Transabdominal Intestinal Ultrasonography in Monitoring and Predicting Outcomes in Ulcerative Colitis—A Systematic Review

<b>Background/Objectives:</b> Intestinal ultrasound (IUS) is increasingly used to monitor ulcerative colitis (UC), but its predictive value remains unclear. This systematic review evaluated the ability of IUS parameters and scores to predict short- and long-term treatment response, remission, and adverse outcomes in hospitalized and outpatient UC populations. <b>Methods:</b> A systematic review was conducted according to Cochrane and PRISMA guidelines. MEDLINE and Embase were searched for prospective studies assessing IUS as a predictor of clinical or endoscopic response, remission, relapse, or adverse outcomes in adult UC. Two reviewers independently performed screening, data extraction, and QUADAS-2 assessment. <b>Results:</b> Eighteen prospective studies were included: eleven outpatient studies and seven involving hospitalized patients treated with intravenous corticosteroids (IVCS). In hospitalized patients, bowel wall thickness (BWT) was the most consistent predictor of treatment failure, rescue therapy, colectomy, and clinical response. Baseline BWT showed variable performance, but once IVCS was initiated, early BWT change within 48-72 h was the strongest marker of disease trajectory. Non-responders had higher BWT and smaller reductions. A BWT ≥ 4 mm, absolute reduction ≤ 1 mm, or relative reduction ≤ 20% at 48 h reliably identified patients needing rescue therapy (area under the curve (AUC) values of 0.77 (95% confidence interval (CI) 0.71-0.74), 0.71 (95% CI 0.56-0.86), and 0.74 (95% CI 0.60-0.88)). Colectomy risk was similarly predicted: BWT < 3 mm at 48 h was associated with no colectomies, whereas BWT ≥ 4 mm or persistently elevated BWT at day 6 markedly increased risk (Odds ratio (OR) 9.5-fold (95% CI 1.4-64.0) and OR 8.3 (95% CI 1.7-40.0), respectively). Other sonographic features (loss of haustration, increased vascularity) added supplementary but less consistent value. In outpatients, BWT also demonstrated the strongest predictive accuracy. BWT ≤ 3.6 mm at 2 weeks and <3.0 mm at 6 weeks were associated with early endoscopic remission (area under the receiver operating characteristic (AUROC) of 0.87 (95% CI 0.71-1.00) and 0.82 (95% CI 0.63-1.00), respectively). Dynamic changes with ≥23-25% relative reduction predicted clinical or endoscopic response (AUROC of 0.81 (95% CI 0.61-1.00) and OR of 13.9 (95% CI 1.13-1986.85), respectively). Persistent BWT > 3.5 mm or minimal reduction (<20% or <1 mm) indicated a low likelihood of long-term remission. Composite vascularity-based indices, particularly the Milan Ultrasound Criteria (MUC), strengthened prediction: MUC ≤ 4.3 or ≥2-point reduction at 12 weeks predicted long-term remission (AUROC 0.88 (95% CI 0.750-0.952) and 0.82 (95% CI 0.68-0.91), respectively), while MUC ≥ 7.7 indicated high risk of treatment failure or colectomy (AUROC 0.77 (95% CI: 0.73-0.82)). <b>Conclusions:</b> Across clinical settings, BWT consistently emerged as the strongest IUS predictor of UC treatment outcomes. Early BWT change within 48-72 h in hospitalized patients and absolute BWT values at 2-6 weeks in outpatients showed high predictive accuracy for response, remission, and colectomy. Composite indices incorporating vascularity further improved prediction. These findings support the incorporation of IUS into early treatment-response algorithms and underscore the need for standardized cut-offs and multicenter validation.