Transradicular lumbar disc herniation: An extreme variant of intraradicular disc herniation.

Intradural or intraradicular lumbar disc herniation (IDH) is a relatively rare condition often diagnosed intraoperatively. We encountered an extreme variant of IDH - a transradicular herniation as the disc material extruded through the lumbar nerve root through a split essentially transecting the nerve root. While failure to recognize intradural and intraradicular disc herniation can lead to failed back surgery, the variant described in the present case could lead to iatrogenic injury and complication if not recognized. A unique case of transradicular lumbar disc herniation in a 25-year-old patient is presented with the depiction of intraoperative images supplementing the text.

INTRODUCTION

Lumbar disc herniation (LDH) is one of the most common conditions seen and treated by spine surgeons. Intradural lumbar disc herniation (IDH) though unusual, is a well-described entity with diagnosis often made during surgery.1234 We present an extreme variant of LDH which we named as transradicular disc herniation. To the best of our knowledge, this presentation has not been described before.

CASE REPORT

A 25-year-old woman presented with low back pain for 2 years treated with a number of selective L4/L5 transforaminal steroid injections and physical therapy with good relief. She had sudden exacerbation of pain in the left buttock and leg for a month, which was refractory to conservative management. Neurological examination revealed presence of left sided extensor hallucis longus weakness (4/5) with numbness involving left L5 dermatome. Magnetic resonance imaging (MRI) demonstrated a left L4/L5 disc herniation [Figure 1]. Due to severe unrelenting pain, minimally invasive left L4/L5 microdiscectomy was performed. After performance of the left sided laminotomy with resection of the ligamentum flavum and identification of the dura, a disc fragment was seen extruding through what was considered an annulus defect [Figure 2, left]. However, on further cephalad exposure with delineation of the anatomy, what was initially considered to be the annulus turned out to be the nerve root and the disc was seen herniating through a split in the left L5 nerve root essentially transecting the nerve root longitudinally [Figure 2, right]. There was no obvious cerebrospinal fluid (CSF). All herniated disc fragments were removed, and it was ensured that the nerve was free at the end of the procedure [Figure 3]. No attempted to repair any dura was made as there was no CSF leak. The patient improved right after surgery and there was no recurrence of symptoms at 1 year followup.

Figure 1

T2W sagittal (left) and axial (right) magnetic resonance imaging of the lumbosacral spine demonstrating a L4/L5 left sided disc herniation with significant mass effect on the thecal sac and severe left neural foraminal narrowing

Figure 2

Intraoperative photographs showing disc fragment extruding through what was considered an annulus defect [Figure 2, left]. The right sided photograph demonstrates the presence of disc herniating through a split in the left L5 nerve root - interradicular disc herniation

Figure 3

Intraoperative photograph following removal of herniated disc fragments with nerve root being seen at the end of the procedure

DISCUSSION

Intradural lumbar disc herniation is a rare clinical entity, generally detected during surgery and reported most often in the lumbar spine.1234 The most widely accepted mechanism for it is supposed to be adhesion between the ventral dura and posterior longitudinal ligament (PLL) leading to subsequent perforation of these firmly adhesive tissues with the increased intradiscal pressure leading to intradural migration of the disc.2567 Mut et al.3 classified this unusual variants of IDH into two types: (1) Type A: Herniation of a disc into the dural sac, (2) Type B: Herniation of a disc into the dural sheath in the preganglionic region of the nerve root which can also be named as true intradural and intraradicular disc herniations, respectively. Though, on the myelographic examination, IDH can be suspected based on total or subtotal block, it is difficult to differentiate IDH from other intradural mass lesion of the lumbar region, including neurofibroma, meningioma, lipoma, epidermoid and arachnoid cyst, metastases, and even large extradural disc herniation.4 Similarly, gadolinium-enhanced MRI can show rim enhancement of the lesion coupled with discontinuity of the PLL.24 The pathogenesis of intraradicular variant of LDH is less understood, but a cadaveric study by Spencer et al.8 demonstrated the existence of dural ligaments fixing the dura and the nerve root at their exit from the main dural sac, to the PLL and vertebral body periosteum, which in certain cases can cause increased nerve root fixation, allowing penetration of a ruptured disc directly into the root leading to an IDH. A congenital weakness in the radicular sheath has also been attributed to the development of intraradicular disc herniation in some cases.9 Regardless of the pathogenesis and attempt to diagnose these variants on imaging findings, most of the cases of intradural or intraradicular disc herniations are diagnosed only during surgery.1234567810 It is important to be suspicious of these as unrecognition of these variants can lead to the unsuccessful surgery, which can be falsely labeled as failed back syndrome.2511 Poor correlation between the radiology and intraoperative findings in the presence of continued symptoms, absence of an extradural disc in presence of clearly visualized disc fragment on MRI despite being at the correct level or finding of a swollen nerve root during surgery are indicators toward the presence of intradural/intraradicular disc herniations.124611

Our case would be classified as a transradicular disc herniation as the disc fragment was not entirely contained within the nerve root, but was visible through a defect in the nerve root essentially transecting the nerve root. The plausible pathogenesis for this might be the same as for intraradicular disc herniation, but failure of the disc herniation to be contained within the nerve root, but its herniation through the dorsal aspect of the root manifesting as transradicular disc herniation. Hence, a transradicular LDH might be an extreme variant of intraradicular disc herniation with similar pathogenesis, but different spectrum of intraoperative manifestation. Though a direct role of prior transforaminal injections cannot be implicated as the nerve root involved in this case was the traversing root at L4/L5 level, which is very unlikely to be punctured by a L4/L5 transforaminal injection, [Figure 4] it might be plausible that fibrosis secondary to the prior injections might have predisposed to the development of an intraradicular disc herniation, which subsequently herniated through the dorsal aspect of the nerve root resulting in a transradicular disc herniation. Even though, the exact pathogenesis is conjectural, the identification of this variant cannot be overemphasized to obviate iatrogenic nerve root injury during lumbar microdiscectomy. In the present case, on initial identification of the disc, the nerve root was mistaken with the annulus through which the disc was considered to be protruding out and was about to be incised. But since, we could not find the top of the annulus, we tried delineating the anatomy further and found that the disc was actually herniating through a split in the nerve root and the annulus was visualized later after removing the disc fragment allowing the nerve root to be mobilized medially with visualization of the annulus and actual disc space. Another interesting feature was that there was no CSF visualized during the procedure, which may be attributed to the possible fibrosis of the nerve root defect through which the extruded disc fragment herniated.

Figure 4

Schematic diagram showing the course of exiting and traversing nerve roots demonstrating that the nerve at risk during a L4/L5 transforaminal injection would be the exiting L4 nerve root as against the traversing L5 nerve root

The present case is reported due to an atypical intraoperative transradicular disc herniation, which could be a potential cause of complications following lumbar discectomy and every attempt should be made to expose all anatomical structures carefully during performance of lumbar microdiscectomy remaining cognizant about these variant. With increasing application of minimally invasive lumbar discectomy with narrow operative exposure, the potential of failure to recognize this unusual variant can be high if not appropriately recognized by clear depiction of the intraoperative normal anatomy.