There are two distinct vascular compression disorders due to compression of another structure by the superior mesenteric artery.
Wilkie syndrome or Superior mesenteric artery syndrome is defined as compression of the third part of the duodenum by the SMA.
Nutcracker or renal vein entrapment syndrome implies compression of the the left renal vein by the SMA.
Superior mesenteric artery (SMA) syndrome is an uncommon but well recognized clinical entity characterized by compression of the third,
or transverse,
portion of the duodenum between the aorta and the superior mesenteric artery.
This results in chronic,
intermittent,
or acute complete or partial duodenal obstruction. [1]
Nutcracker syndrome (NCS),
also known as left renal vein entrapment2-4 is characterized by impeded outflow from the left renal vein (LRV) into the inferior vena cava (IVC) due to extrinsic LRV compression,
often accompanied by demonstrable lateral (hilar) dilatation and medial (mesoaortic) narrowing.
The superior mesenteric artery usually forms an angle of approximately 45° (range,
38-56°) with the abdominal aorta,
and the third part of the duodenum and left renal vein crosses caudal to the origin of the superior mesenteric artery,
coursing between the superior mesenteric artery and aorta.
Any factor that sharply narrows the aortomesenteric angle to approximately 6-25° can cause entrapment and vascular compression.
In addition,
the aortomesenteric distance in superior mesenteric artery syndrome is decreased to 2-8 mm (normal is 10-20 mm).
5-6
The patient with SMA syndrome often presents with chronic upper abdominal symptoms such as epigastric pain,
nausea,
eructation,
voluminous vomiting (bilious or partially digested food),
postprandial discomfort,
early satiety,
and sometimes,
subacute small bowel obstruction.
The symptoms are typically relieved when the patient is in the left lateral decubitus,
prone,
or knee-to-chest position,
and they are often aggravated when the patient is in the supine position.
These maneuvers are thought to reduce the small bowel mesenteric tension at the aortomesenteric angle. [7]
The frequency and severity of the Nutcracker syndrome vary from asymptomatic microhematuria to severe pelvic congestion. Although some patients have severe and persistent symptoms,
many,
especially children,
are asymptomatic.8
Hematuria is the most commonly reported symptom and is attributed to rupture of thin-walled varices,
due to elevated venous pressure,
into the collecting system.9
The diagnosis of superior mesenteric artery (SMA) syndrome is difficult.
Confirmation usually requires radiographic studies,
such as an upper GI series,
hypotonic duodenography,
and CT scanning.
Upper GI endoscopy may be necessary to exclude mechanical causes of duodenal obstruction.
Fluoroscopic findings suggestive of superior mesenteric artery syndrome include dilation of the first and second portions of the duodenum with an abrupt narrowing at the third portion ,
delayed gastroduodenal emptying,
and antiperistaltic waves proximal to the obstruction.
Additionally,
the obstruction of the duodenum may be relieved by a change in position,
especially left lateral decubitus position. [10]
CT scanning is useful in the diagnosis of superior mesenteric artery syndrome and can provide diagnostic information,
including aorta-superior mesenteric artery distances and duodenal distension.
Also,
it can be used to assess intra-abdominal and retroperitoneal fat.
CT criteria for the diagnosis of superior mesenteric artery syndrome include an aortomesenteric angle of less than 22 degrees and an aortomesenteric distance of less than 8-10 mm.
In children,
an angle of less than 20° has been correlated with superior mesenteric artery syndrome.11
Diagnostic methods in NCS include blood examinations,
urinalysis,
urine culture,
cytology,
urethrocystoscopy,
CT urography,
and renal biopsy.
In some cases,
renal biopsy precedes the final diagnosis of NCS.
Radiographic features of NCS are similar on ultrasound,
Doppler ultrasound,
CT,
MRI,
and conventional angiography:
· reduced aortic-SMA angle (the normal angle between aorta and SMA is approximately 45° (38-65°)
· left renal vein stenosis
· collateral pathways: main collateral pathway is the left gonadal vein which will display early enhancement during portal venous phase
· pressure gradient >3 mm Hg on renal venography
· compression ratio (CR) given by the equation CR=P-C/C i.e.
diameter of pre-compressed vein (P) minus diameter of compressed vein (C) divided by the diameter of the compressed vein (C).
A compression ratio above 2.25 is highly sensitive and specific for Nutcracker syndrome 12