A 65-year-old man presented to the emergency department for atraumatic acute abdominal pain and vomiting. He was a chronic tobacco user with no medical or surgical history or any regular medication.

Upon arrival, he presented with normal vital parameters. Abdominal examination revealed epigastric tenderness. He was treated initially with intravenous paracetamol before performing an ultrasound (US) which demonstrated the presence of a heterogenous mass in the epigastric region and the presence of a hyperechoic, well-organised heteregenous mass within the right iliac fossa, with no Doppler signal.

Thereafter, abdominal computed tomography (CT) with contrast was performed and demonstrated a well-organised and hyperdense mesenteric haematoma measuring with mean density of 57 Hounsfield unit (HU) approximately 9 cm × 5 cm × 4 cm. Two small aneurysms were identified originating from the upper and lower PDAs. There was marked CA compression by the MAL resulting in post-stenotic fusiform dilatation of the CA (Figure 1).

A 64-year-old woman with persistent abdominal pain for two weeks was admitted to our emergency department. The patient complained of diffuse abdominal pain and haematemesis; digital rectal exam indicated melena. Clinical abdominal examination revealed a peri-umbilical palpable mass with localised tenderness. Eso-gastro-duodenal endoscopy (EGDE) displayed external compression of the second-part duodenum without any mucosal abnormality. Biochemistry demonstrated regenerative microcytic and hypochromic anaemia.

An abdominal US was performed, highlighting the presence of free hyperechoic fluid in the pouch of Douglas. No mass was identified.

Subsequent abdominal CT demonstrated a homogenous hyperdense haematoma (43 HU) of approximately 14 cm × 10 cm × 8 cm with haemorrhage within the stomach (average density of 62 UH). There was moderate CA compression by the MAL with no visible PDA aneurysm (Figure 2).

A 44-year-old woman was admitted for acute abdominal and back pain, as well as haematemesis and melena. Her previous medical and surgical history was unremarkable and she was not on any regular medication.

Upon arrival, the patient had a tachycardia and was tachypnoeic. She was normotensive, had normal oxygen saturation (98%) and was afebrile. Abdominal examination elicited diffuse tenderness.

Eso-gastro-duodenal endoscopy did not show any abnormalities. Her blood tests were normal apart from a modestly raised C-reactive protein (20 mg/dL) and mildly decreased haemoglobin levels (9 g/dL).

Abdominal CT revealed a retroperitoneal haematoma with rupture of a small aneurysm originating from the inferior pancreatic-duodenal artery (IPDA), secondary to stenosis of the CA. Additionally, there was also a Standford B dissection of the aorta. It is postulated that the above scenario was caused by the decreased blood flow in the CA compressed by the intimal flap (causing dynamic ischaemia), which then resulted in further decreased flow in the coeliac trunk, increased blood flow in SMA branches and finally rupture of the pre-existing aneurysm (Figure 3).

The pathophysiology is clearly established. In MALS, blood flow to the splanchnic organs is diverted to the SMA and then to the PDA because of decreased flow in the CA. The PDAs cannot sustain this high flow, causing high-flow true aneurysms, which can eventually result in spontaneous rupture.² Aneurysm regression or stability is reported after CA reconstruction with stent placement, MAL incision or bypass placement.

There are other causes of PDA aneurysms other than MALS such as atherosclerosis, pancreatitis, mycotic and bacterial infection, trauma or fibromuscular hyperplasia.^2,8

The radiological diagnosis of MALS is firstly based on detecting MAL thickening equal to or greater than 4 mm,⁹ but the definitive finding for the diagnosis is focal narrowing of the proximal CA with a characteristic ‘hooked appearance’, best seen on a sagittal image.¹⁰ This sign helps differentiating MALS from other causes of CA stenosis, such as atherosclerosis. Associated findings include post-stenotic dilatation or collateral vessel formation from the SMA branches.⁶

Computed tomography and magnetic resonance imaging (MRI) should ideally be performed in the end-inspiratory and expiratory phases. True and permanent compressions persist or increase during end-expiration, whereas transient physiological compression is seen only during end-inspiration.⁶

Percutaneous angiography is the reference standard for the diagnosis of MAL syndrome and shows findings similar to CT such as superior indentation, hooking and post-stenotic dilatation of the CA. Furthermore, angiography can assess the stenosis in both end-inspiration and expiration. Retrograde filling of the CA from the PDA can also be assessed.⁶

Doppler US can demonstrate peak systolic velocities greater than 200 cm/s correlating to a degree stenosis of at least 70% with a sensitivity of 75% and specificity of 89%.^7,11

Median arcuate ligament syndrome may cause true post-stenotic aneurysms, which can be located anywhere in the distal arteries, but appear to predominantly involve the PDA and dorsal pancreatic artery (DPA) – the two main collateral networks between the CA and the SMA.^12,13

True aneurysms of the PDA are rare, accounting for only 2% of all visceral aneurysms.¹³ Almost half of all PDA aneurysms are associated with CA stenosis.¹⁴ Compression by the MAL has been shown to be responsible for stenosis in about 10% – 30% of cases.¹⁴ They can also be induced by periarterial inflammation, usually associated with pancreatitis, trauma, infection or atherosclerosis.¹³

More than 60% of patients with PDA aneurysms initially present with a rupture.¹³ Rupture risk is not related to the aneurysm size, suggesting that all aneurysms should be treated, regardless of their size.¹⁵ In the literature, ruptured aneurysm diameters ranged between 0.7 cm and 2 cm. On the other hand, unruptured aneurysm sizes were between 0.7 cm and 6 cm.¹⁶ The rupture risk of PDA aneurysms is about 65%. Among the case histories of 88 patients with PDA aneurysm published in the literature, 53 patients had rupture and 26 of them unfortunately died.¹⁵

In the third reported case, there was a Standford B aortic dissection where the CA originated from the false lumen. We postulate that this caused dynamic ischaemia and reduction of blood flow through the CA. The intimal flap reduces the CA ostium size and potentiates blood reduction caused by the abnormal MAL, resulting in further increased blood flow in the SMA branches, and finally, rupture of the small IPDA aneurysm. To the best of our knowledge, no such case was reported elsewhere.

The issues of treatment options regarding PDA aneurysms are still controversial. Recently, endovascular treatment has become popular but open surgical repair is also an option for young patients with no malignancy, for whom long-term survival is expected.¹⁵