Dr. Johnson is in the Department of Radiology, Brooke Army Medical Center, Fort Sam Houston, TX.

A ccurate and timely radiographic evaluation can be crucial for preventing the severe complications of small bowel volvulus. The upper gastrointestinal series is the usual method of detecting midgut volvulus associated with malrotation. ^1-4 Although the exact incidence is unknown, however, a small but significant subset of patients can have either focal or extensive small bowel volvulus without malrotation. In the absence of an enteric tube, bowel distension, or scoliosis, ³ the duodenum and ligament of Treitz (LOT) are positioned normally. Therefore, a positive contrast examination to the level of the LOT may miss this condition. Moreover, since small bowel volvulus distal to a normal duodenum and LOT may not be anticipated, distal gas patterns that would ordinarily suggest volvulus may be minimized or dismissed as spurious, so that vascular complications develop as the patient is evaluated by other modalities.

With these pitfalls in mind, this article summarizes the typical radiographic features of small bowel volvulus originating distal to the LOT in patients found at surgery to have normal rotation of the gastrointestinal tract and normal mesenteric development.

Plain film evaluation

Although midgut volvulus with malrotation can produce normal plain film findings, ⁵ this collection without malrotation does not include any normal plain radiographs. The specific positive findings correlate with the content (gas or fluid) and extent of the volvulus, and are similar to the alterations when malrotation is a predisposition.

A gas-filled volvulus should be suspected when a small bowel loop assumes a "coffee-bean" configuration ⁶ (figure 1A). When correlated with direct inspection, the coffee bean represents either the tightly folded bowel loop associated with focal volvulus or a closed loop at the distal end of small bowel spiraling around the superior mesenteric artery (figure 2). The depicted loop is twisted focally in both situations. The "lip" in the center of the bean can be traced to an incisura, which represents the site of the twist. ⁶

In infants, this loop interrupts the uniformly distributed circular and tubular gas-filled bowel loops normally seen on supine films. Focal distension (figure 1 A) and/or isolation from other small bowel loops suggest that this configuration is significant, rather than a normal variant. Retention of gas on sequential supine and prone radiographs excludes simple obstruction and distension of transverse or sigmoid colon by elevation as alternative causes of focally distended bowel during infancy. ⁷

The pseudotumor associated with a fluid-filled volvulus is distinguished easily from intussusception, normal variations in the gas pattern, an uncomplicated fluid filled duplication, and medical conditions that alter the gas pattern. Medical causes of bowel distension in infants, such as infection and milk-soy protein intolerance, ^7,8 typically produce uniform bowel distension. Interruption of uniform distension by a mass effect implies an acute abdomen with obstruction--most commonly from intussusception ⁷ or small bowel vovulus. ⁶ An intraluminal mass and an age over 3 months suggests intussusception. A soft tissue mass associated with small bowel obstruction in a younger baby implies small bowel volvulus producing a pseudotumor sign of Frimann-Dahl ⁶ (figure 3). Although an uncomplicated small bowel duplication can resemble a pseudotumor associated with volvulus, this lesion does not typically produce the complete small bowel obstruction seen in figure 3B. Simulation of the pseudotumor and small bowel obstruction by focal inflation of a tortuous transverse colon on the supine film (a normal variant in neonates and infants) can be excluded by displacing gas into the right and left colon in the prone position ⁷ (figure 4).

Positive contrast studies

We recommend adding a small bowel series to a normal upper gastrointestinal series when the above gas patterns suggest a volvulus distal to a normal duodenum and LOT (figures 3C to 3E). After filling with positive contrast (figure 3E), a pseudotumor also has a coffee-bean configuration analogous to the gas-filled coffee bean (figure 1A) and the appearance seen at surgery (figure 2). Like the gas-filled coffee bean, the pseudotumor may represent a focal volvulus or a closed loop (figures 2, 3A, 3B, and 3E) at the distal end of small bowel spiraling around the superior mesenteric artery (figures 3C and 3 D).

Although omphalormesenteric duct remnants, ⁹ adhesions, internal hernias, polyps, foreign body impactions, mesenteric defects, duplications, ⁵ and even mesenteric cysts ^2,10 have been associated with small bowel volvulus without malrotation, we have noted a more striking association with colonic obstruction from meconium plugs (figure 3F), small left colon (figure 1B), and Hirschsprung disease (figure 5). Violent peristalsis around hairpin turns in the small bowel proximal to the colonic obstruction may provoke this complication. ⁶

In light of this potential association, it is important to search for colonic obstruction as a potential cause of small bowel volvulus without malrotation. Conversely, the small bowel should be inspected carefully for a pseudotumor or gas-filled coffee bean in any newborn with colonic obstruction from colon inertia and/or Hirschsprung disease.

Occasionally, the spiral may extend into the duodenum. In such cases, a straight descending duodenum that is firmly anchored to the pancreas ¹¹ followed by a horizontal spiral distal to that point should suggest that the spiral originates distal to the LOT and that it is restrained from a more vertical position by a normally anchored LOT that is displaced by the volvulus and/or gastrointestinal distension ³ (figure 5). When the duodenum spirals because of malrotation, the spiral is usually more vertical because of a low and medial LOT and the descending duodenum typically participates in the spiral (figure 6).

Sonography

Sonography may disclose an unanticipated volvulus during an evaluation for abdominal pain, possible pyloric stenosis, or possible intussusception. Alternatively, sonography may be employed intentionally ^12,13 when complete small bowel obstruction is not recognized on the prone plain film ⁷ (figure 3B) so that other possibilities, such as the uncomplicated small bowel duplication originally postulated for this patient, are considered.

The sonographic appearance of small bowel volvulus (with or without malrotation) correlates with the position of the transducer in relationship to the coil (figures 3C and 3D) or pseudotumor (figures 3A and 3E). When the probe is held axial (transverse) to the abdomen and the coil (figure 3C), the "whirlpool sign" is depicted ^12,13 (figure 3G). Color doppler interrogation can be utilized to depict the vascular components of the whirlpool. ^l3 Clockwise motion of the superior mesenteric vein around the superior mesenteric artery as the transducer is moved caudally is considered a definite sign of midgut volvulus. ^l3 The more distal aggregation of twisted, fluid-filled loops responsible for the pseudotumor produce a coffee-bean (figure 3H) or "dumbbell" (figure 3I) configuration depending on the orientation of the probe in relationship to the pseudotumor. The coffee bean is depicted when the transducer is directly over the pseudotumor. The dumbbell may be produced by the very tight twist at the entrance of the coil into the pseudotumor.

The relative positions of the superior mesenteric artery and the superior mesenteric vein do not reliably predict the presence or absence of a predisposing malrotation. ¹⁴ This distinction does not affect the initial clinical management, however.

Computed tomography

CT has the potential to clearly depict the small bowel circling and encompassing two concentric vascular coils, which in turn surround the superior mesenteric artery. As axial sections are evaluated in a cranial to caudal direction, the superior mesenteric vein rotates around the superior mesenteric artery. ¹⁵ In patients without malrotation, this spiral may be counterclockwise (figures 7A to 7E) rather than the typical clockwise rotation seen when malrotation is a predisposition. An additional "wheel" between this venous spiral and the superior mesenteric artery is produced by twisted small branches of the superior mesenteric artery (figures 7A to 7E). Impaired arterial flow is inferred when the superior mesenteric artery (the "hub" of these wheels) is abnormally small (figures 7A to 7E). Three dimensional reconstruction can display the length of the superior mesenteric vein spiral directly. ¹⁵

Conclusion

Small bowel volvulus without malrotation may occur for no obvious reason or secondary to known predispositions. In babies, this condition may be provoked by colonic obstruction from colon inertia and Hirschsprung disease. Since volvulus may be more emergent but less obvious than the colonic obstruction, it is particularly important to anticipate a potential association of volvulus with a second distal obstruction, and to give a convincing account of plain film findings that suggest this complication.

Although an upper gastrointestinal series is the preferred method for confirming volvulus predisposed by malrotation, this exam, as well as the barium enema, are not adequate for depicting small bowel volvulus without malrotation. Therefore, when clinical or plain film findings include this possibility, a small bowel series should be added to the upper gastro-intestinal series. Although other imaging studies have been used in this situation, usually a small bowel series is the most rapid way to confirm this diagnosis.

In addition to the potential for a second more distal obstruction, three other unique findings may be demonstrated when these patients are evaluated:

1) restriction of the volvulus to the area between a normally positioned ligament of Treitz and the cecum;

2) an unaffected descending duodenum while the rest of the duodenum spirals in a horizontal rather than vertical orientation; and

3) counterclockwise rather than clockwise rotation of the small bowel, superior mesenteric vein, and proximal branches of the superior mesenteric artery around the superior mesenteric artery. AR

The views expressed herein are those of the author and do not necessarily reflect the views of the Department of the Army or the Department of Defense.

Acknowledgment

The author wishes to thank Ms. Suzy M. Kai, Editorial Assistant, Cardiology Service, for her help typing and preparing this manuscript.

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