Patients with Crohn’s disease (CD) develop several complications, including penetrating disease during the long-term follow-up period. Chronic intestinal inflammation that occurs in CD can lead to the development of intestinal complications including strictures, fistulas, and abscesses.¹ Each complication is associated with other complications; perilesional abscess can often develop at the fistula site during the course of CD. Patients with CD who develop intra-abdominal abscess (IAA) tend to suffer from recurrent IAA during the follow-up period. The American College of Gastroenterology (ACG) recommends that objective assessment using endoscopic or cross-sectional imaging be performed periodically to minimize errors of under- or over-treatment because the symptoms of CD do not correlate properly with the presence of active inflammation.¹ Computed tomography (CT) and magnetic resonance imaging (MRI), which are representative cross-sectional imaging modalities, play a key role in the evaluation of disease severity and complications, such as IAA, bowel perforation, stricture, fistula, and hydronephroureterosis,^1–3 whereas ileocolonoscopy is a crucial examination method for the diagnosis and follow-up evaluation of CD. Pelvic MRI has a main role in the investigation of perianal fistulas and abscesses.^4,5 Thus, we reviewed the various CD-associated complications that can be revealed by radiologic examinations.

IAAs may result from the perforation of a hollow viscus into the peritoneum and from gastrointestinal fistulas. IAA is a common complication in patients with CD, occurring in up to 10% of these patients.^6,7 The American College of Radiology suggests that if an IAA is suspected, cross-sectional imaging (i.e., CT) of the abdomen and pelvis should be performed.¹ In CT images, IAAs show a rim-enhancing mass with perilesional infiltration adjacent to CD-associated active inflammation (Fig. 1–4). Asymmetric mural hyperenhancement, especially on the mesenteric side of the small bowel, is a specific CT finding of active inflammation in CD.² An IAA is sometimes mistaken for a normal small bowel and thus can be missed in the radiologist’s reading of contrast-enhanced CT images. IAAs tend to occur adjacent to the CD-associated severely active inflammatory small bowel or other CD lesions, such as fistulizing lesions. The most common site of IAA is the right side of the abdomen, especially around the terminal ileum.⁸ It was found that 26% to 46% of patients who had previous IAAs developed recurrent abscesses during the course of CD.^9–11 Thus, radiologists need to carefully to evaluate a patient who has fistulizing CD or had previous abscess, regardless of the presence of abscess in CT images. Radiologists must also take note of abscess-muscle fistulas or enterocutaneous fistulas in CT images, which can delay healing of the abscess or recurrent abscess (Fig. 5, 6). Patients who develop IAAs can be treated medically with antibiotics only, with antibiotics plus percutaneous drainage, or with antibiotics plus surgery.¹ Currently, the first-line treatment of IAAs is antibiotic therapy with or without percutaneous drainage.¹² The European Crohn’s and Colitis Organisation (ECCO) and the European Society of Colo-Proctology (ESCP) consensus recommend that IAAs smaller than 5 cm in size be treated with antibiotics only and that large abscesses (> 5 cm) be treated by percutaneous drainage with antibiotic therapy and follow-up by imaging.¹² Percutaneous drainage may avoid surgery, be less invasive, and preserve bowel length to minimize the risk of short bowel syndrome.^13,14

Figure 1. A 3-cm intra-abdominal abscess in a 38-year-old male patient with Crohn’s disease. Postcontrast-enhanced axial (A) and coronal (B) computed tomography images show fluid and air collection with rim enhancement in the right lower quadrant mesentery, consistent with an intra-abdominal abscess (white arrows). Multifocal segmental mural hyperenhancement (white arrowheads) and increased mesenteric vascularity (“comb sign”) with enlarged mesenteric lymph nodes (black arrow) are noted in the involved segment of the distal and terminal ileum. These findings are suggestive of active Crohn’s disease.

Figure 2. Several intra-abdominal abscesses in a 45-year-old male patient with Crohn’s disease. Postcontrast-enhanced axial (A) and coronal (B) computed tomography images show an intra-abdominal abscess (white arrows) in the right lower quadrant mesentery. There is another small abscess (black arrow) with peripheral enhancement on the right side of the larger abscess. Note that segmental mural hyperenhancement of the inner layer of the involved distal and terminal ileum (white arrowheads). These findings are suggestive of intra-abdominal abscess and adjacent active inflammatory Crohn’s disease.

Figure 3. A 3.6-cm intra-abdominal abscess in a 29-year-old male patient with Crohn’s disease. Postcontrast-enhanced axial (A) and coronal (B) computed tomography images show an air-containing abscess (white arrows) in the right lower mesentery. The coronal image exhibits severe wall thickening of the right proximal colon and deformity of the ileocecal valve (black arrow). A small amount of fluid (white arrowhead) is noted in the right paracolic gutter, consistent with localized peritonitis due to active Crohn’s disease with penetrating disease.

Figure 4. A 5-cm intra-abdominal abscess in a 22-year-old male patient with Crohn’s disease. Postcontrast-enhanced axial (A) and coronal (B) computed tomography images show fluid and air-containing abscesses (arrows) with rim enhancement in the adjacent pelvic loop of ileum. Coronal image demonstrates mild wall thickening and asymmetric mural hyperenhancement (arrowhead) along the mesenteric border of the pelvic ileal loop, consistent with active inflammatory Crohn’s disease.

Figure 5. Intra-abdominal abscess-muscle fistula and musculocutaneous fistula in a 21-year-old male patient with Crohn’s disease. (A) Postcontrast-enhanced axial computed tomography image shows a fistula between the intra-abdominal abscess and right iliopsoas muscle (white arrow). The patient underwent percutaneous drainage treatment. (B, C) Axial T2-weighted magnetic resonance images with fat saturation after 2 months. The residual abscess is noted in the right lower abdomen (white arrowhead). A tram track-like structure crossing from the peritoneal cavity to the anterior abdominal wall (subcutaneous fat layer and transversus abdominis) can be seen, consistent with musculocutaneous fistula (black arrows).

Figure 6. Enterocutaneous fistula in a 22-year-old male patient with Crohn’s disease, (A, B) Postcontrast-enhanced axial and coronal computed tomography images show a percutaneous drain catheter (arrows) inserted for the treatment of an intra-abdominal abscess. The catheter is inserted through the right rectus abdominis muscle. (C, D) Postcontrast-enhanced axial and coronal computed tomography images after 3 months of percutaneous drainage treatment. An enterocutaneous fistula (arrowheads) developed at the previous drain catheter insertion site.

Fistulas occur in approximately 30% of patients with CD, with the perianal area as the most common location. Enteroenteric, enterocolic, enterovesical, and rectovaginal fistulas can also develop in patients with CD.

Perianal fistulas occur in up to one-fourth of patients with CD.¹ These fistulas are categorized as either simple (mainly in the anal sphincter area with a single tract, located distal to the dentate line) or complex (transsphincteric, suprasphincteric, or intersphincteric; may have multiple fistula tracts).¹ Patients with symptomatic perianal fistulas usually undergo rectal MRI, which reveals the extent and complications of the fistula. In previous studies, endoscopic ultrasound and MRI examinations showed comparable accuracy for the diagnosis of perianal fistulas.^15,16 When a patient undergoes an MRI examination, it is important to cover the full range of the perianal fistula. Because the external opening of the perianal fistula is often seen at the upper thigh level, the scan range of the pelvic MRI should include the upper thigh level and extend more caudally than usual rectal MRI to avoid incomplete coverage of the lesion. The radiologist can sometimes encounter an invisible external opening of the perianal fistula in MRIs due to incomplete scan range coverage. In such cases, the patient may undergo repetitive MRI to evaluate the end of the fistula tract. Fat suppression T2-weighted and contrast-enhanced fat suppression T1-weighted images (WIs) are especially useful for evaluating the extent of the fistula tract. Contrast-enhanced MRI and T2WI can be helpful in differentiating perianal fistulas with active inflammation from perianal fistulas with fibrotic change.^17,18 Perianal fistulas with active inflammation generally appear as hyperintense tracts on T2WI (Fig. 7, 8), whereas fistula with fibrosis can be seen as a dark signal intensity on T2WI.¹⁹ Although inactive, healing fistulas may appear as loss of gadolinium enhancement with a linear tract on contrast-enhanced T1WI,¹⁹ and lesions with fibrosis can demonstrate progressive enhancement of variable degrees depending on the amount of fibrosis.¹⁸ Rectovaginal fistulas occur in approximately 9% of female with CD with anal involvement.²⁰ Perianal fistulas are considered complex when the lesion has a tract along the perineum and external opening at the vagina (Fig. 7, 8). When radiologists review perianal fistulas in pelvic MRIs, they may describe the number of fistulas and all internal and external openings with complexity.

Figure 7. Complex-type intersphincteric perianal fistula in a 21-year-old female patient with perianal Crohn’s disease. (A, B) Axial T2-weighted fat-suppressed magnetic resonance images show an intersphincteric perianal fistula in the left posterior aspect of the anus (white arrows). (C, D) The white arrowhead indicates the superior aspect of the fistula where the internal opening connects to the left aspect (3’o clock, white arrowhead) of the rectum, and the black arrow indicates the inferior aspect of the fistula where the external opening connects to the posterior aspect (6’o clock) of the perineum.

Figure 8. Complex-type intersphincteric perianal fistulas in a 24-year-old female patient with perianal Crohn’s disease. (A) Axial T2-weighted magnetic resonance image (MRI) without fat saturation shows internal openings of two intersphincteric fistulas in the 11 o’clock and 2 o’clock directions (white arrows). (B) Coronal T2-weighted MRI without fat saturation shows the inferior extension of fistulas into the level of the lower rectum (white arrowheads). (C) Axial T2-weighted MRI without fat saturation shows the external opening of the left-sided fistula connected to the perineum (black arrow). (D) Axial diffusion-weighted image and (E) apparent diffusion coefficient map also show two fistulous tracts on both sides (black arrowheads).

Enteroenteric and enterocolic fistulas, which are one of internal fistulas, especially those connected between bowel segments, rarely require therapy and are often asymptomatic. CT examination is helpful in evaluating the presence and extent of internal fistulas (Fig. 9). The aggravation of asymmetric inflammation at the mesenteric side with sacculations along the antimesenteric side may result in enteroenteric and enterocolic fistulas. However, patients with internal fistulas often develop IAAs that affect treatment options, such as anti-tumor necrosis factor (anti-TNF) agents.⁹ Because the anti-TNF agentthat is used to treat severely active CD leads to apoptosis of macrophages, natural killer cells, and T-lymphocytes and reduces chronic pathologic inflammation, it may increase the risk of infections.²¹ As a result, anti-TNF agents can induce delayed healing when the patient has an abscess or other infections. IAA can sometimes be underestimated in contrast-enhanced CT images because the lesion can be mistaken for the adjacent bowel segment, which has a similar enhancement, thus inducing a delayed diagnosis of the abscess. Therefore, when enteroenteric or enterocolic fistulas present in CD, it is important to assess for adjacent bowel segments, which is commonly related to active inflammatory changes or other complications, such as abscess.

Figure 9. Enteroenteric fistulas in a 31-year-old male patient with Crohn’s disease. Postcontrast-enhanced axial (A) and coronal (B) computed tomography images show mural hyperenhancement of the distal ileal loop and right colon. These bowel loops are tethered and angulated to form asterisk-shaped ileocolic and ileoileal fistula complexes (arrows).

An enterovesical fistula can demonstrate recurrent urinary frequency and recurrent cystitis regardless of proper treatment (Fig. 10). It develops in 2%–8% of patients with CD.^22,23 The right bladder dome is the predominant site of CD-related enterovesical fistulas, which is the site of the bladder closest to the terminal ileum.²⁴ Superior elongation of the bladder with connection to the ileum is a common imaging finding of enterovesical fistula-associated CD. Sagittal and coronal CT images are more useful than axial CT scans for detecting enterovesical fistulas. Because the uterus and vagina protect the bladder from inflammation extension, enterovesical fistulas occur three times more frequently in male than in female.

Figure 10. Internal fistulas in a 33-year-old male patient with Crohn’s disease. (A) Postcontrast-enhanced coronal computed tomography image shows tethered distal ileal loop with mural hyperenhancement, forming an enterocolic fistula (white arrows). (B) There are tiny air bubbles in the urinary bladder (black arrow) and fistulous tract between the distal ileum and urinary bladder, consistent with an enterovesical fistula (white arrowheads).

The most common indication for surgical resection of the bowel in CD is intestinal obstruction due to a fibrostenotic stricture.¹ The most common area of de novo strictures is the ileum and ileocolonic region, probably due to the ileum having a diameter smaller than that of the colon.^25,26 In one patient, CT imaging revealed long segmental bowel dilatation with abrupt luminal narrowing manifesting as a transitional zone showing severe active CD involvement; the patient also had bowel obstruction symptoms (Fig. 11, 12). The National Cooperative CD Study in the USA described at least one small bowel stricture in 25% and at least single colonic stricture in 10% of patients with CD.²⁷ The postoperative recurrence of CD at the anastomosis site commonly occurs after intestinal resection for the complication of a stricture, particularly in patients with ileocolonic anastomosis.^28,29

Figure 11. Small bowel obstruction due to inflammatory stricture in a 21-year-old male patient with Crohn’s disease. (A) Postcontrast-enhanced coronal computed tomography image shows dilated small bowel loop with small bowel feces sign (white arrow). (B) The mesenteric border of the pelvic ileal loop is mildly thickened and shows asymmetrical mural hyperenhancement (white arrowheads). (C) Upstream small bowel loops are markedly distended (black arrow). (A–C) These findings suggest small bowel obstruction at the pelvic ileal loop owing to stricture associated with active inflammatory Crohn’s disease.

Figure 12. Active inflammation with stricturing Crohn’s disease and small bowel obstruction in a 17-year-old male patient. Postcontrast-enhanced coronal computed tomography image shows segmental bowel wall thickening and mural enhancement in the proximal jejunum (white arrow) (A), distal jejunum (black arrow) (B), and ileum (black arrowhead) (C), findings consistent with active inflammatory Crohn’s disease. The upstream proximal jejunum is markedly dilated (white arrowheads), consistent with small bowel obstruction due to stricturing Crohn’s disease.

Ureteral stricture with obstructive uropathy can develop after inflammation and fibrosis in the retroperitoneum. A common site for the development of ureteral strictures in patients with CD is the ureteropelvic junction at the sacroiliac joint level, which may necessitate urological intervention.³⁰ Stent placement (usually as a transient double-J catheter³⁰) and transient percutaneous nephrostomy³¹ are helpful in patients until proper surgical or medical treatment can be performed.³² Percutaneous nephrostomy can prevent renal damage before surgery, and double-J catheter placement can restore regular ureteral flow and minimize the risk of ureteral damage during the surgery.^30,31 The associated inflammatory penetrating behavior of terminal ileal involvement, regardless of urolithiasis, predominantly occurs on the right side.³³ The incidence rate of ureteral stricture-related hydronephrosis is approximately 6% in those with CD.²⁴ The radiologist may consider inflammation of CD extension into the right ureter when the patient present with right hydronephroureterosis without urolithiasis and adjacent bowel inflammation in CT images (Fig. 13, 14).

Figure 13. Hydronephroureterosis in a 24-year-old male patient with Crohn’s disease. (A, B) Postcontrast-enhanced coronal computed tomography images show intra-abdominal abscess (black arrows; A), distal ileal wall thickening with symmetric bowel wall hyperenhancement (black arrows; B), and extended inflammation in the right ureter (white arrowheads; B) abdomen due to Crohn’s disease. This small bowel inflammation caused intra-abdominal abscess and ureteral obstruction in the right middle ureter (white arrowheads) and right hydronephrosis (white arrows).

Figure 14. Hydronephroureterosis in a 35-year-old male patient with Crohn’s disease. (A, B) Postcontrast-enhanced coronal computed tomography images show segmental wall thickening at pelvic ileal loop and terminal ileum with slightly asymmetric bowel wall hyperenhancement and pelvic inflammation (black arrows), causing right ureteral stenosis, upstream right ureteral dilatation (white arrowheads), and right hydroneprhosis (white arrows), findings consistent with active inflammatory Crohn’s disease with hydronephroureterosis. (C) The patient underwent percutaneous nephrostomy and ureteral stent insertion for the stricture.

Appendiceal CD is sometimes misdiagnosed as primary acute appendicitis in radiologic CT reports. A patient with active CD involving the appendix presents with clinical symptoms of acute appendicitis or periappendiceal abscess.^34–36 However, CT images of appendiceal CD (i.e., CD involving the appendix) show wall thickening of the appendix with periappendiceal inflammation combined with mural hyperenhancement of the adjacent other bowel and most of the cecum showing severe ileocolonic inflammation.³⁵ Mural hyperenhancement of the cecum is different from secondary changes of the cecum due to primary acute appendicitis in CT images, which reveals mainly submucosal edema in the cecum. Therefore, these image features can be useful in distinguishing CD with appendiceal involvement from primary acute appendicitis. In particular, when a less experienced radiologist is likely to misdiagnose acute appendicitis or periappendiceal abscess in CT images in patients with appendiceal CD, patients who have not been diagnosed with CD (i.e. without prior history of CD) are clinically suspected of having acute appendicitis. When radiologists suspect acute appendicitis, they may determine whether the lesion is primary appendicitis or due to other disease involvement (Fig. 15). Because CT interpretation may affect the surgical plan (e.g., laparoscopic appendectomy, ileocecectomy, or delayed operation with percutaneous catheter drainage insertion), appropriate interpretation is needed for accurate patient diagnosis and treatment.

Figure 15. Intra-abdominal abscess around appendix mimicking perforated appendicitis in a 28-year-old male patient with Crohn’s disease. (A, B) Postcontrast-enhanced coronal computed tomography images show intra-abdominal abscess (white arrows) and segmental mural hyperenhancement of the right colon (white arrowhead) and distal ileum (black arrowhead), suggesting severe active inflammation of Crohn’s disease and penetrating disease. During the initial presentation, the patient was misdiagnosed with acute appendicitis and periappendiceal abscess with secondary bowel change. However, the patient had persistent abdominal pain and anemia after the appendectomy. After the computed tomography image was reviewed again, the patient was diagnosed with active Crohn’s disease involvement with penetrating disease. (C) The patient underwent percutaneous drainage due to recurred abscess. Tubography shows fistula formation (black arrows) between the complicated fluid collection and terminal ileum.

None.

No potential conflict of interest relevant to this article was reported.