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Case report
Isolated aortic origin of the intersegmental spinal branch
  1. Philippe Gailloud
  1. Correspondence to Dr P Gailloud, Division of Interventional Neuroradiology, The Johns Hopkins Hospital, 600 N Wolfe Street, Baltimore, MD 21287, USA; phg{at}jhmi.edu

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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 reports

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).

Figure 1

Right L1–T12 ipsilateral intersegmental trunk with right T12 isolated intersegmental spinal branch. (A) Spinal digital subtraction angiography (SpDSA), right L1 injection, anteroposterior projection. The dorsospinal artery (DA) of the right L1 (arrow) supplies the dorsal components of both L1 (white arrowhead) and T12 (black arrowhead). (B) SpDSA, right L1 injection, lateral projection. Same legends. (C) SpDSA, right T12 injection, anteroposterior projection. The long white arrow points at the right T12 stem; note the presence of a hemivertebral blush, which mainly results from the opacification of the anterior-lateral group of osseous branches. The stem can be followed to the origin of the retrocorporeal artery (white arrow), a branch of the spinal component of the DA that provides the posterior-median group of osseous branches and fans into the retrocorporeal anastomotic arterial network (small white arrowheads).

Case 2

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).

Figure 2

Right T11–T12 ipsilateral intersegmental trunk with right T12 isolated intersegmental spinal branch. (A) Spinal digital subtraction angiography (SpDSA), right T11 injection, anteroposterior projection. The T11–T12 trunk includes the T11 intercostal and T12 subcostal branches (small black arrows), a spinal branch for T11 (white arrowhead) and a dorsal branch for T12 (black arrowhead). The dorsal branch of T11 and the spinal branch of T12 are missing. In the venous phase of this injection (B) a hemivertebral blush is documented at T11 (black asterisk), but is only partly visible at T12 (white asterisk). (C) SpDSA, right T12 injection, anteroposterior projection. The T12 aortic stem continues as the T12 spinal branch, providing in particular a conspicuous retrocorporeal artery (small white arrow). A left anterior spinal artery contributor is opacified via the retrocorporeal anastomotic network. Note a small anastomosis with the subcostal artery coming from the incomplete T11–T12 trunk (small white arrowhead). An almost complete hemivertebral blush is visible. (D) SpDSA, right T10 injection, anteroposterior projection. The T10 intersegmental artery provides the T11 dorsal component missing from the right T11–T12 trunk (black arrow).

Case 3

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).

Figure 3

Right T9–T8 ipsilateral intersegmental trunk with right T8 isolated intersegmental spinal branch. (A) Spinal digital subtraction angiography (SpDSA), right T9 injection, anteroposterior projection. The right T9–T8 trunk includes intercostal (lateral) branches for both levels (small white arrows) and a dorsospinal artery for T9 (white arrow), but only a dorsal component for T8 (white arrowhead). Note the absence of vertebral blush at T8. (B) SpDSA, right T8 injection, anteroposterior projection. The T8 intersegmental artery continues as the T8 spinal branch, which provides a prominent anterior spinal artery contributor (black arrow). An injection of T8 with the field of view centered lower on the spine was obtained in order to investigate the distal portion of the anterior spinal artery. (C) This injection showed immediate opacification of the perimedullary venous system at the time of the arterial phase, a finding consistent with the diagnosis of perimedullary arteriovenous fistula (Merland type I). (D) Artistic representation of the variant in its anatomic surroundings. The color coding indicates the actual origin of the various components of the right T8 and T9 (green = T9 origin, brown = T8 origin). The same configuration without the presence of an anterior spinal artery contributor corresponds to case No 1.

Discussion

Developmental anatomy

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).

Figure 4

Intersegmental artery (ISA) development in the chick embryo (reproduced from Evans6). Original legend, with modifications in italics: AC, arteria centralis (sulcal artery); I, probable intercostal artery; PC, posterior cardinal vein; RB, ventral radicular branch of the intersegmental artery; S, first extramyotomal or skin branch of the intersegmental artery; SA, intersegmental artery; SCP, spinal ganglion's capillary plexus; SP, superficial capillaries without the myotome; SV, dorsal segmental vein. A simple capillary loop is present at 50 h (A). A capillary network then appears along the lateral and anterior aspects of the neural tube (B, 60 h; C, 78 h). At 116 h (D), a ventral radicular branch has developed, which forms an anastomotic chain along the anterior aspect of the neural tube with vessels coming from adjacent levels. The first central or sulcal arteries are identified (A.C.). S and I indicate the secondary branches that will develop into the spinal and intercostal arteries of the adult ISA configuration.

Figure 5

Branching pattern of intersegmental arteries (ISA). Thoracic ISA evaluation by flat panel catheter angiotomography in two patients; in the left panel, a classic branching pattern is observed, including an aortic stem (arrow), a lateral (or intercostal) branch (arrowhead) and a dorsospinal artery (DA) (grey arrow) that branches off a spinal component (small arrowhead) and a dorsal component (small arrow). In the right panel, the spinal and dorsal branches originate separately from the aortic stem; there is therefore no DA. The fundamental constituents of the ISA are the aortic stem and the spinal branch (see figure 4). The lateral and dorsal musculocutaneous branches develop later; their points of origin from the primitive ISA dictate its adult anatomy and variations. The long arrow shows the site of disconnection in the formation of an isolated DA (left panel) or an isolated intersegmental spinal artery (right panel).

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.

Clinical implications

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.

References

Footnotes

  • 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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