It may be seen in Crohn's disease, hypertrophic pyloric stenosis, carcinoid tumor and colon cancer.
In people with Crohn's Disease, the string sign is caused by incomplete filling of the intestinal lumen, which results from irritability and spasm associated with severe ulceration.
In such cases, the string sign is most frequently seen at the terminal ileum.
Often, there are several of these strings seen (called the "railroad track sign").
The use of the upper GI series for the diagnosis of HPS, which was the primary diagnostic tool for this condition in the 1980s and 1990s, has been largely replaced by the use of ultrasound, which is less invasive and can visualize the thickened pylorus, giving actual measurements of this thickening.