Anatomy, Anterior Spinal Artery

Structure and Function

The ASA supplies the anterior 2/3 of the spinal cord, accounting for approximately 75% of total spinal cord perfusion.[1] The rami of the ASA arise from the intracranial V4 segment of the vertebral artery distal to the origin of the posterior inferior cerebellar artery (PICA). The ASA most commonly originates from the vertebral arteries prior to their anastomosis forming the basilar artery, fuses at the pyramidal decussation, and follows a recurrent course through the premedullary cistern to reach the cervical cord (see Image. Diagram of the Brain Blood Circulation). Caudally, the ASA forms an arch after anastomosing with the posterior spinal artery (PSA) at the level of the filum terminale.

Svensson, Klepp, and Hinder initially postulated that the ASA represents a continuous vessel arising from the vertebral arteries and extending to the filum terminale. Gharagozloo and colleagues later proposed that the ASA is discontinuous and that spinal cord perfusion occurs segmentally via the vertebral, intercostal, and lumbar arteries.[2] A figure from the same plate depicts small blood vessels coursing over the surface of the cervical spinal cord, possibly representing the first published depiction of ASAs and anterior spinal veins.[3]

The ASA most commonly exhibits a bilateral origin, observed in approximately 75% of cases. Bilateral origins can be further classified by the relative caliber of the contributing rami into balanced (approximately 40%), right-dominated, and left-dominated configurations. Unilateral origins and branching from an intervertebral transverse arch also occur.[4] The distance from the apex of the vertebrobasilar junction to the origin of the ASA averages 7.34 mm (± 2.71 mm), whereas the distance from the PICA origin averages 9.02 mm. The mean caliber of the ASA is 1.145 mm (± 0.12 mm), with the ratio of ASA rami to the vertebral artery measuring 0.17.[5] Both the rami and the ASA within the premedullary cord commonly give rise to multiple perforating branches, averaging 3 per segment.

Anatomically, the ASA and its accompanying vein occupy a subpial location, in contrast to the posterior spinal vessels, which reside within the subarachnoid space.[6] The ASA courses beneath the “linea splendens,” a fasciculated and porous region of epipia covering the anterior median sulcus. This structure permits cerebrospinal fluid to permeate from the subarachnoid space into the pia. The true pia mater, or pia intima, adheres tightly to the glia limitans of the spinal cord and extends to the base of the anterior median sulcus.[7]

The spinal artery divides at the level of the intervertebral foramina into ventral, middle, and dorsal branches. The ventral and dorsal branches supply the dura mater and vertebral bone. The middle branch bifurcates to join the anterior and posterior nerve roots and subsequently forms radicular, radiculopial, and radiculomedullary arteries, as described by Tanon. The radicular artery is short, whereas the radiculopial artery reaches the surface of the spinal cord. The radiculomedullary artery supplies the spinal cord subpially over multiple spinal segments.

Sulcal arteries perfuse the anterior gray matter, the anterior portion of the posterior gray matter, and the inner 1/2 of the white column in a centrifugal pattern. Radial perforating arteries, originating from the PSAs and the pial plexus, supply the posterior portion of the posterior gray matter and the outer half of the white column in a centripetal manner.[8] The ASA thus demonstrates a radiculomedullary pattern of arterial contribution, whereas the PSA network exhibits a radiculopial pattern.

Embryology

Gastrulation is the process by which a single-layered blastula develops into a specialized, multilayered structure.[9][10] The resulting trilaminar structure consists of 3 germ layers: ectoderm, mesoderm, and endoderm. The ectoderm gives rise to the embryological origins of the nervous system, known as the neuroectoderm. The neuroectoderm contributes to the formation of the notochord, which induces the development of the neural plate.

The neural plate arises as a thickening of the ectoderm through a process termed "neurulation." This process is regulated by growth factors derived from the notochord and endoderm, which provide a scaffold for spinal support, along with signals from the mesoderm.[11] The neural plate invaginates to form the neural tube, which separates completely from the neuroectoderm during the 3rd week of development. The spinal cord and brain develop from the neural tube.[12][13]

Cells at the margins of the neural tube form the neural crest, which also originates from the embryonic ectoderm. Additional patterning is mediated by the sonic hedgehog (Shh) ligand and the translation of homeotic (HOX) genes. These molecular signals direct the subsequent development of the vertebral column and the segmentation of the body into a 3-dimensional form.

The primitive spine develops segmentally from blocks of mesoderm located on either side of the notochord, arranged as repeating somites, together with the neural tube. This repetitive organization of segments is referred to as "metamerism." Up to 44 somites form between the 3rd and 6th weeks of development. Subsequent regression reduces this number to 31. Paired segmental, or metameric, arteries arising from the dorsal aorta supply the neural tube and all components of the metameres via their dorsomedial and dorsolateral branches.

Spinal vasculature begins to develop at approximately 23 days, nearly coinciding with closure of the cranial and dorsal neuropores at 25 to 27 days. During the first weeks of embryonic life, the neural tube receives nutrition via diffusion from a perineural primitive vascular plexus, the vasa corona, located within the meninx primitiva. As embryonic metabolism increases, longitudinal capillary tracts develop on either side of the median sulcus from connections within the vasa corona. These tracts are termed "ventral longitudinal arteries." Branches arising from the developing cord enter the sulcus and sprout into the vasa corona. Dorsomedial branches of the segmental arteries supply the ventral longitudinal arteries, the future ASA, and the anterior roots.

Posterior longitudinal channels form later on the dorsolateral surface of the neural tube from the vasa corona plexus and subsequently develop into the PSAs. Final development of the PSAs occurs between the 15th and 20th weeks of gestation. The delayed formation results from the ventral longitudinal arteries supplying most of the gray matter, which precedes the formation of white matter tracts. The emergence of longitudinal arteries supplying multiple metameres shifts the segmental blood supply toward the adult spinal arterial pattern.

Craniocaudal midline fusion of the ventral longitudinal arteries within the neural tube occurs after 6 weeks, forming the ASA. Failures of this fusion can manifest as duplications of the ASA, most commonly within the cervical cord. Desegmentation of the segmental feeders occurs concurrently, with regression of most primitive segmental arteries supplying the neural tube. Upon completion, only 4 to 8 ventral segmental arteries supply the ASA, while 10 to 20 dorsal segmental arteries supply the vasa corona. Segmental arteries that persist as radiculomedullary arteries form “hairpin” connections with the longitudinal tracts and, ultimately, with the ASA. Longitudinal anastomoses between the segmental arteries of each metamere develop around the forming spine. The vertebral artery within its canal represents the most prominent of these longitudinal connections.

Vascularization of tubular organs, including the neural tube, follows the Lierse law of angiogenesis. Primitive anterior tracts course centripetally toward a subependymal vascular plexus, which accompanies a zone of cellular proliferation, or matrix, surrounding the central canal. The subependymal plexus largely regresses by the end of the 8th week. A gradually unstructured pattern of internal vascularization develops thereafter.

The primitive ASA forms during this period, notably as the lumbar trunk. Formation of the anterior median fissure results from proliferation and cellular differentiation within the matrix, combined with expansion of the anterior horn region. Kadyi described a specific role for the primitive meninges, termed the "anterior piae matris," in providing supplementary nutrition to the neural tube via extrinsic supply, distinguishing prechoroidal and choroidal stages. Subsequent intrinsic restructuring establishes a central, or centrifugal, supply through sulcal and sulco-commissural arteries, complementing the extrinsic vascular input, as described by Adamkiewicz.

The formation of primitive ASAs follows the fusion of paired longitudinal neuronal arterial axes. Three primary theories explain the transition from paired arteries to a single vessel. The 1st proposes medial displacement and subsequent fusion of the primitive arteries, mirroring the developmental pattern of the primitive aortae. The 2nd describes the paired longitudinal channels as initially forming a rope-ladder-like pattern with transverse midline anastomoses, after which 1 channel predominates and persists.

The 3rd posits that the primary embryonic vascular tract regresses as metabolic demands increase, followed by the transient reappearance of paired longitudinal tracts through vascularization from the secondary spinal meninges, supplying each hemicord. These mechanisms collectively result in architectural reconfiguration toward an unpaired ASA. The embryological centripetal vascular pattern gradually converts to a centrifugal configuration.[14]

Physiologic Variants

The ASA shows substantial variability in anatomical origins. These differences reflect developmental variation in the fusion and persistence of longitudinal arterial axes.

Type I variants of the ASA involve 2 rami that fuse to form a single vessel. Type Ia represents symmetric prespinal rami, each originating from 1 of the 2 vertebral arteries, which fuse at the anterior median fissure to form a single ASA and constitutes the most common variant. Type Ib consists of 2 rami arising from the vertebral arteries that initially fuse but later divide into 2 separate ASAs descending parallel to the pyramids up to the C3 myelomere. Type Ic involves the formation of a vascular arch by the 2 rami of the vertebral arteries, from which 2 separate ASAs subsequently arise.

Type II variants involve a single ASA arising from 1 of the 2 rami. Type IIa refers to 1 ASA originating from a single ramus. Type IIb occurs when both rami form, but 1 is very short and supplies only the ventral surface of the medulla, with the dominant ramus continuing as the ASA. Type IIc describes 1 dominant ramus forming the main trunk of the ASA, with the smaller ramus joining via an end-to-side anastomosis.

Type III variants consist of 2 independent ASAs arising directly from the vertebral arteries and descending separately to the C3 myelomere. Additional variations include a prespinal artery originating at the convergence of the vertebral arteries, while the other arises directly from a vertebral artery or the anterior inferior cerebellar artery.

Anatomical variations of the ASA can confer morphofunctional advantages by improving spinal cord perfusion when an embolus affects only 1 hemicord, as observed in Brown-Sequard syndrome. These same variations may introduce complications during microvascular or endovascular procedures involving the relevant anatomical territories.[15]

Cadaveric studies demonstrate at least 1 dominant anterior thoracic artery—distinct from the artery of the lumbar (artery of Adamkiewicz at T8–L2 in 75% of cases) and cervical (C5–C6) enlargements—in approximately 95% of individuals. Overlying osseous or muscular branches may obscure these arteries, leading to underreporting in spinal angiography studies. This distribution challenges conventional concepts of watershed zones in the thoracic spinal cord and carries significant implications during trauma, ischemic events, and surgical or endovascular interventions.[16]

Occlusion of the vertebral arteries combined with inadequate collateral circulation via the posterior communicating arteries can result in flow reversal within the ASA through the C4 segmental artery. This flow reversal creates collateral channels.[17][18]

Surgical Considerations

The pattern, site, and distance of the ASA origin from the vertex of the vertebrobasilar junction and the PICA exhibit considerable variability. Awareness of these anatomical relationships is critical for endovascular and microsurgical procedures within this region.[19]

Clinical Significance

Etiologies of Anterior Spinal Artery Injury 

Injuries to the ASA may result from a variety of pathological and vascular conditions. Traumatic events can directly disrupt ASA perfusion, leading to ischemia of the anterior 2/3 of the spinal cord.[20] Degenerative spinal pathologies, including spondylosis or disc herniation, can compress or compromise arterial flow. Atherosclerotic disease may reduce luminal diameter and predispose to thrombosis. Cardiac emboli and cardiac arrest can obstruct or reduce ASA perfusion. Systemic hypotension decreases perfusion pressure, increasing susceptibility to ischemic injury.[21]

Aneurysms and pseudoaneurysms may compress or rupture into adjacent ASA branches.[22][23][24][25][26][27] Vasculitis can inflame and occlude the arterial wall. Aortic dissection may extend into the ASA origins, disrupting flow.[28][29] Arteriovenous malformations create steal phenomena, diverting blood away from the ASA.[30] Fibrocartilaginous embolism may occur following intervertebral disc injury.[31][32] Kyphotic deformities combined with posterior osteophytes can mechanically compromise arterial patency.[33] Arterioectatic spinal angiopathy, sickle cell disease, hypercoagulable states, and cocaine abuse also contribute to ASA injury through vascular occlusion, embolism, or vasospasm.[34]

Iatrogenic injuries to the ASA arise from clinical procedures. Spinal anesthesia can directly puncture or compress the ASA branches.[35] Digital subtraction angiographic procedures or microvascular and endovascular interventions in the proximal vertebrobasilar system carry a risk of arterial disruption. Aortic repair surgery may compromise the ASA origin or segmental feeders.[36][37] Kyphoplasty with retropulsion of bone fragments and spinal deformity correction surgeries can mechanically injure the artery or its radiculomedullary branches.[38][39][40]

Complications of ASA injury include ASA syndrome, characterized by motor paralysis and selective sensory deficits (see Image. Clinical Features of Anterior Spinal Artery Syndrome).[41] Acute onset of back pain localized to the level of spinal involvement is a typical finding. Bilateral motor weakness and sensory deficits occur below the affected segment due to compromise of the bilateral corticospinal and spinothalamic tracts. Proprioception, vibratory sense, 2-point discrimination, and fine touch, mediated by the dorsal columns, remain intact because these structures are supplied by the PSAs. Involvement of the lateral horns between the T1 and L2 levels can produce autonomic dysfunction.[42]

The Ondine curse is a form of central hypoventilation characterized by failure of automatic respiration. Cervical ASA compromise can disrupt phrenic nerve function and precipitate this condition.[43]

Imaging of the Anterior Spinal Artery

Spinal angiography remains the gold standard for visualizing the ASA, but the procedure is invasive and requires dedicated, trained personnel. The normal cervical ASA, its continuity, and radiculomedullary feeders is often visualized using magnetic resonance angiography, including maximum-intensity projection and 3-dimensional fast low-angle shot techniques. Limitations of this modality include prolonged acquisition times and difficulty distinguishing the ASA from the anterior median vein.

The hallmark radiological finding of ASA syndrome is hyperintensity involving the anterior 2/3 of the spinal cord on T2-weighted magnetic resonance imaging (MRI) axial images.[44] Peripheral and posterior regions of the cord are relatively spared due to collateral perfusion from the vascular pial plexus and PSAs.[45][46] Paired anterior-horn T2-hyperintensities, termed "owl's eyes," "snake eyes," or "snake bite sign," are also characteristic.[47] These findings reflect cell loss in the anterior horns accompanied by cystic necrosis at the junction of the posterior ventrolateral column and the central grey matter.[48]

A thin, pencil-like hyperintensity may extend vertically across multiple spinal segments in sagittal views. Dense edema can produce a "white cord" sign.[49] MRI can further assist in identifying the underlying cause of ASA syndrome. Differential diagnoses include transverse myelitis, complete spinal cord transection from severe cord edema, central cord syndrome, Guillain-Barré syndrome, Brown-Sequard syndrome in the context of vascular variations, multiple sclerosis, compressive myelopathies, and spinal vasculopathies such as arteriovenous malformations.