Main pathological agent is purely a mechanical one. The 3rd part of duodenum (D3) passes from (R) to (L) across the IVC and abdominal aorta with vertebral column as the most posterior hard structure. SMA takes off from L1 vertebral level and passes inferiorly behind the head of pancreas. It then passes between the neck and superior surface of the uncinate process of pancreas to come anterior to the uncinate process. At this level (L) renal vein, uncinate process and D3 lie between the SMA anteriorly and the Aorta posteriorly. If the angle between Aorta and SMA take off is very narrow, these 3 structures (viz. (L) renal vein, uncinate process and D3) are prone to get pinched. Anatomy books name this situation as "NUT-CRACKER".
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| Smaller the angle α more severe the symptoms. Symptoms can be eased by putting the patient in knee-elbow position, thereby making angle α less acute and taking off the traction on SMA by the weight of Tr colon, the jejunal and ileal vessels, middle colic artery, (R) colic artery and the gut attached to these structures. |
1. Gastro-colic time of over 3 Hrs
2. Dilatation of small bowel more than 3 CMs on Ba meal FT or 2.5 CMs on CT with contrast
3. Distance between adjacent volvulae conneventes more than or equal to 4 MM. in jejunum and duodenum.
Criteria to diagnose SMA syndrome:
1. Sharp vertical cut off to the (R) of mid-line on vertebral column to the column of Ba meal.
2. Disappearance of to-fro movement of Ba column beyond the D3-D4 area.
3. Prolonged hold-up of Ba at D2 and (R) side of D3 level of duodenum
4. Proximal duodenal dilatation with normal jejunal loops.
5. Disappearance of symptoms in knee-elbow position.
Some anatomical considerations while assessing D2-D3-D4.
With patient in supine position, the Ba has to ascend into the D3 from D2 as D2 is the most dependent portion. Anterior to the aorta, is the summit of the duodenal D3 part from where the Ba descends into distal D3 and D4 and this is aided by gravity, causing quicker run off in distal D3-D4 segment. The gradient Ba column is expected to climb at D2-D3 is approx 3-5 CMs (from the D2 in right paracolic gutter to the D3 crossing aorta).
Thus, many NORMAL Ba Meal FTs show a little hold up of the Ba in D2 and proximal D3 and thinner Ba in distal D3-D4-DJ and jejunum. The same is seen in those loops of small bowel which cross vertebral column from (R) to (L) or from (L) to (R) and this can be very well observed in the Gastrograffin film displayed in this blog on page " 3 Surgeries (2002 to 2009) ". In that film, along with D3 there is a loop of jejunum parallel to D3 which shows thinning of contrast and a litle distorsion of volvulae conneventes at the place where it is crossing the vertebral column.
Following 2 radiographs are of a different patient and are displayed here to show how a 'normal' duodenum looks on the Ba meal FT.
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| This patient's Ba meal FT was done to assess IC Jn. She had no obstructive symptoms. Note the end-on appearance of volvulae at D2-D3 area with narrowing of D3 against vertebral column and further D3-D4 seen behind the gastric outline. |
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| The Follow Through of the same patient. Watch the D2-D3 and pre-vertebral area of the D3 in particular. |
These radiographs are quite instructive about the D3 assessment.
