**2. Etiology**

There are various etiologies of mid-GI bleeding, but their frequency depends on patient's age and underlying comorbidities [6]. Below the age of 40, the most common causes include Crohn's disease, Dieulafoy's lesion, small bowel tumors, Meckel's diverticulum, and polyposis syndrome. Small bowel tumors could be benign or malignant [7]. Benign ones include small gastrointestinal stromal tumors (GIST), benign neuroendocrine tumors (particularly small carcinoid), hemangioma, adenoma, leiomyoma, lipoma, and neurofibroma. Malignant ones include large GIST, adenocarcinoma, lymphoma, malignant neuroendocrine tumors, leiomyosarcoma, and metastatic tumor from melanoma, lung, or breast [8–11].

Rarely, polyposis syndromes involving the small bowel may present with mid-GI bleeding. These include familial adenomatous polyposis, Peutz-Jeghers syndrome, and generalized juvenile polyposis. Over the age of 40, the most common causes of mid-GI bleeding include angioectasia, nonsteroidal anti-inflammatory drug (NSAID)-induced ulcers, Dieulafoy's lesion, and small bowel tumors. On rare occasions, other small bowel lesions can cause gastrointestinal bleeding. These include small intestinal diverticuli, small intestinal varices, hereditary hemorrhagic telangiectasia, Kaposi sarcoma, intestinal tuberculosis, blue rubber bleb nevus syndrome, hematobilia, hemosuccus entericus, aortoenteric fistula, infectious enteritis, radiation enteritis, ulcerative jejunoileitis due to celiac disease, cryptogenic multifocal ulcerous stenosing enteritis [12], amyloidosis, Behcet's disease, pseudoxanthoma elasticum, and Ehlers-Danlos syndrome [13]. The incidence of small bowel neuroendocrine tumors (SBNET) has been increasing over the last few decades, and they are now considered as the most common primary malignancy of small bowel. Adenocarcinomas are generally seen in the proximal small bowel, whereas SBNET and lymphoma are commonly located in the distal small bowel. Sarcomas (GIST and non-GIST mesenchymal tumors: leiomyosarcoma, liposarcoma, fibrosarcoma, Kaposi's sarcoma, angiosarcoma) are evenly located throughout the small bowel.
