Median Arcuate Ligament Syndrome (MALS): Respiratory Maneuver and PSV Thresholds on Duplex

Pathophysiology and Clinical Context

MALS (median arcuate ligament syndrome) is extravascular compression of the celiac artery by the median arcuate ligament of the diaphragm and/or fibers of the celiac nerve plexus. Normally, the ligament passes above the ostium of the celiac artery; however, when the ligament is positioned low or the artery arises high, the ligament compresses the vessel from above, especially on expiration when the diaphragm rises and the ligament shifts caudally. Thus, respiratory dependence is the key feature. The condition is more common in young, often lean women; clinically presenting with abdominal pain, weight loss, and nausea.

Respiratory Maneuver: Technique

In MALS, compression of the celiac artery ostium is maximal on expiration and decreases on inspiration due to diaphragmatic descent. Accordingly, celiac artery PSV increases on expiration and decreases on inspiration—duplex readily detects these changes. PSV must be measured in both respiratory phases, since expiratory compression occurs in 13–51 % of healthy individuals, and according to CT/angiography data, 13–50 % of patients with such morphology are asymptomatic.

Duplex Criteria and PSV Thresholds

According to Gruber H. et al., mean expiratory PSV in MALS was 425 cm/s versus 210 cm/s in controls.

Morphologic Features on Imaging

Ascending angulation or superior indentation of the celiac artery is characteristic, typically within 5 mm of the ostium, creating a 'hooked appearance'. Maximum compression occurs at end-expiration; may persist at end-inspiration. On color Doppler, post-stenotic dilatation and/or flow turbulence are possible. If indentation/angulation persists at end-inspiration in a symptomatic patient, MALS diagnosis is more reliable.

Supporting Findings and Adjunct Modalities

Additional features include post-stenotic dilatation, retrograde filling via the superior mesenteric artery and pancreatoduodenal arcade. On CT/MRI, a hooked, narrowed proximal celiac artery with post-stenotic dilatation and collaterals are seen; the ligament itself is considered abnormal at thickness >4 mm. On ultrasound, the sagittal plane is most informative.

Limitations of the Method

MALS diagnosis remains controversial and non-standardized. In cohort studies, symptom resolution after surgery is achieved in only a portion of patients (~29 %). Given the high prevalence of asymptomatic compression in healthy individuals, duplex criteria should be interpreted only in clinical context with mandatory respiratory maneuver.