Transverse subisthmic course of the innominate artery in an adult: detailed anatomy and additional variation.

A rare morphology of an aberrant innominate artery (IA) is reported here, together with additional arterial variation encountered in the respective specimen. The IA originated in the aortic arch on the left side of the trachea, coursed on that side of the trachea to reach the left thyroid lobe, turned in at a right angle to pass anterior to the trachea and immediately inferior and parallel to the thyroid isthmus, and finally it divided inferior to the right thyroid lobe into the right subclavian and common carotid arteries. The right common carotid artery immediately turned at a right angle to ascend in the neck. Thus the terminal branches of the IA had origins in a higher position than is usually expected. This aberrant course of the IA determined a step-like morphology in the sagittal plane of the left common carotid artery. Additional variations were also encountered: (a) a lateralised right external carotid artery with the superior thyroid artery initially coursing over the internal carotid artery; (b) the right vertebral artery coursing over the inferior thyroid artery and entering the transverse process of the fifth cervical vertebra; (c) the left subclavian and vertebral arteries were tortuous. Knowledge of the presence of this IA variant, with a transverse subisthmic segment, appears to be important in various surgical approaches, such as tracheostomies, thyroidectomies, and mediastinoscopies; in addition, the variations of the IA and the vertebral arteries are relevant for lower cervical spine approaches. Nevertheless, the lateralised external carotid artery may lead, if unidentified, to hemorrhagic complications during carotid space approaches. It is important for surgeons to be aware that if an aberrant IA is identified it may not be the only variation in that patient.