Prior to the injection procedure, review of imaging studies is useful to evaluate pathological changes (e.g., stenosis) that may mitigate against a given injection site or favor a specific location (disc degeneration that correlates to the patient's symptoms). A pain diagram is completed by all patients so that a graphic depiction of pain distribution is available. After the patient has been placed in a prone position, fluoroscopic imaging is performed to optimize needle placement, based on the osseous anatomy. A multidirectional, high-resolution C-arm is preferred for flu-oroscopic localization so that the image intensifier and x-ray tube can be manipulated to optimize the trajectory without the need to change the patient's position. The target anatomy is identified on the skin by using a radiopaque marker (or intrinsic light laser source), with the C-arm unit oriented 20 to 30° caudal and lateral to the midline. After the skin entry site has been marked (usually with a small skin impression or marker), a wide area is prepped and draped in sterile fashion. Under intermittent fluoroscopic guidance, a 22-gauge spinal needle with a beveled tip is advanced to the epidural space by a dorsal, oblique, paramedian approach (Figure 9.1). The needle is advanced to the superior margin of the spinal lamina, immediately subjacent to the interlaminar gap. Fluoroscopy is performed intermittently to monitor the position of the needle as it is advanced.
After contact with the superior laminar margin, the needle is withdrawn slightly, and the bevel is oriented caudally. The needle tip is
Figure 9.1. Needle placement for interlaminar lumbar epidural injection. After contact with the lamina, the needle is guided superiorly into the intralaminar gap through the ligamentum flavum into the dorsal epidural space.
then guided over the lamina, through the ligamentum flavum, and into the dorsal epidural space at the midline. There is increased resistance as the needle traverses the ligamentum flavum. One should proceed cautiously upon encountering this structure, with incremental advancements of the needle interspersed with fluoroscopic visualization of needle position. When the negative resistance of the epidural compartment is encountered, the contrast agent will easily advance into the epidural space (Figure 9.2). A small air bubble in the tubing adjacent to the needle hub may facilitate this determination, but it is important not to inject a large volume of air into the epidural space, even though it is generally well tolerated in this compartment. If the needle tip is within an artery, there is potential for arterial gas embolism, a rare but serious complication.32 This should be avoidable by using realtime epidurography.
A variation of this technique uses a so-called epidural needle, with a tapered, rounded tip and a side hold. The epidural needle may be advanced directly toward the interlaminar gap. Although this technique does avoid contact with the periosteum (which occasionally may be painful), it does not provide the depth control that is gained from contact with the lamina. After needle placement and negative aspiration for CSF, 4 to 6 mL of nonionic contrast is injected, under direct fluoroscopic observation. Images are obtained to document epidural distribution of the injectable and to exclude subarachnoid injection due to inadvertent dural puncture, before injection of therapeutic substances.33
Epidurography is performed with nonionic iodinated contrast that is approved for myelography. This renders an inadvertent thecal puncture essentially harmless. A volume of contrast medium is injected sufficient to achieve dispersal within the epidural space and to reveal the presence of adhesions, loculations, and even spinal canal stenosis. This provides important anatomical information and may explain a limited or compartmentalized block caused by limited distribution of the in-jectate.34 Filming is performed in at least two planes, typically anteroposterior (AP) and lateral projections. Transforaminal epidurograms may be filmed by using an oblique projection. An oblique projection also is useful for cervical and occasionally thoracic epidurograms, where lateral projections are often suboptimal because of adjacent structures with markedly disparate densities.
After filming, the therapeutic substances are injected through the same needle without a change of position. Typically, a water-soluble steroid preparation (2-3 mL of betamethasone preparation or equivalent steroid dosage) is injected, followed by an injection of 3 to 5 mL of 1% lidocaine or 0.5% bupivacaine. Filming is repeated after the injection of therapeutic substances. The postinjection films document distribution of the injectate. These images may provide an explanation of a compartmentalized result. For this reason, the author does not perform a limited epidurogram (<4 mL of contrast). The contrast study is filmed and interpreted, with films documenting before and after the installation of therapeutic materials. These films are retained as part of the patient's medical records.
Figure 9.2. (A) AP radiograph following needle placement and injection of nonionic contrast media reveals opacification of the epidural space cephalad and caudal to the injection site. (B) Lateral radiograph demonstrates contrast within the dorsal and ventral epidural compartments during injection.
Figure 9.2. Continued. (C) Lateral radiograph following injection of therapeutic mixture reveals further dispersal of the previously injected contrast circumferen-tially within the epidural compartment.
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