Introduction. Transforaminal injection of steroids has been associated with infrequent but devastating neurological complications and death. Direct injection of particulate steroids into the vertebral artery or medullary arteries is the leading theory in the majority of these complications. Practice guidelines have been published to minimize the likelihood of directly encountering vulnerable arterial structures. However, retrograde flow into a vertebral or medullary artery has not been considered in the literature.
Case. This case demonstrates retrograde flow into a common arterial trunk with subsequent antegrade flow of intravenous contrast into a thoracic spinal artery during thoracic transforaminal injection.
Discussion. Antegrade flow of particulate steroids through direct cannulation of a vertebral or medullary artery has been advocated as one explanation for complications involving brain or spinal cord infarction. The possibility of retrograde flow into a common arterial trunk with subsequent antegrade flow into vulnerable arteries should also be considered as a possible mechanism by which embolic spinal cord or brain injury may occur.
Conclusion. Retrograde flow into medullary or vertebral arteries without direct cannulation can occur, and provides an alternative mechanism of potential injury to the spinal cord or brain during transforaminal injections.
Transforaminal injections of steroids have been implicated in causing serious neurological complications and death. Most incidents have occurred after cervical injections [1–7] but some have resulted from lumbar injections [7–9]. Although thoracic transforaminal injections are practiced , only one case report concerns a complication following T12-L1 injection .
Various conjectures have been raised to explain the mechanism of injury. Direct injection into a vertebral artery or medullary artery may occur if needles are placed ventrally in the intervertebral foramen. Circumstantial evidence currently favors arterial embolization of corticosteroid preparations as the mechanism of neurological injury. However, even when needles have been correctly placed dorsally in the foramen, inadvertent intra-arterial injection has been demonstrated in the course both of cervical and of lumbar transforaminal injections [6,12,13].
The assumption in the prevailing model of injury is that, when steroids are injected into an artery, the embolic material flows orthograde along the artery (i.e. downstream). Retrograde flow has not been considered in the literature, ostensibly because operators believe that arterial pressure would ensure downstream flow of any injectate. The following case demonstrates an example of retrograde flow into a common arterial trunk with secondary antegrade flow into a thoracic medullary artery. The phenomenon observed allows for a modified mechanism of injury involving reinforcing radicular arteries or vertebral arteries.
In a 35-year-old female with a left paracentral T6-7 disc protrusion and left-sided thoracic radicular pain, a left T6-7 transforaminal injection was undertaken using a posterior parasagittal approach following established guidelines . Under multi-planar, real-time fluoroscopic guidance, the needle was placed dorsal, and very slightly caudal, to the dorsal root ganglion of the left T6 spinal nerve, just outside the intervertebral foramen (Figure 1a). Upon injection of a test-dose of contrast medium, appropriate peri-radicular spread was observed, but what appeared to be opacification of a thoracic spinal artery was evident on anterior-posterior fluoroscopy (Figure 1b). Digital subtraction angiography confirmed that this was an artery, with rapid pulsatile washout of injected contrast (Figure 1c).
When digital subtraction angiography (DSA) was repeated, additional features emerged. On the lateral view, a small artery filled in a retrograde direction, starting from a caudal and dorsal position in the intervertebral foramen, and continuing ventrally across the lateral aspect of the vertebral body into an intercostal artery (Figure 2a). Subsequently, the contrast medium continued to opacify the intercostal artery, in a retrograde direction toward the thoracic aorta, but also filled a spinal artery in an orthograde direction toward the ventral and cephalad margin of the intervertebral foramen (Figure 2b). Subsequent lateral DSA imaging demonstrated filling of an artery in the ventral spinal canal, consistent with a medullary artery (Figure 2C). This latter vessel was distinctly separate from the original artery that was filled.
The procedure was terminated, and the patient suffered no ill effects.
Dommisse has cautioned that the intervertebral foramina represent a “major arterial distribution point” and are of “critical value in the blood supply of the cord”. Alleyne's microscopic dissections of the great medullary artery of Adamkiewicz illustrated arterial branches dorsal to the spinal nerves, continuous with the Artery of Adamkiewicz through a common trunk . This case demonstrates that retrograde flow can occur within arteries following transforaminal injections. Furthermore, injectate can subsequently flow in an antegrade direction within a spinal or medullary artery that was not directly cannulated by the needle (Figure 2d). Although this phenomenon was observed at a thoracic level, it raises an intriguing possibility if applied to cervical levels.
It would seem feasible that a cervical transforaminal injection could back-fill a radicular (or other) artery and enter a vertebral artery. Subsequently, injectate could flow in an antegrade direction either into a nearby medullary branch of the vertebral artery or along the full course of the vertebral artery into the brainstem. If embolic material were so injected it could cause infarction of the spinal cord or of the brainstem. Embolic events in the territories supplied by the vertebral artery or a medullary artery, therefore, could occur without the artery itself actually having been punctured.
Physicians have already been urged to be alert to possible antegrade injection of a medullary artery or the vertebral artery [5,10,12,16]. The present case calls on them also to be alert to possible retrograde filling to a common arterial trunk.