## 5 Clinical Presentation and Instrumental Diagnosis  
CI may manifest with different signs and symptoms depending on the severity of the ischemia. Often, the symptomatology may initially be non-specific and misdiagnosed. The most common clinical presentation includes abdominal pain and diarrhea with rectal hemorrhage, associated with worsening of the general clinical condition, often resulting in an initial increase of infection markers [49]. In most cases, the onset of this complication occurs within the first postoperative week [18]. In case of an rAAA, the diagnosis is even more complex due to the critical general condition of the patient, as a result of the possible onset of respiratory failure, cardiovascular instability, renal failure, and increased catecholamine requirements, irrespective of CI [25]. Morphologically, CI may involve only the mucosa with the possibility of complete healing, or the mucosa and the musculature leading to fibrosis, or it may be transmural, resulting in intestinal gangrene and subsequent perforation. The gangrenous form more often occurs after aortic surgery, whereas the non-gangrenous form occurs in 80% of cases of idiopathic ischemic colitis. There are no specific laboratory tests for a certain diagnosis of CI. Increased infection markers and lactate levels are the most indicative parameters of possible CI, but normal values in the presence of specific symptoms do not exclude a diagnosis for CI [50]. Research into biomarkers for intestinal ischemia has identified several promising candidates, including intestinal fatty acid-binding protein (I-FABP), glutathione S-transferases (GSTs), and D-lactate. These biomarkers hold potential for the early diagnosis and assessment of intestinal injury, although none is sufficiently accurate to be used as an exclusive diagnostic tool [51]. I-FABP is a small cytoplasmic protein expressed in mature enterocytes located on the villi of the small intestine. In the case of mucosal injury, it is released into the circulation, making it a potential early indicator of intestinal ischemia. In a study by [PERSON] et al., patients with confirmed ischemia exhibited significantly elevated mean I-FABP levels (149.74 ng/mL) compared to non-ischemic patients (36.78 ng/mL). However, elevated I-FABP levels have also been observed in other gastrointestinal conditions, such as [PERSON]'s disease and celiac disease, which may affect its specificity [52]. Glutathione S-transferases (GSTs) are enzymes distributed throughout the intestinal tract that are released into the bloodstream following cellular injury, including ischemic damage. Circulating GST levels have been shown to correlate with the severity of histological injury. Although promising in experimental models, further clinical studies are needed to establish GSTs as reliable biomarkers for intestinal ischemia in humans [53]. D-lactate is produced by bacterial fermentation in the gastrointestinal tract and is normally present in the bloodstream at minimal levels. Intestinal ischemia can increase gut permeability, allowing D-lactate to enter the circulation. In the same study by [PERSON] et al. [52], mean D-lactate levels in ischemic patients were significantly higher (52.73 \(\upmu\)g/mL) compared to non-ischemic controls (15.58 \(\mu\)g/mL). However, elevated D-lactate levels can also occur in conditions associated with increased intestinal permeability or bacterial overgrowth, which may limit its specificity. Given the limitations of individual biomarkers, combining multiple markers may improve diagnostic accuracy and offer a more comprehensive evaluation of intestinal integrity, supporting an earlier diagnosis of ischemia [54]. As previously reported, in the case of clinical suspicion (suggestive symptoms and supportive laboratory markers), the gold-standard diagnostic tool is the sigmoidoscopy, which can determine whether the ischemia is transmural or not [21].