Epidural steroid injection has been used for treating pain due to lumbosacral radiculopathy.1 The injections can be delivered into interlaminar, caudal, and transforaminal locations. Because of its advantage of selectively injecting drugs into a specific nerve root, transforaminal injection method is preferred.1,2 There are 3-4 types of well-known approaches in the transforaminal injection method, and among those, the most used is the infrapedicle transforaminal injection which has the injection needle approach the safe triangle below the underside of the pedicle so that contrast dye usually spreads to L4 root in case of L4-5 level, and if the L5-S1 level, mostly spreads to L5 root.3 Preganglionic approach accesses the suprapedicle level, and contrast dye is known to spread over to L5 root in case of L4-5 level, and to S1 root in case of L5-S1 level.4

In the case of infrapedicle approach, there are numerous cases where the injected drug would flow into the cephalic root of the nerve root, and in the case of severe foraminal stenosis, in many cases the injection needle is unable to reach the desired location after penetrating the intervertebral foramen.5 Furthermore, with regard to infrapedicle approach, there are even cases where drugs cannot be injected into the epidural and preganglionic part of the supraadjacent intervertebral disc.4 At that point, a retrodiscal (RD) approach can be used as an alternative method.

In determining the level of retrodiscal (RD) approach, it is critical to know to which nearby nerve root the contrast dye spreads. Current research is not clear on spread patterns of RD EPB contrast dye.

In this study, the authors investigated the spreading pattern and location of contrast dye and the clinical features of the transforaminal epidural block according to the retrodiscal (RD) approach, relatively unknown to the present.

Transforaminal epidural injection method has an excellent curative effect in treating chronic lumbar nerve root pain syndrome, and has been used extensively in a clinical setting due to its satisfactory effect in the long term.6

When transforaminal epidural injection method is performed, the contrast dye must spread over to the proximal region along the anterior lateral part of a specific nerve.7,8,9 However, a considerable amount of the drug injected via infrapedicle approach sometimes flows into the distal region along the spinal nerve, and it becomes difficult to insert the injection needle in the case of severe spinal stenosis when the needle is supposed to be located at the region more anterior to the intra-transforaminal epidural nerve root.10

As an alternative to this, the retrodiscal (RD) approach can be applied. The researchers have concluded that the epidural block by L5-S1 retrodiscal (RD) approach can reduce the stimulation to the nerve root during the procedure compared to the infrapedicle approach. This is a helpful method especially when the disc sequestrum is stuck in the medial intervertebral foramen or when spinal stenosis is severe.10

With regard to determining the level of RD approach, it is very important to be aware of which nearby nerve roots the contrast dye is spreading to. However, studies done on which nerve root the drugs spread to are insufficient. The authors found that the contrast dye mainly spreads over the upper nerve root regardless of the level of the procedure when performing a procedure with the RD approach (Table 2). This is likely due to the anatomical structure of the spinal epidural space. Although contralateral spread of the contrast dye was not observed in this study, the authors noted that in a considerable number of cases, the contrast spread to even the nerve roots below the level of the targeted nerve root apart from the targeted nerve root. When over 0.5 ml of contrast dye was injected, the contrast can spread to more than one of the nearby (superior, inferior, and/or contralateral) nerve roots.7,11 Because 1.0 ml of contrast dye was administered in this study, it was observed that the contrast spread to not only the targeted nerve roots but also to the nerve roots below the level of the targeted nerve roots.

The authors found that the contrast dye in the RD approach usually spread to the proximal target nerve root (40/63, 63.5%) (Table 3). In case of the contrast dye spreading to the distal root, the authors observed that the resulting changes in pain (VAS change) before and after procedure was less compared to the patients with a spread to the proximal root. This is attributed to the inability of the contrast dye to rise to the proximal nerve root and its flowing down to the distal root following the spinal nerve by the synechia of epidural space including the root sleeve.

This study has its limitations. First, when recruiting the subject group for this study, all patients with severe spinal stenosis or herniated intervertebral disc were included, and because some patients with severe synechia of epidural space were included among them, there were many cases where the injected drugs spread to the distal nerve root. Therefore, if a study is conducted in the future to judge the effect of the contrast spreading pattern on pain reduction, it should be carried out after excluding patients with severe spinal stenosis or severe herniated intervertebral disc. Second, the level of pain was recorded with VAS before the procedure and two weeks after the procedure. It seems that using another scale besides VAS would be helpful to enhance the objectivity of the results. Furthermore, the mid- to long-term effect of the procedures could be identified if follow-up is conducted not only two weeks but also six months or more after the procedure.

When performing lumbosacral transforaminal epidural block by the retrodiscal (RD) approach, the contrast dye mostly spread to the cephalic root regardless of the level of the procedure. The pain-reducing effect two weeks after the procedure was better when the contrast dye spread over to the relevant proximal nerve root rather than to the distal root.