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Interpreting a radiograph of the abdomen
Imaging
Interpreting a radiograph of the abdomen
Abnormal distribution of gas All gas shadows must be attributed to a hollow viscus; a gas shadow should be assumed to be abnormal if it cannot be attributed to a gas-containing structure. The most common cause of an abnormal gas shadow on the supine radiograph is free peritoneal gas (pneumoperitoneum, see below). Gas can also escape into the retroperitoneum (Figure 1), and the causes for this are listed in Table 1. Retroperitoneal gas can track superiorly into the mediastinum, and inferiorly into the buttock and thigh. Gas in the soft tissues of the left thigh is a classical site for gas from a diverticular perforation. An abnormal distribution of gas may also be found in the: • bowel wall (see below) • biliary tree (Figure 2; e.g. following sphincterotomy, stent insertion, gallstone ileus) • portal veins (portal gas from ischaemia of the small bowel) • kidneys (emphysematous pyelonephritis) • pancreas (infected necrosis or abscess) • gallbladder • urinary bladder.
Iain Morrison
Abstract The plain abdominal radiograph is commonly used in the investigation of the acute abdomen, and the admitting surgeon makes early decisions based on it. The standard films obtained are a supine abdominal and erect chest views. The relative merits of the erect abdominal film are discussed, since the presence of fluid levels does not necessarily indicate obstruction to the small bowel. The abnormalities seen on a supine abdominal X-ray are divided into broad categories as follows; abnormal distribution of gas – especially pneumoperitoneum, retroperitoneal gas, and gas in the bowel wall; dilatation of the bowel – namely stomach, small bowel and large bowel; abnormal bowel wall pattern – including ischaemia, infarction and inflammatory bowel disease; and some inflammatory conditions of the abdomen, including appendicitis.
Pneumoperitoneum The erect radiograph of the chest will usually show free peritoneal gas, but relies on correct radiographic technique. There are several conditions which mimic pneumoperitoneum on a chest radiograph, such as basal atelectasis, diaphragmatic humps, colon interposed between diaphragm and liver (Chilaiditi’s syndrome) and subdiaphragmatic fat. Therefore, one
Keywords abdomen; acute abdomen; radiograph; radiology; X-ray
The plain radiograph of the abdomen is commonly used in the investigation of the acute abdomen, and the admitting surgeon makes early decisions based on it. Correct interpretation is essential because neither a radiologist nor a senior surgeon may be available to offer advice. The supine radiograph of the abdomen is the standard film provided by the Radiology Department in the UK. It shows calcifications, the distribution of abdominal gas, the calibre and mucosal pattern of gas-filled bowel, and displacement of bowel loops by soft tissue masses or fluid. The erect radiograph of the chest will show a small amount of gas under the diaphragm; free gas is more difficult to detect on an erect radiograph of the abdomen. In the past, the erect radiograph of the abdomen was used to show fluid levels in order to diagnose obstruction of the small bowel. Fluid levels can be very misleading, and cannot be used to differentiate between obstruction and ileus. Multiple levels of fluid can be seen in some normal subjects, and the frequent false-positive and -negative findings have led to the withdrawal of the erect radiograph of the abdomen from routine use in most institutions in the UK.
Iain Morrison MBBS MFRCP FRCR is a Consultant Radiologist at Kent and Canterbury Hospital, UK. He qualified from St Thomas’ Hospital and trained in radiology at St Bartholomew’s Hospital, London. He has a special interest in abdominal radiology. Conflicts of interest: none declared.
SURGERY 26:6
Figure 1 Radiograph from an intravenous urogram performed for investigation of loin pain. Gas in the retroperitoneum due to perforated diverticulitis. a Gas shadows are in the layers of the abdominal wall on the left, and b gas is around and above the kidneys.
250
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Causes of retroperitoneal gas • • • • • • •
Signs of pneumoperitoneum on the supine radiograph of the abdomen
Perforation of a duodenal ulcer Diverticular perforation Following colonoscopic biopsy Following percutaneous biopsy Emphysematous pyelonephritis Infected complication of pancreatitis Retroperitoneal abscess
• Gas over the liver • Gas in Morrison’s pouch • Rigler’s (double-wall) sign • Gas outlining features of the anterior abdominal wall ○ Falciform ligament ○ Umbilical ligaments • Air triangles • Football or air dome sign
Table 1
Table 2
must know the signs of free gas on a supine radiograph of the abdomen.
Dilation of the bowel
Signs of pneumoperitoneum on the supine radiograph of the abdomen (Table 2): Rigler’s sign is produced by gas on both sides of the bowel wall, making the bowel wall very distinct because inner and outer surfaces are shown, and the bowel loops look ‘ghostlike’ (Figure 3). Gas may also outline structures on the inner surface of the abdominal wall (falciform ligament) and the medial and lateral umbilical ligaments; the latter produce an ‘inverted-V’ appearance. Perforations of the appendix and diverticulitis may not produce any signs on plain radiograph because they are frequently ‘walled-off’ by omentum in the iliac fossae.
Gastric dilation Gastric dilation may be due to mechanical obstruction (e.g. peptic stricture, carcinoma), ileus (acute gastric dilation, usually seen in the elderly or those with significant other abdominal comorbidity) or gastric volvulus. Usually, the enlarged stomach retains its shape, but can change to a spherical shape in volvulus (Figure 4). There is a danger of gangrene in gastric volvulus. The stomach can appear distended due to swallowing air or after intubation. Dilation of the small bowel In general, the small and large bowel can be distinguished on a plain radiograph. Dilated loops of small bowel tend to be multiple and lie centrally in the abdomen; there are fewer large bowel loops and they lie peripherally. Dilation of large bowel is clearly larger, but dilated small bowel (30–50 mm) may be difficult to distinguish from large bowel in some circumstances. The valvulae conniventes traverse the width of the bowel, but colonic haustra can also appear to do so depending on the orientation of the loop. Valvulae occur far more frequently per length of bowel compared to haustra, and they are thinner (Figure 5), but valvulae can appear thicker if the bowel is oedematous. Dilated large bowel often contains solid faeces, whereas small bowel does not.
Gas in the bowel wall (mural gas) Mural gas may be seen in any bowel viscus, and usually is a very ominous sign of bowel necrosis and impending perforation. The gas can track into the portal venous system. Gas may also be seen in emphysematous gastritis, emphysematous cholecystitis, emphysematous enterocolitis (in premature babies) and emphysematous cystitis. However, there are instances where the presence of gas is more benign; pneumatosis cystoides intestinalis is a rare condition of unknown cause that results in multiple cysts in the bowel wall (usually in the left hemicolon). The cysts can rupture, producing free peritoneal gas, but without peritonitis.
Figure 2 Aerobilia. Air is in branching structures in the liver (arrow). Only the first two or three generations of duct branches contain air. The air does not reach the periphery of the liver in the biliary tree (air in the portal venous system is carried to the peripheral vessels).
SURGERY 26:6
Figure 3 Supine radiograph of the abdomen showing Rigler’s sign (a sign of free peritoneal gas). a Gas is on both sides of the bowel wall. b The loops in the central abdomen have a ‘ghostlike’ appearance.
251
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Causes of obstruction of the small bowel that may be seen on an abdominal radiograph • Hernia (femoral or inguinal) • Volvulus • Gallstone ileus • Intussusception • Malrotation • Neoplasm • Appendix abscess Table 3
Chronic obstruction of the small bowel can cause massive dilation, mimicking dilation of the large bowel. The distinction between obstruction of the small bowel and ileus is difficult radiologically, and clinical signs often help. If there is obstruction of the small bowel, the calibre of the large bowel is often reduced, which is not usually the case with ileus. Ileus may consist of a few dilated loops local to a site of inflammation (e.g. in pancreatitis, appendicitis). Obstruction of the small bowel Obstructed small bowel may be filled predominantly with gas and the radiological signs are reasonably obvious; however, the loops may be partly gas-filled or only fluid-filled. The radiological
Figure 4 Gastric volvulus. The twisted stomach forms a large, rounded mass in the upper abdomen. Gas is in the gastric wall (arrow), indicating necrosis.
Figure 6 Obstruction of the large bowel due to carcinoma of the sigmoid colon. The gas-filled loops of colon are clearly seen (transverse colon and flexures, arrows), but the fluid-filled loops are not (ascending and descending colons). The level of obstruction is further distal than is apparent from the radiograph (sigmoid colon).
Figure 5 Obstruction of the small bowel due to adhesions. There are numerous, dilated loops of small bowel that are filled with gas. They are a arranged centrally in the abdomen, and b show valvulae conniventes.
SURGERY 26:6
252
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Figure 7 Ulcerative colitis with toxic megacolon. a The transverse and descending colons show thickening of the bowel wall. The transverse colon is dilated. b Irregular mucosal islands are present, but c the haustral pattern is lost (indicating inflammation affecting the entire thickness of the wall and the risk of perforation).
Figure 8 Caecal volvulus. The grossly enlarged loop is caecum (arrow), twisted on a long mesentery. Distal large bowel is not visible, and a competent iliocaecal valve ensures that there is no reflux of gas into the small bowel.
diagnosis becomes increasingly difficult the less gas is present. If there is only a small amount of gas, bubbles can lie between the valvulae conniventes, producing a ‘string of beads’ appearance which is highly suggestive of obstruction of the small bowel. The radiograph shows a ‘gasless abdomen’ if gas is absent in the dilated loops. This sign is suggestive of obstruction of the small bowel, but can also be seen in normal subjects. The loop may stand out obviously if there is a small bowel volvulus (or a strangulated loop). Occasionally, the strangulated loop is filled with fluid, appearing as a mass on the radiograph. Signs of strangulation of bowel loop are far more evident on CT. Occasionally, the cause of obstruction of the small bowel can also be determined on the plain radiograph (Table 3).
Toxic megacolon – in the setting of inflammatory bowel disease, colonic dilation indicates a fulminating course with full-thickness mural inflammation, including neuromuscular degeneration. Mucosal islands may be identified and there is a lack of haustral markings, a significant finding in the ascending and transverse colons (Figure 7). Pseudo-obstruction – there are numerous causes of colonic distension without obstruction, and it is not possible to distinguish these from true obstruction of the large bowel on the plain radiograph. In order to exclude pseudo-obstruction, suspected obstruction of the large bowel is confirmed by means of an unprepared enema, CT or by colonoscopy. Caecal volvulus – the caecum becomes enlarged, but can lie in the left upper quadrant, right lower quadrant or in the central abdomen (Figure 8). The key to distinguishing caecal volvulus from sigmoid volvulus lies in the distribution of gas in the other loops. Often, in caecal volvulus, there is dilation of the small bowel but the distal colon is empty. The more proximal obstructed loops of large bowel are also seen in sigmoid volvulus. Sigmoid volvulus (Figure 9) – it can be difficult to distinguish the enlarged sigmoid loops of distal mechanical obstruction from the loop of sigmoid volvulus. However, in sigmoid volvulus, the loop tends to be very enlarged, extending superiorly under the diaphragm to overlie the vertebra T10, to extend laterally to project lateral to the lateral flank fat stripe, and to lack haustral markings. Some authors say that a caecal volvulus can be distinguished from a sigmoid volvulus according to the
Dilation of the large bowel Obstruction of the large bowel leads to dilation of the bowel as far as the obstructing lesion (Figure 6). Occasionally, the dependent descending colon is entirely filled with fluid, so it does not appear on the plain radiograph. Therefore, the level of obstruction (as judged by gas on the plain radiograph) may be more proximal than the true level. Multiple loops of small bowel filled with gas will also be seen if the iliocaecal valve allows gas back into the small bowel, and distinguishing obstruction of the large bowel from ileus will be more difficult. The cause of obstruction of the large bowel cannot be identified on plain radiograph, except in large bowel volvulus, which can have characteristic appearances (see below).
SURGERY 26:6
253
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Signs of appendicitis seen on an abdominal radiograph • Appendicolith • Localized ileus of ileum and/or caecum • Blurring and widening of the properitoneal fat line • Scoliosis concave to right (especially children) • Obstructive small bowel pattern • Generalized ileus • Soft tissue mass indenting the caecum Table 4
Ischaemia of the large bowel is characteristically seen in the splenic flexure and descending colon, causing marked thickening of the mucosa due to intramural haemorrhage. The thickened folds can form rounded indentations into the luminal gas (‘thumbprinting’). The more proximal colon frequently dilates. A stricture is produced in the chronic phase, which may also be seen on the plain radiograph. Inflammatory colitis – acute colitis usually renders the affected segments of large bowel empty of faeces and filled with gas. The condition of the mucosa is indicated by the mucosal edge and thickness of the bowel wall, and the distribution of the active disease can be assessed. ‘Fuzzy’ mucosa and thickened haustra indicate active disease, and islands of mucosa may be seen. Loss of haustra on the ascending and transverse colons indicates that the full thickness of the bowel wall is involved, and there is a high risk of dilation and perforation (Figure 7). Daily radiographs of the abdomen may be needed in the acute setting in order to detect the development of these ominous signs.
Figure 9 Sigmoid volvulus. The dilated sigmoid loop is seen in the a right and b central abdomen. c Dilated splenic flexure is also seen and the d caecum is visible behind the very dilated loop in the right iliac fossa.
direction of the central fold, but this is not true and the sign can be very misleading.
Inflammatory conditions of the abdomen Abnormal pattern of the bowel wall
Appendicitis is a clinical diagnosis; suspected appendicitis is not an indication for an abdominal radiograph. Ultrasound or CT (if available) increasingly play a role in indeterminate cases. However, especially in older patients, the signs may be obscured, and a plain radiograph of the abdomen may form part of the investigation of abdominal pain. Thus, the signs of appendicitis on plain film (Table 4) should be known. Pancreatitis – numerous radiological signs have been described for pancreatitis but, in most patients, the plain radiograph is normal or almost normal, with maybe one or two mildly dilated loops of small bowel in the upper abdomen (indicating localized ileus). Diverticulitis – usually, plain radiographs of the abdomen are not helpful in the diagnosis of acute diverticulitis. A pericolic abscess must reach a great size before the soft-tissue mass or extra-luminal gas shadows become apparent. Perforation does not usually produce sufficient gas to be visible. Associated obstruction of the large bowel may be evident. ◆
The mucosal pattern on a plain radiograph is not as well shown as on a double-contrast barium examination, but the luminal gas (if present) profiles the mucosa, and the fat outside the bowel combines with the gas within to indicate the thickness of the bowel wall. Such indicators are very useful in diagnosing ischaemia of the small and large bowel and inflammatory bowel disease. Ischaemia and infarction of the small bowel is very serious, but the radiological findings are often non-specific, usually consisting of mildly dilated, gas-filled loops of small bowel. On careful examination, there may be more characteristic signs which can point to the diagnosis. The bowel loops may show mural thickening due to oedema and haemorrhage. As gangrene develops, gas bubbles or linear tracks may be seen within the wall, and gas may enter the portal venous system. Gas is carried into the liver with blood flow. Portal venous gas can be confused with gas in the biliary tree, but portal venous gas fills more peripheral vessels than biliary gas. Signs of perforation of the small bowel may be seen.
SURGERY 26:6
254
© 2008 Elsevier Ltd. All rights reserved.
Interpreting a radiograph of the abdomen
Abnormal distribution of gas All gas shadows must be attributed to a hollow viscus; a gas shadow should be assumed to be abnormal if it cannot be attributed to a gas-containing structure. The most common cause of an abnormal gas shadow on the supine radiograph is free peritoneal gas (pneumoperitoneum, see below). Gas can also escape into the retroperitoneum (Figure 1), and the causes for this are listed in Table 1. Retroperitoneal gas can track superiorly into the mediastinum, and inferiorly into the buttock and thigh. Gas in the soft tissues of the left thigh is a classical site for gas from a diverticular perforation. An abnormal distribution of gas may also be found in the: • bowel wall (see below) • biliary tree (Figure 2; e.g. following sphincterotomy, stent insertion, gallstone ileus) • portal veins (portal gas from ischaemia of the small bowel) • kidneys (emphysematous pyelonephritis) • pancreas (infected necrosis or abscess) • gallbladder • urinary bladder.
Iain Morrison
Abstract The plain abdominal radiograph is commonly used in the investigation of the acute abdomen, and the admitting surgeon makes early decisions based on it. The standard films obtained are a supine abdominal and erect chest views. The relative merits of the erect abdominal film are discussed, since the presence of fluid levels does not necessarily indicate obstruction to the small bowel. The abnormalities seen on a supine abdominal X-ray are divided into broad categories as follows; abnormal distribution of gas – especially pneumoperitoneum, retroperitoneal gas, and gas in the bowel wall; dilatation of the bowel – namely stomach, small bowel and large bowel; abnormal bowel wall pattern – including ischaemia, infarction and inflammatory bowel disease; and some inflammatory conditions of the abdomen, including appendicitis.
Pneumoperitoneum The erect radiograph of the chest will usually show free peritoneal gas, but relies on correct radiographic technique. There are several conditions which mimic pneumoperitoneum on a chest radiograph, such as basal atelectasis, diaphragmatic humps, colon interposed between diaphragm and liver (Chilaiditi’s syndrome) and subdiaphragmatic fat. Therefore, one
Keywords abdomen; acute abdomen; radiograph; radiology; X-ray
The plain radiograph of the abdomen is commonly used in the investigation of the acute abdomen, and the admitting surgeon makes early decisions based on it. Correct interpretation is essential because neither a radiologist nor a senior surgeon may be available to offer advice. The supine radiograph of the abdomen is the standard film provided by the Radiology Department in the UK. It shows calcifications, the distribution of abdominal gas, the calibre and mucosal pattern of gas-filled bowel, and displacement of bowel loops by soft tissue masses or fluid. The erect radiograph of the chest will show a small amount of gas under the diaphragm; free gas is more difficult to detect on an erect radiograph of the abdomen. In the past, the erect radiograph of the abdomen was used to show fluid levels in order to diagnose obstruction of the small bowel. Fluid levels can be very misleading, and cannot be used to differentiate between obstruction and ileus. Multiple levels of fluid can be seen in some normal subjects, and the frequent false-positive and -negative findings have led to the withdrawal of the erect radiograph of the abdomen from routine use in most institutions in the UK.
Iain Morrison MBBS MFRCP FRCR is a Consultant Radiologist at Kent and Canterbury Hospital, UK. He qualified from St Thomas’ Hospital and trained in radiology at St Bartholomew’s Hospital, London. He has a special interest in abdominal radiology. Conflicts of interest: none declared.
SURGERY 26:6
Figure 1 Radiograph from an intravenous urogram performed for investigation of loin pain. Gas in the retroperitoneum due to perforated diverticulitis. a Gas shadows are in the layers of the abdominal wall on the left, and b gas is around and above the kidneys.
250
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Causes of retroperitoneal gas • • • • • • •
Signs of pneumoperitoneum on the supine radiograph of the abdomen
Perforation of a duodenal ulcer Diverticular perforation Following colonoscopic biopsy Following percutaneous biopsy Emphysematous pyelonephritis Infected complication of pancreatitis Retroperitoneal abscess
• Gas over the liver • Gas in Morrison’s pouch • Rigler’s (double-wall) sign • Gas outlining features of the anterior abdominal wall ○ Falciform ligament ○ Umbilical ligaments • Air triangles • Football or air dome sign
Table 1
Table 2
must know the signs of free gas on a supine radiograph of the abdomen.
Dilation of the bowel
Signs of pneumoperitoneum on the supine radiograph of the abdomen (Table 2): Rigler’s sign is produced by gas on both sides of the bowel wall, making the bowel wall very distinct because inner and outer surfaces are shown, and the bowel loops look ‘ghostlike’ (Figure 3). Gas may also outline structures on the inner surface of the abdominal wall (falciform ligament) and the medial and lateral umbilical ligaments; the latter produce an ‘inverted-V’ appearance. Perforations of the appendix and diverticulitis may not produce any signs on plain radiograph because they are frequently ‘walled-off’ by omentum in the iliac fossae.
Gastric dilation Gastric dilation may be due to mechanical obstruction (e.g. peptic stricture, carcinoma), ileus (acute gastric dilation, usually seen in the elderly or those with significant other abdominal comorbidity) or gastric volvulus. Usually, the enlarged stomach retains its shape, but can change to a spherical shape in volvulus (Figure 4). There is a danger of gangrene in gastric volvulus. The stomach can appear distended due to swallowing air or after intubation. Dilation of the small bowel In general, the small and large bowel can be distinguished on a plain radiograph. Dilated loops of small bowel tend to be multiple and lie centrally in the abdomen; there are fewer large bowel loops and they lie peripherally. Dilation of large bowel is clearly larger, but dilated small bowel (30–50 mm) may be difficult to distinguish from large bowel in some circumstances. The valvulae conniventes traverse the width of the bowel, but colonic haustra can also appear to do so depending on the orientation of the loop. Valvulae occur far more frequently per length of bowel compared to haustra, and they are thinner (Figure 5), but valvulae can appear thicker if the bowel is oedematous. Dilated large bowel often contains solid faeces, whereas small bowel does not.
Gas in the bowel wall (mural gas) Mural gas may be seen in any bowel viscus, and usually is a very ominous sign of bowel necrosis and impending perforation. The gas can track into the portal venous system. Gas may also be seen in emphysematous gastritis, emphysematous cholecystitis, emphysematous enterocolitis (in premature babies) and emphysematous cystitis. However, there are instances where the presence of gas is more benign; pneumatosis cystoides intestinalis is a rare condition of unknown cause that results in multiple cysts in the bowel wall (usually in the left hemicolon). The cysts can rupture, producing free peritoneal gas, but without peritonitis.
Figure 2 Aerobilia. Air is in branching structures in the liver (arrow). Only the first two or three generations of duct branches contain air. The air does not reach the periphery of the liver in the biliary tree (air in the portal venous system is carried to the peripheral vessels).
SURGERY 26:6
Figure 3 Supine radiograph of the abdomen showing Rigler’s sign (a sign of free peritoneal gas). a Gas is on both sides of the bowel wall. b The loops in the central abdomen have a ‘ghostlike’ appearance.
251
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Causes of obstruction of the small bowel that may be seen on an abdominal radiograph • Hernia (femoral or inguinal) • Volvulus • Gallstone ileus • Intussusception • Malrotation • Neoplasm • Appendix abscess Table 3
Chronic obstruction of the small bowel can cause massive dilation, mimicking dilation of the large bowel. The distinction between obstruction of the small bowel and ileus is difficult radiologically, and clinical signs often help. If there is obstruction of the small bowel, the calibre of the large bowel is often reduced, which is not usually the case with ileus. Ileus may consist of a few dilated loops local to a site of inflammation (e.g. in pancreatitis, appendicitis). Obstruction of the small bowel Obstructed small bowel may be filled predominantly with gas and the radiological signs are reasonably obvious; however, the loops may be partly gas-filled or only fluid-filled. The radiological
Figure 4 Gastric volvulus. The twisted stomach forms a large, rounded mass in the upper abdomen. Gas is in the gastric wall (arrow), indicating necrosis.
Figure 6 Obstruction of the large bowel due to carcinoma of the sigmoid colon. The gas-filled loops of colon are clearly seen (transverse colon and flexures, arrows), but the fluid-filled loops are not (ascending and descending colons). The level of obstruction is further distal than is apparent from the radiograph (sigmoid colon).
Figure 5 Obstruction of the small bowel due to adhesions. There are numerous, dilated loops of small bowel that are filled with gas. They are a arranged centrally in the abdomen, and b show valvulae conniventes.
SURGERY 26:6
252
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Figure 7 Ulcerative colitis with toxic megacolon. a The transverse and descending colons show thickening of the bowel wall. The transverse colon is dilated. b Irregular mucosal islands are present, but c the haustral pattern is lost (indicating inflammation affecting the entire thickness of the wall and the risk of perforation).
Figure 8 Caecal volvulus. The grossly enlarged loop is caecum (arrow), twisted on a long mesentery. Distal large bowel is not visible, and a competent iliocaecal valve ensures that there is no reflux of gas into the small bowel.
diagnosis becomes increasingly difficult the less gas is present. If there is only a small amount of gas, bubbles can lie between the valvulae conniventes, producing a ‘string of beads’ appearance which is highly suggestive of obstruction of the small bowel. The radiograph shows a ‘gasless abdomen’ if gas is absent in the dilated loops. This sign is suggestive of obstruction of the small bowel, but can also be seen in normal subjects. The loop may stand out obviously if there is a small bowel volvulus (or a strangulated loop). Occasionally, the strangulated loop is filled with fluid, appearing as a mass on the radiograph. Signs of strangulation of bowel loop are far more evident on CT. Occasionally, the cause of obstruction of the small bowel can also be determined on the plain radiograph (Table 3).
Toxic megacolon – in the setting of inflammatory bowel disease, colonic dilation indicates a fulminating course with full-thickness mural inflammation, including neuromuscular degeneration. Mucosal islands may be identified and there is a lack of haustral markings, a significant finding in the ascending and transverse colons (Figure 7). Pseudo-obstruction – there are numerous causes of colonic distension without obstruction, and it is not possible to distinguish these from true obstruction of the large bowel on the plain radiograph. In order to exclude pseudo-obstruction, suspected obstruction of the large bowel is confirmed by means of an unprepared enema, CT or by colonoscopy. Caecal volvulus – the caecum becomes enlarged, but can lie in the left upper quadrant, right lower quadrant or in the central abdomen (Figure 8). The key to distinguishing caecal volvulus from sigmoid volvulus lies in the distribution of gas in the other loops. Often, in caecal volvulus, there is dilation of the small bowel but the distal colon is empty. The more proximal obstructed loops of large bowel are also seen in sigmoid volvulus. Sigmoid volvulus (Figure 9) – it can be difficult to distinguish the enlarged sigmoid loops of distal mechanical obstruction from the loop of sigmoid volvulus. However, in sigmoid volvulus, the loop tends to be very enlarged, extending superiorly under the diaphragm to overlie the vertebra T10, to extend laterally to project lateral to the lateral flank fat stripe, and to lack haustral markings. Some authors say that a caecal volvulus can be distinguished from a sigmoid volvulus according to the
Dilation of the large bowel Obstruction of the large bowel leads to dilation of the bowel as far as the obstructing lesion (Figure 6). Occasionally, the dependent descending colon is entirely filled with fluid, so it does not appear on the plain radiograph. Therefore, the level of obstruction (as judged by gas on the plain radiograph) may be more proximal than the true level. Multiple loops of small bowel filled with gas will also be seen if the iliocaecal valve allows gas back into the small bowel, and distinguishing obstruction of the large bowel from ileus will be more difficult. The cause of obstruction of the large bowel cannot be identified on plain radiograph, except in large bowel volvulus, which can have characteristic appearances (see below).
SURGERY 26:6
253
© 2008 Elsevier Ltd. All rights reserved.
Imaging
Signs of appendicitis seen on an abdominal radiograph • Appendicolith • Localized ileus of ileum and/or caecum • Blurring and widening of the properitoneal fat line • Scoliosis concave to right (especially children) • Obstructive small bowel pattern • Generalized ileus • Soft tissue mass indenting the caecum Table 4
Ischaemia of the large bowel is characteristically seen in the splenic flexure and descending colon, causing marked thickening of the mucosa due to intramural haemorrhage. The thickened folds can form rounded indentations into the luminal gas (‘thumbprinting’). The more proximal colon frequently dilates. A stricture is produced in the chronic phase, which may also be seen on the plain radiograph. Inflammatory colitis – acute colitis usually renders the affected segments of large bowel empty of faeces and filled with gas. The condition of the mucosa is indicated by the mucosal edge and thickness of the bowel wall, and the distribution of the active disease can be assessed. ‘Fuzzy’ mucosa and thickened haustra indicate active disease, and islands of mucosa may be seen. Loss of haustra on the ascending and transverse colons indicates that the full thickness of the bowel wall is involved, and there is a high risk of dilation and perforation (Figure 7). Daily radiographs of the abdomen may be needed in the acute setting in order to detect the development of these ominous signs.
Figure 9 Sigmoid volvulus. The dilated sigmoid loop is seen in the a right and b central abdomen. c Dilated splenic flexure is also seen and the d caecum is visible behind the very dilated loop in the right iliac fossa.
direction of the central fold, but this is not true and the sign can be very misleading.
Inflammatory conditions of the abdomen Abnormal pattern of the bowel wall
Appendicitis is a clinical diagnosis; suspected appendicitis is not an indication for an abdominal radiograph. Ultrasound or CT (if available) increasingly play a role in indeterminate cases. However, especially in older patients, the signs may be obscured, and a plain radiograph of the abdomen may form part of the investigation of abdominal pain. Thus, the signs of appendicitis on plain film (Table 4) should be known. Pancreatitis – numerous radiological signs have been described for pancreatitis but, in most patients, the plain radiograph is normal or almost normal, with maybe one or two mildly dilated loops of small bowel in the upper abdomen (indicating localized ileus). Diverticulitis – usually, plain radiographs of the abdomen are not helpful in the diagnosis of acute diverticulitis. A pericolic abscess must reach a great size before the soft-tissue mass or extra-luminal gas shadows become apparent. Perforation does not usually produce sufficient gas to be visible. Associated obstruction of the large bowel may be evident. ◆
The mucosal pattern on a plain radiograph is not as well shown as on a double-contrast barium examination, but the luminal gas (if present) profiles the mucosa, and the fat outside the bowel combines with the gas within to indicate the thickness of the bowel wall. Such indicators are very useful in diagnosing ischaemia of the small and large bowel and inflammatory bowel disease. Ischaemia and infarction of the small bowel is very serious, but the radiological findings are often non-specific, usually consisting of mildly dilated, gas-filled loops of small bowel. On careful examination, there may be more characteristic signs which can point to the diagnosis. The bowel loops may show mural thickening due to oedema and haemorrhage. As gangrene develops, gas bubbles or linear tracks may be seen within the wall, and gas may enter the portal venous system. Gas is carried into the liver with blood flow. Portal venous gas can be confused with gas in the biliary tree, but portal venous gas fills more peripheral vessels than biliary gas. Signs of perforation of the small bowel may be seen.
SURGERY 26:6
254
© 2008 Elsevier Ltd. All rights reserved.