or in patients with gastrointestinal or
hepatobiliary disease that markedly alters gastrointestinal transit and
intestinal absorptive capacity.
A simpler test now available is one which
measures faecal elastase levels. Absence indicates chronic pancreatitis and is
specific.
Imaging
investigations
Ultrasonography
In the hands of a skilled radiologist, ultrasonography can outline the
pancreas with accuracy. However, in patients who are fat and those with much gas
or fluid in the bowel, the images of the pancreas are poor and an accurate
diagnosis cannot be achieved. In the investigation of jaundice ultrasonography
remains the initial preferred investigation. Not only will ultrasound determine
whether or not the bile duct is dilated, but it will also define the presence or
absence of a mass in the pancreas and the coexistence of gallstones or gross
disease within the liver such as metastases (Fig. 55.6).
Computerised
tomography (Figs 55.7,
55.8, 55.9, 55.10,
55.11, 55.12)
Pancreatic carcinomas of 1—2 cm in size can usually be demonstrated
whether in the head, body or tail of the pancreas. A specific pancreatic
protocol should be followed using a spiral CT scanner. Following rapid injection
of intravenous contrast, scanning is performed at 5-mm intervals in the
arterial phase and 10-mm intervals during the venous phase. Prior to injection
of contrast an unenhanced CT scan is essential to determine the presence of
calcification within the pancreas and gall bladder. The stomach and duodenum
should be outlined with water and distended to define the duodenal loop. All
significant pathologies within the pancreas can be diagnosed on high-quality
spiral CT scans. Endocrine tumours which previously could not be imaged can now
be seen with clarity (Fig. 55.12) and this is now probably the preferred
technique for defining such tumours. CT scanning is also of value in the
therapeutic setting to drain cysts or abscesses, or to guide percutaneous
biopsies.
Magnetic
resonance imaging
With the recent developments in MRI the pancreas can now be clearly
displayed in a manner similar to the best CT image. In addition, clear images of
the bile duct and the pancreatic duct, together with fluid collections, can be
defined. Magnetic resonance pancreatography and cholangiography may well
replace diagnostic endoscopic pancreatography and cholangiography. MRI is
noninvasive and cheaper (Fig. 55.13). Using special contrast agents flow within
a duct can be delineated; using the technique in conjunction with secretin
emptying of the pancreatic duct can be demonstrated to show the absence or
presence of obstruction.
Endoscopic
retrograde cholangiopancreatography
By using this technique with a side-viewing fibreoptic duodenoscope the
ampulla of Vater can be clearly seen. Images of contrast injected into the
biliary and pancreatic ducts can display the anatomy and pathology of these
ducts. Changes seen in chronic pancreatitis include pancreatic duct strictures,
dilatation of the main pancreatic duct with stones (Figs 55.14
,55.15, 55.16, 55.17,
55.18 55.19), abnormalities of pancreatic duct side branches,
communication of the pancreatic duct with cysts and bile-duct strictures. In
pancreatic carcinoma, the main pancreatic duct may be narrowed or completely
obstructed at the site of the tumour with dilatation upstream but with a normal
duct system downstream. This, in conjunction with bile-duct obstruction or a
stricture, results in the so-called‘ double duct sign’. Collection of bile or pancreatic juice at
endoscopy and brushing of these ducts can yield cells which confirm the
suspected diagnosis of carcinoma (Fig. 55.20).
Plain
X-ray