MALS: Respiratory Tests and Ultrasound Criteria for Celiac Trunk Compression
Pathophysiology and Clinical Portrait
MALS (median arcuate ligament syndrome; synonyms — celiac axis syndrome, Dunbar syndrome) is an extravascular compression of the celiac artery by the median arcuate ligament of the diaphragm (and/or fibers of the celiac nerve plexus). The ligament is a fibrous arch connecting the diaphragmatic crura around the aorta, forming the aortic hiatus; it typically arises above the origin of the celiac artery, slightly above the L1 level. When positioned low or when the artery originates high, the ligament compresses the celiac artery from above, especially on exhalation when the diaphragm rises and the ligament shifts caudally. On inhalation, the diaphragm descends, reducing or eliminating the compression. It often affects young, frequently slender women.
Respiratory Tests: Principle of Investigation
The main feature of MALS is respiratory dependence. With compression on exhalation, the PSV of the celiac artery increases, while on inhalation it decreases. Duplex examination reliably identifies these velocity changes. Velocities are measured in the celiac artery during two phases of breathing. It is important to document the breathing phase used during visualization, as this helps assess the reliability of the findings. If indentation/angulation persists at the end of inhalation (when diaphragmatic excursion shifts the ligament downward), the diagnosis of MALS in a symptomatic patient can be made with greater confidence.
Ultrasound Diagnostic Criteria
| Criterion | Value |
|---|---|
| Expiratory PSV of the celiac artery | >320–350 cm/s |
| PSV on inhalation | <200–250 cm/s |
| Exhalation/Inhalation ratio | ≥2.0 |
| CA/Ao ratio | >3.0 |
| Origin morphology | Focal stenosis of the origin, 'hooked' deformation |
According to Gruber H. et al. (Med Ultrason, 2012), a combination of an angle deviation >50° and expiratory PSV >350 cm/s provides Se 83%, Sp 100%, PPV 100%; the average expiratory PSV was 425 cm/s compared to 210 cm/s in controls.
Additional Signs
Supporting findings include post-stenotic dilation, retrograde filling through the superior mesenteric artery and pancreaticoduodenal arcade. CT/MRI data describe a hooked and narrowed appearance of the proximal segment of the celiac artery with post-stenotic dilation and collaterals; the median arcuate ligament itself can sometimes be visualized and is considered pathological if its thickness is >4 mm.
Differential Diagnosis and Interpretation
Do not confuse two syndromes: MALS — compression of the celiac artery by the ligament (arterial issue, respiratory dynamics); Wilkie's syndrome — compression of the duodenum between the aorta and the SMA (intestinal obstruction, narrow aorto-mesenteric angle). In MALS, velocities are measured in the celiac artery during two phases of breathing; in Wilkie's syndrome, the aorto-mesenteric angle and distance are assessed.
It is important to emphasize that imaging findings are compatible with MALS but are not diagnostic by themselves: 13–50% of patients with such morphology are asymptomatic. Diagnosis requires the clinical symptoms to match the imaging data.
Frequently asked questions
During which phase of breathing does compression intensify in MALS?
On exhalation: the diaphragm rises, the median arcuate ligament shifts caudally and compresses the celiac artery. On inhalation, the diaphragm descends, reducing or eliminating the compression. Accordingly, PSV increases on exhalation and decreases on inhalation.
What threshold velocities are considered criteria for MALS?
Expiratory PSV of the celiac artery >320–350 cm/s, decrease on inhalation <200–250 cm/s, or an exhalation/inhalation ratio ≥2.0, as well as a CA/Ao ratio >3.0 (according to the author's lecture material and SRU/EFSUMB).
What is the diagnostic accuracy of the combined criterion by Gruber?
A combination of an angle deviation >50° and expiratory PSV >350 cm/s gives Se 83%, Sp 100%, PPV 100% (Gruber H. et al., Med Ultrason, 2012). The average expiratory PSV was 425 cm/s compared to 210 cm/s in controls.
Are ultrasound findings alone sufficient for diagnosing MALS?
No. Imaging findings are compatible with MALS but do not diagnose it by themselves — 13–50% of patients with such morphology are asymptomatic. Clinical symptoms must correspond to the imaging data for diagnosis.
How to distinguish MALS from Wilkie's syndrome?
MALS involves compression of the celiac artery by the ligament with respiratory dynamics, measuring velocities in the celiac artery during two phases of breathing. Wilkie's syndrome involves compression of the duodenum between the aorta and the SMA; the aorto-mesenteric angle and distance are assessed.