Study on the variations of the ventral abdominal aortic branches : a computed tomography based study

The abdominal aorta [AA] begins at the aortic hiatus [at the 12th thoracic vertebra [T12]] and ends at the fourth lumbar vertebra [L4]. The main ventral branches of the AA are Coeliac Axis [CA], Superior Mesenteric Artery [SMA] and Inferior Mesenteric Artery [IMA]. Variations occur in branching pattern and the level of origin. Methods This is a cross-sectional, descriptive Computed Tomographic imaging [CT] based study done at the Teaching Hospital Anuradhapura, Sri Lanka from November 2018 to March 2019. Consecutive patients undergoing CT abdomen at the radiology department were selected. All images were analysed by the author in conjunction with the radiologist. Images of patients less than 25 years, incomplete records, non-clear images and images with the non-identifiable origin of the were also excluded.


Introduction
The abdominal aorta [AA] begins at the level of the twelfth thoracic vertebra [T12] at the aortic hiatus of the diaphragm and ends by dividing into common iliac arteries [CIA] at the level of the fourth lumbar vertebra [L4]. The main anterior visceral branches of the AA are coeliac axis [CA], superior mesenteric artery [SMA] and inferior mesenteric artery [IMA]. Generally, CA originates at the level of T12, SMA arises at the level of L1 approximately 1.5 cms below the CA and the IMA originates at the level of L3. CA divides into common hepatic artery [CHA], splenic artery [SA] and left gastric artery [LGA]. These branches supply the liver, stomach, duodenum, pancreas and spleen. The absence of LGA results in the hepatosplenic trunk [HST] and the absence of SA results in the hepato gastric trunk [HST]. SMA supplies the entire small intestine other than the first part of the duodenum and it also supplies part of the large intestine [up to the left 1/3 of the transverse colon]. IMA supplies left [distal] 1/3 of the transverse colon, descending colon and the upper part of the rectum.
Variations of the level of origin and branching patterns of these arteries are known to occur due to the changes which take place during embryonic development [1]. The reported incidence of variations of the CA and the SMA are between 3% to 7.2% [2] [3]. Awareness of these variations is important to avoid disasters during interventions and surgeries. Such an instance is the absence of a superior mesenteric artery which is associated with dilatation of the inferior mesenteric artery. In such a situation ligation of the IMA, when repairing abdominal aortic aneurysm, results in lower intestinal ischaemia. Awareness of variations in CA and SMA is also important in liver transplantation [also in organ retrieval], liver resection, surgeries of the stomach and chemoembolization of liver tumours. Also, awareness of these variations in anatomy will prevent unnecessary dissection and associated morbidity during surgery and will prevent prolongation of surgeries.
This study aimed to describe the prevalence of variations of the level of origin and branching pattern of the anterior visceral branches of the aorta among patients presenting to the The Sri Lanka Journal of Surgery 2021; 39(1): 26-29 Teaching Hospital of Anuradhapura [THA] Sri Lanka.

Methods
This was a cross-sectional descriptive study. Consecutive patients undergoing CT abdomen at the radiology department were selected. The study was done at the THA, Sri Lanka, from November 2018 to March 2019. Contrast-enhanced computed tomographic angiography [CTA] images in an arterial phase were analysed. All CT scans were performed by 160 detectors Toshiba Aquilion scanner machine. All images were transferred to the console and 3D reconstructions were made before analysis. The CTA images were analysed by both the author and the radiologist in the CT scan working station [console room]. Data on the patient's age, gender, details on branching level [Vertebral level] variations, branching pattern variations and the vertebral level of aortic division were obtained. Patients younger than 25 years were excluded. Because the vertebral level of origin is known to vary with the growth of the individuals [4]. Besides incomplete records, non-clear images and images with the non-identifiable origin of the arteries were also excluded from the analysis.  The reported incidence of CMT is about 2.7% to 5.4% [1]. While the reported incidence of HGT is between 0.08% to Therefore as discussed above branching variations occur in the anterior visceral branches of the abdominal aorta especially about the level of origin and the branching pattern especially in CA and its branches [2.9%] as in this series. The awareness of these variations in the branches of the aorta is important especially during emergency surgeries. The practice of pre interventional imaging and determination of the variations is important to prevent unwanted complications. 7.2% [2] [6]. HST occurs at a rate of 0.02% [7]. In this series, CA originated at the level of T12 in 68.3% of individuals. Other similar series also report a similar level of origin in about 38% -64% of cases [5] [8]. The SMA is fairly constant in respect to the level of origin i.e. 90.2% at the L1 level. The reported level of origin of IMA is at L3 in 47 -66% of cases [9]. Knowing the level of origin of the CA and its branches variation is extremely important especially in upper abdominal and liver surgeries. Accidental ligation of variant arteries e.g in the case of Celio mesenteric trunk can result in disastrous complications [10]. Similarly in the case of bleeding pre-operative knowledge of the variations is important to prevent unnecessary complications. Besides in endovascular aneurysm repair, knowing the exact level of origin and the variations is important to prevent inadvertent occlusion of the origin and post-procedure organ ischemia. Another example is the absence of a superior mesenteric artery which is associated with dilatation of the inferior mesenteric artery. In such a situation ligation of the IMA, when repairing abdominal aortic aneurysm, results in lower intestinal ischaemia [11].

Results
Division of aorta occurs above the level of L4 vertebral level in 18.2% [as in this series] to as high as 53.84% in some series [10]. Knowing this level is also important in clinical practice. When radiotherapy is utilised to treat carcinoma of the cervix, usually the upper limit is determined as L4-L5 intervertebral disc. This level is chosen to cover the iliac lymph nodes. If there is a higher division of the aorta [i.e. at L3 level in 18.2% as in this series], a significant length of CIA and therefore a significant number of iliac lymph nodes would be missed if the radiotherapy field is not shifted upwards. Therefore knowing the level of aortic division is important.