A STUDY ON BRANCHING PATTERN OF ARCH OF AORTA WITH ITS EMBRYOLOGICAL SIGNIFICANCE AND REVIEW OF LITERATURE

Address for Correspondence: Dr. N. Shakuntala Rao, Professor, P.E.S. Institute of Medical Sciences and Research, Kuppam, Andhra Pradesh 517425, India. E-Mail: drshakuntala@gmail.com Introduction: The Arch of Aorta normally provides three major branches from its convex surface, before descending down as the thoracic aorta. The branches arranged from right to left are the brachiocephalic trunk, the left common carotid artery and the left subclavian artery. The right common carotid artery and the right subclavian artery are given off by the brachiocephalic trunk. Any variation in the origin and arrangement of these branches are considered to be deviations from the normal process of development of the aortic arches. Objective: To observe the variation in the branches arising from the aorta and to correlate with embryological development and discuss clinical significance. Materials and Methods: The study involved 36 formalin fixed cadavers, 12 female and 24 male cadavers, which were used for routine dissection and teaching purposes for the first year medical students in the department of Anatomy, P.E.S Institute of Medical Sciences and Research. Results: The arch of aorta was normal in 33 of the 36 cadavers. There were three cadavers which showed variation in the origin of branches from the arch of aorta. The variations were different in each of the cadavers and the same were discussed in detail while comparing with available literature. Conclusion: The variation in the arch of aorta is due to involution of some parts of the arterial arches during development. In this study the origin of right subclavian artery from the distal part of arch of aorta and passing posterior to esophagus is a significant variation to be noted. We also observed that In the present study it was not present close to the left subclavian artery but deviated to the right behind the esophagus from the distal part of the arch and did not form a ring. This can have clinically significant symptoms and signs related to pathology in the esophagus or during deglutition.


INTRODUCTION
The Arch of Aorta normally provides three major branches from its convex surface, before descending down as the thoracic aorta.The branches arranged from right to left are the brachiocephalic trunk, the left common carotid artery and the left subclavian artery.The right common carotid artery and the right subclavian artery are given off by the brachiocephalic trunk.Standard textbook analysis of variation reports the usual pattern of three branches in 65%.There may be four separate branches in 2-5% only and a great variety of patterns are seen in 5%.Sometimes left common carotid artery shared the brachiocephalic trunk in 27% of individuals.

MATERIALS AND METHODS
The study involved 36 formalin fixed cadavers, 12 female and 24 male cadavers, which were used for routine dissection and teaching purposes for the first year medical students in the department of Anatomy, P.E.S Institute of Medical Sciences and Research.Observation in variations of the arch of aorta were made when the superior mediastinum was opened up by conventional dissection method as per Cunningham manual of Practical Anatomyto demonstrate the structures.The arch of aorta was examined and the branches arising from it were noted in all the cadavers.The variations were photographed and noted down.S. Kadir et al [9] has cited Right subclavianartery courses to the right behind the esophagus in 80% of cases,between the esophagus and trachea in 15% and anterior to the trachea or mainstream bronchus in 5%.The variation that passes anterior to trachea or mainstem bronchus is called "arterialusoria" and is one of the most common vascular ring variations of the AA.

OBSERVATIONS
The most frequent variation that accompanies the right aberrant subclavian artery has been a common trunk for carotid arteries 20.6-29% as reported by K.L. Natsis,et al (10) in their study in 633 angiographies.They have described eight types of aortic arches, (I-VIII) after investigating aortic arches of 633 patients who underwent digital subtraction angiography.
In the present study a type-III variation was found which describes the left vertebral artery as arising from the arch of aorta (Fig. 2).In Type-IV common carotid arteries originate from common trunk.In this type the order of vessels from right to left is right subclavian artery, right and left common carotid arteries leaving from a common trunk and left subclavian artery.In Type-V: common carotid arteries originate from a common trunk and a right aberrant subclavian artery is present.In the present study a common trunk was seen for carotid arteries The right subclavian artery originated far below the arch at the commencement of the descending aorta and coursed upwards behind the esophagus (Fig. IA& IB).
In the study by Pasaoglu [5] Satyapal et al [12] in their study on three hundred and twenty aortic arch angiograms found variations in (5.3%).They analysed their results based on primary branches.They noted that (3.4%) had two primary branches,(0.3%)had three primary branches and (1.6%) had four primary branches.They concluded that a variation if clinically silent and is not recognized can have fatal consequences during trauma.One author W. Voster [13] who opined that a variation such as acommon trunk for BCT and LCCA may actually be useful clinically during catheterization of LCCA which originates from BCT as there is no need of catheter exchange.

CONCLUSION
The variation in the arch of aorta is due to involution of some parts of the arterial arches during development.When the right horn of aortic sac disappears before giving rise to the brachiocephalic trunk the part of third aortic arch remains, thus giving rise to a common trunk for both the carotid arteries.Such a variation was noted in this study.In this study the origin of right subclavian artery from the distal part of arch of aorta and passing posterior to esophagus is a significant variation to be noted 1-2% showed symmetric right and left brachiocephalic trunks [1].During the normal development of the pharyngeal arch arteries,the right fourth pharyngeal artery becomes the proximal part of the right subclavian artery.The right dorsal aorta and the right seventh intersegmental artery form the distal part of the right subcclavian artery.When there is abnormal involution of this segment of the artery cranial to the seventh intersegmental artery then it results in the anomalous formation of the right subcclavian artery from the right seventh intersegmental artery and distal part of right dorsal aorta [2].Unrecognized variations in the vascular anatomy may cause complications in open surgery on the aortic arch (3) Any variation in the origin and arrangement of these branches are considered to be deviations from the normal process of development of the aortic arches.

Fig. 1B :
Fig. 1B: Shows Right common carotid artery (Rt.CCA) arising from descending aorta and passing posterior to esophagus.
only the types I,II,III ofNatsis et al(10)  were present.They have stated that in their study that the incidence of double aortic arch (0.1%), right aortic arch(0.3%)and right vertebral artery which originated from the aortic sac as a last branch (0.1%) was observed.They have also stated that these variations were not included in the classification by Natsis et al.Pasaoglulale has stated that they observed 17 cases (1.9%) with aberrant right SCA and none of them had associated variations of the AA.All the cases were asymptomatic and the aberrant right SCA was an incidental observation.In all the cases the aberrant right SCA passed behind the esophagus.C.Matula,S.et  al [11]  The segment of the vertebral artery that arises from the subclavian to its entry into foramen transversarium is called prevertebral or pretransverse segment.It is frequently affected by atherosclerosis.Abnormal origin from arch of aorta can cause variations in haemodynamics of cerebral circulation.In the present study a type-III variation of Natsis was found which is clinically significant according to the report by C.Matula et al.K.S.
. The right fourth pharyngeal arch artery and the right dorsal aorta disappear cranial to the seventh intersegmental artery.This results in the formation of right subclavian artery from the right seventh intersegmental artery and distal part of right dorsal aorta.Differential growth shifts the origin of the right subclavian artery cranially close to the left subclavian artery and thus forms a vascular ring behind the trachea and esophagus.In this study it was not present close to the left subclavian artery but deviated to the right behind the esophagus from the distal part of the arch and did not form a ring.This can have clinically significant symptoms and signs related to pathology in the esophagus or during deglutition.
Shakuntala Rao, K Meera, K. Sujatha, H.R. Krishna Rao.A STUDY ON BRANCHING PATTERN OF ARCH OF AORTA WITH ITS EMBRYOLOGI-CAL SIGNIFICANCE AND REVIEW OF LITERATURE. N.