A water-soluble contrast challenge (more widely known as a Gastrografin challenge) is a combined diagnostic study and therapeutic intervention utilized in the evaluation and management of small bowel obstruction. It is used when clinical or imaging features determine there to be small bowel obstruction due to adhesions, without any complications such as perforation or a closed-loop configuration.
A 100 mL dose of water-soluble contrast, usually Gastrografin, is administered via a nasogastric tube, with abdominal radiographs acquired in the subsequent 24 hours. The findings can predict the need for operative management and determine whether the obstruction has resolved; seeing the contrast in the large bowel at 24 hours means the challenge has been successful and that the obstruction has been relieved. It is also considered to have a therapeutic effect; in theory, an increase in luminal fluid content via an osmotic effect may promote enough of a pressure gradient across the transition point to relieve the obstruction.
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Indications
uncomplicated small bowel obstruction due to adhesional band (no ischemia, no perforation, no closed-loop configuration)
Contraindications
There is limited data on the use of the challenge in the setting of small bowel obstruction caused by something other than adhesions, such as hernias, as such, this is still considered to be a contraindication. The challenge has been used in the post-operative setting to differentiate between obstruction and ileus, although there is limited data on this.
Clinical or radiologic features of complex small bowel obstruction, such as pyrexia, abnormal bowel wall enhancement or pneumatosis, or a prolonged period of time having elapsed from the onset of symptoms, are a contraindication to conservative management and therefore also a contraindication to a water-soluble contrast challenge.
Procedure
Most of the literature is focussed on the use of diatrizoate meglumine or diatrizoate sodium (Gastrografin), and this explains why the procedure is widely known as the Gastrografin challenge. A recent publication has found similar results with iohexol 2. Most publications quote 100 mL of undiluted contrast.
A nasogastric tube must be in situ, and allowed to be on free suction for at least two hours prior to administration of the contrast to allow gastric decompression. Once the contrast has been administered, the nasogastric tube should be clamped for two hours. This ensures that the contrast is not immediately suctioned out of the stomach by the NG tube 1.
Timing of the abdominal radiographs varies, with authors advocating for different timepoints, and one or more radiographs, but a consensus towards 8-hour and 24-hour radiographs has emerged, with passage of contrast into the large bowel on either image being a sign of passing the challenge (i.e. no absolute small bowel obstruction). If the contrast reaches the large bowel on the 8-hour radiograph, there is no need for a 24-hour radiograph. If the contrast has not reached the large bowel at 24 hours, or a small volume has but there is still substantial small bowel distension, it is considered an unsuccessful challenge, and a need to assess the patient for possible surgical intervention.
Radiographic features
With ongoing small bowel obstruction, the contrast will fill dilated small bowel loops, and due to mixing with native small bowel fluid, will not appear very dense, but nonetheless appears denser than small bowel seen on other radiographs. When passage into the large bowel occurs, opacity progresses in the expected anatomic fashion, from the cecum round to the rectum, and the contrast is more conspicuously visible as it becomes concentrated.
A frequently encountered problem is that contrast is not clearly visible on the 24-hour radiograph. Unless some or all of the contrast was aspirated from the stomach by the nasogastric tube early in the challenge, the most plausible explanation is ongoing obstruction, with marked dilution of the contrast in the small bowel, and this indicates a failed or unsuccessful challenge.
In the case of patients with ileostomies, contrast within the stoma bag indicates a successful challenge.
Complications
A disadvantage of Gastrografin is the potential to cause aspiration pneumonitis or pulmonary edema if aspirated, and this risk can be mitigated by the use of a nasogastric tube.
Outcomes
Meta-analyzes have indicated a sensitivity of 92-96% and a positive predictive value of 98-99% of successful non-operative management of small bowel obstruction if contrast was present in the large bowel at 24 hours 3,4.
From a therapeutic perspective, studies have shown a reduced rate of operative intervention and reduced length of hospital stay in patients administered the water-soluble contrast challenge 3,4.