Saturday, October 27, 2012

Pelvic Artery Ectasia/Aneurysm

Although isolated aneurysms of the distal abdomen are not uncommon, and aneurysms extending into the iliacs are not uncommon... an isolated aneurysm of the common iliac artery is rare... very rare (about 0.03%)

"Max" is the maximum value of normal (approx 2 s.d.)
But ectasia of the larger pelvic arteries is not rare at all. So what is normal?  what is ectasia? what is aneurysm? At some point you have to mention that an artery is larger than normal, but not really aneurysmal... and then the patient may get follow-up studies to follow its size. Where do you make that cut-off between "normal" and "follow"?  Is it just arbitrary?

Different literature sources (ultrasound, cardiology, interventional radiology, vascular surgery) do not agree on an exact number... for instance, in different literature sources, the mean normal common iliac artery size ranged between 12 - 15 mm ... but since the absolute difference between these values is small... and since the range is not too wide... a lot of the variation in the literature can be swallowed up in differences in interobserver measurement technique (up to 3mm with ultrasound?) or differences in modality.  In one study, ultrasound underestimated normal-sized aortas relative to CT, and overestimated large aneurysms, and it's not really clear that either could be used as a gold standard.

A Terrarecon image I made of an internal iliac artery aneurysm  measuring 12 mm (green arrow).  There is a diffuse aneurysm in the visualized aorta and a right common iliac aneurysm as well -- an isolated iliac artery aneurysm is rare,  occuring only 0.03% of the time.

An aneurysm is usually defined as an arterial diameter 1.5x the normal diameter. The most commonly accepted absolute cut-off for aneurysm in the distal abdominal aorta is 35mm. Values suggested for calling aneurysmal dilatation of the common iliac artery is >1.85 cm in men and > 1.5 cm in women.

The situation is complicated by the fact that aortic and iliac artery sizes normally increase gradually with age... for instance, in one source, for a male in his 30s the distal abdominal aorta was averaged at 16 mm (+/- 2mm), for males in their 70s, it's 20 mm (+/- 2mm).

Some authorities believe that asymptomatic common iliac artery aneurysms should only be treated at 3cm or larger... making the fine line between normal and ectasia -- far below this size -- more a question of follow-up, rather than immediate treatment.

Where does the external iliac artery fit into all this?  Ectasia/aneurysm of the common and internal iliac arteries is much more common than involvement of the external iliac artery, and normal measurements of the external iliac artery are few.  One article investigating aorta and iliac artery ectasia found that 10 mm was an approximate top normal for the external iliac artery in a control group.

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