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Two consecutive intersegmental arteries (ISA) sharing a common origin from a single intersegmental trunk is a frequent anatomic variation. Ipsilateral intersegmental trunks are divided into complete and incomplete forms. In the classic form of incomplete trunk, the dorsospinal artery (DA), missing at one of the levels supplied by the trunk, arises separately from the aorta. This configuration is known as an isolated DA or a direct emergence of the DA from the aorta. Three angiographic observations of a different form of incomplete ipsilateral trunk are reported here, in which only the spinal component of the DA is arising separately from the aorta. This variant corresponds to the adult persistence of the ISA in its most primitive form. If overlooked, an isolated spinal branch may represent a diagnostic pitfall during angiography, in particular when the variant provides blood supply to the spinal cord or to a spinal vascular malformation.
Two consecutive intersegmental arteries (ISA) sharing a common aortic origin from a single intersegmental trunk is a frequent anatomic variation.1 These ipsilateral intersegmental trunks are divided into complete and incomplete forms.2 To be complete, an ISA trunk must have a complete set of branches for each of the vertebral levels it supplies; these include (i) an anterior component (or aortic stem) providing the anterior-lateral group of osseous arteries for the vertebral body, (ii) a lateral component (named lumbar, subcostal or intercostal artery depending on the level considered) and (iii) a dorsospinal artery (DA), itself consisting of a dorsal component vascularizing the paraspinal musculature, and a spinal component providing the radicular artery.
In the classic form of incomplete unilateral trunk, the DA is missing at one of the levels supplied by the trunk, arising instead separately from the aorta. This configuration, reported by Chiras and Merland3 in 1979, is known as an isolated DA or a direct emergence of the DA from the aorta.2– ,5 This variant is clinically relevant as the isolated DA frequently provides a contributor to the anterior or posterior spinal arteries.2 We report here three angiographic observations of a different form of incomplete ipsilateral trunk, in which only the spinal component of the DA is arising separately from the aorta. The mode of formation and the clinical implications of this variation are discussed.
Case No 1
Incomplete right L1–T12 ipsilateral intersegmental trunk with a right T12 isolated intersegmental spinal artery discovered incidentally during diagnostic spinal digital subtraction angiography (SpDSA) in a 35-year-old patient (figure 1).
Incomplete right T11–T12 ipsilateral intersegmental trunk with a right T12 isolated intersegmental spinal artery discovered incidentally during diagnostic SpDSA in a 48-year-old patient (figure 2).
Incomplete right T9–T8 ipsilateral intersegmental trunk with a right T8 isolated intersegmental spinal artery discovered during diagnostic SpDSA in a 56-year-old patient (figure 3).
Dorsal rami start emerging from the posterior wall of the left and right primitive aortas in the early embryonic phase, at the 6 somite stage.6 These dorsal rami—that is, the future ISAs—initially consist of a series of vascular loops that join one primitive aorta to the ipsilateral posterior cardinal vein; later, the arterial and venous sides of the loops develop into the ISAs per se and their corresponding veins (figure 4A, B).6 Capillaries sprouting from the arterial limb of these loops connect to similar vessels coming from adjacent levels to form two longitudinal plexi along the left and right ventrolateral surfaces of the neural tube. The plexi soon extend dorsally to cover the lateral surfaces of the neural tube as well (figure 4C).6 Transverse pathways developing within this capillary network form the primitive anterior and posterior radicular arteries, which are the first and, at this time, only branches of the ISAs. Capillaries extending further medially from each ventrolateral plexus establish a second set of longitudinal chains along the lateral edges of the floor plate of the neural tube. These chains correspond to the primitive anterior spinal arteries, which will later coalesce more or less completely into a single ventromedian anterior spinal artery. In view of this developmental history, it appears that the dorsal arteries stemming from the primitive aortas and their capillary plexi for the spinal cord represent the fundamental anatomy of the ISA; at the adult stage, they respectively correspond to the stem of the ISA and the spinal component of the DA. Additional branches appearing later include a dorsal branch (ie, the dorsal component of the DA) and a lateral branch equivalent to the posterior intercostal, subcostal or lumbar arteries of the traditional nomenclature (figure 4D). The adult anatomy of the ISA depends on the branching pattern of these secondary arteries; in particular, the DA of the classic nomenclature exists only when the origin of the lateral branch is proximal to the take off of the spinal branch (figure 5).
Our three angiographic observations of isolated spinal branches stemming from the aorta can be viewed as the adult persistence of the ISA in its most primitive form—that is, an aortic stem corresponding to a dorsal ramus of the primitive aorta and a spinal branch associated with the early capillary plexus covering the neural tube.
Ipsilateral intersegmental trunks are more complex than generally assumed; the simple dichotomy between complete trunks and incomplete trunks with an isolated DA is unsatisfactory and may be misleading. Given the developmental history of the ISA, variants other than the one reported here are possible, involving various combinations of ISA branches (aortic stem, lateral, dorsal and spinal branches) at two or more levels (eg, case No 2 involves more than two levels).
Our third observation, in which the isolated spinal branch was supplying a vascular malformation of the spinal cord, underlines the potential pitfall represented by ipsilateral trunks in general, and emphasizes the attention with which one must analyze these variants at the time of angiography in order to avoid overlooking a lesion or having to repeat a procedure. The absence of hemivertebral blush at one of the levels supplied by an ipsilateral trunk is a helpful clue in regard to its incomplete nature (eg, case Nos 2 and 3). However, a hemivertebral blush is not always observed, in particular in older patients. Identifying the spinal branch of the ISA or one of its components—that is, a radicular or a retrocorporeal artery—remains the most reliable angiographic criterion for the distinction between complete and incomplete unilateral trunks.
In summary, we have reported a new type of ISA variation in which the spinal component of the DA originates separately from the aorta. Through its radicular branch, this isolated spinal component can participate to the normal blood supply of the spinal cord, and will at times be involved in the vascularization of spinal cord vascular lesions, such as in our third patient. An isolated spinal branch stemming for the aorta is a relatively small vessel that is at risk of being overlooked during spinal angiography.
Competing interests None.
Patient consent Not obtained.
Ethics approval The study was approved by the Johns Hopkins institutional review board.
Provenance and peer review Not commissioned; externally peer reviewed.
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