Minimally invasive treatment of far lateral lumbar disc herniation: Selective nerve root block with percutaneous transforaminal endoscopic discectomy

Abstract

BACKGROUND

Far lateral lumbar disc herniation (FLLDH) is a special type of lumbar disc herniation with high rate of missed diagnosis. Selective nerve root block (SNRB) has special advantages in identifying the responsible nerve root. Percutaneous transforaminal endoscopic discectomy (PTED) is a minimally invasive and effective method to treat FLLDH. However, no report has investigated PTED combined with SNRB to treat FLLDH.

AIM

To explore the diagnosis and treatment process, surgical technique and clinical efficacy of PTED combined with SNRB to treat FLLDH.

METHODS

This is a multicenter center, retrospective, observational study. Between January 2020 and January 2022, 32 patients were initially diagnosed with FLLDH. All the patients were identified using SNRB to determine the responsible segment and involved nerve roots. Because of poor symptomatic control following SNRB, 2 patients were excluded. 30 patients diagnosed with FFLDH underwent PTED. The clinical characteristics, operative and postoperative outcomes, complication and subsequent follow-up were collected.

RESULTS

30 patients who underwent SNRB combined with PTED were followed up. The average visual analogue scale (VAS)-leg score, VAS-back score, Oswestry disability index (ODI) score at the Follow-up (1 day, 1 month, 3 months and last follow-up) were significantly different compared per-operation. According to the modified Macnab efficacy evaluation standard, the satisfaction degree at the last follow-up was excellent (28, 93.33%), good (1, 3.33%), medium (1, 3.33%) and poor (0, 0%).

CONCLUSION

SNRB provides an effective method for the definite diagnosis of FLDH and responsible nerve roots. Combination therapy offers several advantages including minimal invasiveness, precision, effectiveness, safety and low recurrence rates.

Key Words: Far lateral lumbar disc herniation; Selective nerve root block; Percutaneous transforaminal endoscopic discectomy; Diagnosis; Minimally invasive treatment

Core Tip: This study assessed percutaneous transforaminal endoscopic discectomy (PTED) combined with selective nerve root block (SNRB) for far lateral lumbar disc herniation (FLLDH) in 30 prospectively recruited patients. The clinical characteristics, operative and postoperative outcomes, complication and subsequent follow-up were collected. These results suggest that SNRB provides an effective method for the definite diagnosis of FLDH and responsible nerve roots. SNRB combined with PETD for treating FLLDH offers several advantages including minimal invasiveness, precision, effectiveness, safety and low recurrence rates, thus warranting further investigation.

INTRODUCTION

Far lateral lumbar disc herniation (FLLDH) refers to prolapsed intervertebral disc tissue located inside or outside the foramen[1]. FLLDH was first reported by Abdullah in 1974[2], which is a special type of lumbar disc herniation (LDH) with severe clinical symptoms and a low incidence. FLLDH accounts for 2.6%-11.7% of all LDH and most commonly occurs at L4/5, followed by L3/4 and L5/S1[3]. The incidence of FLLDH is low, while the misdiagnosis and missed diagnosis are high approximately 30%[4,5]. FLLDH has atypical features on magnetic resonance imaging (MRI) and computed tomography (CT), and clinicians are accustomed to focusing on compression in the spinal canal while ignoring the conditions of the intervertebral foramen and extraforaminal areas, leading to the high misdiagnosis rate of FLLDH[6,7]. FLLDH has severe clinical symptoms, a low incidence, and a high clinical missed diagnosis rate and the responsible nerve roots are challenging to clarify. Therefore, a method for definite diagnosis is urgently needed. Selective nerve root block (SNRB) is an effective method to diagnose the segment responsible for lumbar disc herniation[8,9]. When patients have multiple level spinal stenosis and complex clinical manifestations, SNRB becomes an effective method to diagnose the segment responsible for LDH. SNRB is an effective method to clarify the responsible segments and nerve roots of LDH, especially for FLLDH. However, there are few related reports so far. Treatment of FLLDH includes conservative and surgical treatment. Traditional nonfusion surgery, such as fenestration of the intervertebral disc requires sufficient exposure of the intervertebral foramina or intervertebral disc, inevitably destroying one side of the articular process and causing lumbar instability. Lumbar interbody fusion surgery for FLLDH is another commonly used surgical method[10]. There are also some disadvantages include large surgical trauma, lower back muscle atrophy, and adjacent spondylosis[11,12]. But percutaneous transforaminal endoscopic discectomy (PTED) offers benefits such as minimized trauma, accelerated recovery and no impact on lumbar spine stability[13]. PTED is currently considered the preferred surgical approach to treat FLLDH[14-16]. No relevant report exists on the treatment of FLLDH with PTED combined with SNRB. From January 2020 to January 2022, we diagnosed and located the responsible segments and nerve roots in 32 patients with FLLDH who had undergone SNRB. 30 SNRB-positive patients had undergone PTED surgery. This study explored the diagnosis and treatment process, surgical technique and clinical efficacy of PTED combined with SNRB to treat FLLDH.

MATERIALS AND METHODS

Study design and patients

This is a multicenter center, retrospective, observational study. The study was approved by the ethical committee of the Shanghai Changzheng Hospital and the ethical committee of Shanghai Tongren Hospital. Additionally, the patients provided their written informed consent to participate in this study. Between January 2020 and January 2022, 32 patients who were initially diagnosed with FLLDH by symptoms and imaging (CT or MRI) were comprised into the group. All patients showed low back pain and/or radicular pain to the lower limb and underwent SNRB before operation. 30 patients with positive SNRB results had received PTED (Figure 1). According to the relationship between the location of the patient's intervertebral disc herniation and intervertebral foramina, three FLLDH types were identified including the intraforaminal type (type I), intervertebral foramen external type (type II), and compound type (type III) (Figure 2).The clinical characteristics, FLLDH types, postoperative outcomes, complication and subsequent follow-up were collected. The inclusion criteria were as follows: (1) Diagnosed with a single-segment extreme lateral lumbar disc herniation by X-ray, CT and/or MRI; (2) Had received more than 3 weeks of conservative treatment for which the effect was poor; and (3) PTED as follow-up treatment after symptoms were relieved by 70% or more by the nerve root block. The exclusion criteria were as follows: Patients with (1) Multiple level lumbar disc herniation, lumbar spinal stenosis and other types of lumbar disc herniation; (2) Lumbar spine infection, deformity, bony lumbar spinal stenosis or a tumor; (3) Previous history of lumbar surgery; (4) Severe hepatic and renal dysfunction; (5) Cardiac and pulmonary failure; (6) Blood coagulation dysfunction and cognitive dysfunction; (7) neurological diseases and other severe diseases; and (8) Acute infection.

Figure 1 Diagnosis and treatment process. FLLDH: Far lateral lumbar disc herniation; SNRB: Selective nerve root block; PTED: Percutaneous transforaminal endoscopic discectomy.

Figure 2 According to the relationship between the location of the patient's intervertebral disc herniation and intervertebral foramina. A and B: The three far lateral lumbar disc herniation types were the intraforaminal type (type I); C and D: Intervertebral foramen external type (type II); E and F: Compound type (type III).

SNRB technique

According to the patient's symptoms, signs and imaging data, the suspicious responsible segment and possible involved nerve roots were initially identified. Next, we formulated the following puncture plan as follows: The most suspicious responsible nerve root was blocked first. If the remission rate was ≥ 70%, it was considered the responsible nerve root. If not, other nerve roots were chosen to block until the responsible nerve root was clear. If there were more than 2 suspicious responsible nerve roots and the order could not be determined, the block operation was performed in the principle of ‘lower nerve root first and higher nerve root second’. The patient was placed in the prone position. Under fluoroscopy guidance of the C-arm machine, the 18G puncture needle was inserted percutaneously into the upper 1/3 of the outer edge of the intervertebral foramen, reaching the nerve root and avoiding the spinal canal. Additionally, 0.5 mL of iohexol contrast agent was administered for export root radiography positioning. After that, 0.5 mL of 0.5% lidocaine was injected to block the nerve root. If the remission rate was more than 70%, the responsible nerve root could be clearly identified; if not, another nerve root was chosen to block (for S1 nerve root block, sacral 1 nerve root foramen block) until the involved nerve root was clear (Figure 3).

Figure 3 Standardization of selective nerve root block. A and B: Location of the puncture needle using an X-ray; C and D: Nerve root imaging after the injection of 0.5 mL of iohexol contrast agent around the root.

PTED technique

All surgeries were performed by 2 senior surgeons in the 2 hospitals. During the FLLDH operation, the corresponding involved nerve roots were decompressed according to the location of the involved nerve roots in the SNRB. The patient was placed prone on the operating table. Before the operation, the midline of the spinous process of the lumbar spine, the horizontal line of the intervertebral disc segment, the contour line of the iliac bone, and the outer edge of the upper and lower facet joints were located under the assistance of the C-arm. After routine disinfection and draping, 0.75% lidocaine was used for local infiltration anesthesia, and sedatives were used for emotionally stressed people. The site and puncture pathway of the puncture needle were marked according to the surgical segment, target, and patient’s position. Under the guidance of the C-arm machine, the working channel was established. The following operations were performed under the endoscope. First, we explored the area of the Kambin triangle, removed the dorsal ligamentum flavum and the ventral herniated intervertebral disc of the walking nerve root. After decompression of the walking root was completed, the wedge-shaped opening of the surgical channel was turned to the upper and/or outside of the intervertebral foramen, then upper nerve root was explored. The herniated nucleus pulposus around the outlet root was removed. Nerve root pulsation was evaluated by the water pressure. After the nerve root was relaxed, 1 mL of betamethasone (2 mg) was injected around the nerve root. The working channel was pulled out, and the wound was sutured (Figure 4). The patient performed straight leg elevation and low back muscle function exercise until half a year after the operation. The patient avoided strenuous exercise, waist flexion and weight lifting within 3 months. The operation time, hospital stays, infection, intraoperative and postoperative complications were collected.

Figure 4 A typical case of selective nerve root block combined with percutaneous transforaminal endoscopic discectomy for treating far lateral lumbar disc herniation. A56-year-old man was admitted to the hospital with radiating pain in his leftlower limb for 3 months. The diagnosis was far lateral lumbar disc herniation, and the responsible segment and nerve root were the L4/5 and L4 nerve roots. A: The X-ray positive and lateral position of the lumbar spinebefore operation; B: The X-ray hyperextension and flexion position of thelumbar spine before operation; C and D: Magnetic resonance imaging and computed tomography before the operation; E and F: The patients undergoing selective nerve root block before surgery; G and H: X-raypositive and lateral images of channel placement during percutaneous transforaminal endoscopic discectomy operation; I: The microscopic images of the outlet nerve root, walking nerve root and intervertebral space during theoperation; J: The outlet nerve root after decompression; K: The walking nerve heel afterdecompression; L: The nucleus pulposus tissue removed during the operation.

Clinical efficacy evaluation

The patient's clinical symptoms were evaluated before the operation, 1 day, 1 month, 3 months after the operation, and at the last follow-up. The VAS score was used to evaluate the low back and leg pain. The ODI score was used as a clinical objective evaluation standard[1]. The modified MacNab criteria[9] were used to evaluate the efficacy of surgery. The evaluation grades were excellent, good, moderate, and poor.

Statistical analysis

All statistical analyses were performed using SPSS 20.0 and GraphPad Prism 7.0 software. Continuous variables were expressed as means ± SD and categorical variables are summarized as frequencies. For continuous variables, group means were compared using a one-way ANOVA for more than two groups. Analysis for categorical variables was performed using the χ² test. P < 0.05 indicates that the difference was statistically significant.

RESULTS

Location of responsible segment

Thirty-two patients were included into the group. Twenty-five patients (type I, 10 cases; type II, 13 cases; type III, 2 cases) had a remission rate ≥ 70% by a single nerve root block that made a definitive diagnosis and identified involved nerve roots. Five cases (type I, 2 case; type III, 3 case) had a definitive diagnosis for which nerve roots were involved by blocking the outlet nerve root and walking nerve root. 2 patients were excluded due to the inability to establish a clear diagnosis after SNRB (remission rate ≤ 50%) (Figure 1). 30 patients aged 25-75 years (average age, 51.4 years) with a 2-18 month (average, 6.6 months) duration of symptoms before surgery were included. According to FLLDH types, the 30 patients were divided into three groups (type I, 12 cases; type II, 13 cases; type III, 5 cases). There were no significant differences in age, gender, BMI, preoperative course, surgical segment, responsible nerve root, preoperative VAS and ODI score among the three groups (Table 1, P > 0.05). The responsible segments included L3/4 (9.30%), L4/5 (1343.33%), and L5/S1 (8, 26.67%). The responsible nerve roots included 9 L3 nerve roots, 16 L4 nerve roots, 9 L5 nerve roots, and 1 S1 nerve roots (a total of 35 nerve roots) (Table 1).

Table 1 Baseline clinical characteristics and demographic data of 30 patients, n (%).

FLLDH type	I	II	III	P value
No. of patients	12	13	5
Age, years	50.167 ± 16.503	49.308 ± 13.493	59.60 ± 9.182	0.3788
Male sex	5 (41.67)	9 (69.23)	3 (60)	0.3757
BMI	22.952 ± 1.496	24.512 ± 2.953	25.504 ± 2.222	0.1014
Preoperative course	7.583 ± 5.230	6.0 ± 5.0	5.8 ± 4.147	0.679
Surgical segment				0.2
L3/4	2	4	3
L4/5	6	7	0
L5/S1	4	2	2
Responsible nerve root				0.5718
L3	2	4	3
L4	7	7	2
L5	4	2	3
S1	1	0	0
ODI	48.148 ± 11.383	42.073 ± 10.281	57.780 ± 15.072	0.1141
VAS-leg	6.417 ± 1.782	6.692 ± 1.437	7.20 ± 1.924	0.6773
VAS-back	4.083 ± 1.676	3.154 ± 1.405	2.60 ± 1.140	0.1351

FLLDH: Far lateral lumbar disc herniation; BMI: Body mass index; ODI: Oswestry disability index; VAS: Visual analogue scale.

Efficacy of PTED

The operations were all successfully completed, and the operation time ranged from 60 minutes to 200 minutes. The average time was approximately 100.67 minutes. Most patients undergo outpatient surgery with short hospital stays. No deep or superficial infection occurred in any of the operations. The follow-up was more than 3 months after the operation. The average VAS-leg score was 6.667 ± 1.626 before the operation, 1.7 ± 1.022 one day after surgery, 0.8 ± 0.61 one month after surgery, 0.467 ± 0.629, 3 months after surgery, and 0.267 ± 0.521 at the last follow-up. The average VAS-back score was 3.433 ± 1.547 before the operation, 1.533 ± 0.86 one day after surgery, 1.033 ± 0.85 one month after surgery, 0.767 ± 0.728, 3 months after surgery, and 0.433 ± 0.626 at the last follow-up. The average preoperative ODI score was 47.121 ± 12.477, 27.417 ± 10.286 one day after surgery, 21.500 ± 9.414 one month after surgery, 18.667 ± 9.643 three months after surgery, and 18.083 ± 9.207 at the last follow-up. According to the different types of FLLDH, no significant difference was found between the groups (Figure 5). According to the modified Macnab efficacy evaluation standard, 28 cases were excellent (93.33%), 1 was good (3.33%), and 1 was medium (3.33%) at the last follow-up. Complications of patients were rare during follow-up. One patient (3.33%) had hyperalgesia and burning radiculopathy at the corresponding nerve root distribution areas 1 day after surgery, and one week after the operation, the patient's symptoms improved significantly. One patient (3.33%) had significant weakening of the muscles of extensor hallucis immediately after the operation (muscle strength was grade 2). After symptomatic conservative treatment and hyperbaric oxygen chamber treatment, the muscle strength recovered to Grade 4+ within 3 months after surgery. No cases of recurrence or reoperation were observed during the follow-up period.

Figure 5 Postoperative follow-up efficacy. A: Visual analogue scale (VAS)-leg score in different far lateral lumbar disc herniation (FLLDH) types; B: VAS-back Score in Different FLLDH types; C: Oswestry disability index score in different FLLDH types. ¹Represents a statistically significant difference compared to the preoperative state. ODI: Oswestry disability index; VAS: Visual analogue scale.

DISCUSSION

Patients with FLLDH often experience severe radicular pain for the compression of the dorsal root ganglia (DRG)[17]. The incidence of FLLDH in the L4/5 is the highest while the L5/S1 is very rare[18]. The incidence of FLLDH is highest at the L4/L5 level, with lowest at the L5/S1 level[4]. FLLDH has atypical features on MRI and CT, and clinicians are accustomed to focusing on compression in the spinal canal while ignoring the conditions of the intervertebral foramen and extraforaminal areas, leading to the high misdiagnosis rate of FLLDH[6,7]. Montinaro[5] reported that the misdiagnosis rate of FLLDH is approximately 30%. Regarding imaging, detecting FLLDH on plain radiographs and myelography is challenging. The current commonly used clinical imaging diagnostic methods are MRI and CT, but there is still a considerable missed diagnosis rate[19]. The oblique coronal MRI can effectively reduce the missed diagnosis rate of FLLDH[20]. Sung et al[21] found that 3D images can improve the diagnostic accuracy. Presently, studies have reported various methods to improve the diagnostic efficiency of FLLDH, such as radiculography and magnetic resonance hydrography. However, these methods require special equipment or technology and challenges persist in large-scale promotion.

SNRB is an effective method to diagnose the responsible segment and nerve root and was reported by MacNab in 1971[9]. SNRB is an operation of injecting local anesthetics around the nerve root after imaging it under the guidance of X-ray. For patients with multiple level degeneration of the lumbar spine, SNRB helps to clarify the responsible segment and range of nerve roots involved and then reduces the scope of surgery. However, many controversies persist regarding the clinical application of SNRB[22] and its operating specifications and procedures[23]. The actual operation of SNRB varies significantly and no standardized SNRB operation guide exists at present. The classic SNRB includes two parts: The provocation test and block test. During the SNRB puncture process, the position of the puncture needle, injection dose of anesthetic and diffusion coefficient are closely related to the accuracy of SNRB[22,24,25]. The location of the puncture needle and dose of local anesthetic drugs can substantially affect the dispersion of the local anesthetic drugs, leading to the blocking effect of different nerve roots and affecting the judgment of the responsible nerve root. Therefore, we believe that SNRB can be divided into therapeutic and diagnostic SNRB according to the purpose of SNRB. SNRB for treatment emphasizes effectiveness. Choosing the Kambin triangle block is safer and equally effective. At the same time, the injection dose of local anesthetic is appropriately increased, and the effect of the compound long-acting hormone is better. The current SNRB based on diagnosis emphasizes accuracy, the standard SNRB method is used for outlet root block. The standard method is described as follows: 18G puncture needles were used for precise puncture to reduce the chance of complications. The nerve root provocation test and nerve root radiography are used before the block to improve the diagnostic accuracy. The local anesthetic injection dose is strictly limited to 0.5 mL. We chose the local anesthetic lidocaine for lidocaine chosen for its safety profile and short half-life. The short half-life of the lidocaine also helps to observe the relationship between the failure of the nerve root block and metabolism of the local anesthetic to further clarify the responsible segment. The standardized SNRB can provide precise positioning for the decompression of outlet roots and walking nerve roots during surgery and can be used as an effective means to predict postoperative effects. A single-segment disc herniation can cause both the outlet root and walking root of the same segment to be affected at the same time. For FLLDH patients with multiple nerve roots involved at the same time, SNRB can clarify the main involved nerve roots and extent of involvement, aiding in the development of preoperative decompression strategies and comprehensive intraoperative decompression (involving outlet roots and walking roots). SNRB can effectively predict the postoperative effect and improve the postoperative curative effect.

Compared with traditional surgery, PTED has the advantages of less trauma and faster recovery, and its long-term curative effect is equivalent to that of traditional surgery. PTED is currently considered the preferred treatment for FLLDH[26]. During the PTED operation, the location of the working channel and scope of nerve decompression were determined based on the herniated disc's position and the responsible nerve root identified through SNRB. For type I (intraforaminal type, walking nerve roots and outlet nerve roots at the same segment may be affected) FLLDH, the decompression should focus on the area within the intervertebral foramen and fully decompress walking nerve roots and outlet nerve roots. For type II (the shape of the intervertebral foramina, which affects the outlet nerve root) FLLDH, the decompression should focus on the outlet nerve roots. For type III (mixed, walking nerve roots and outlet nerve roots at the same segment may be affected) FLLDH, the intraforaminal decompression should be performed first. After walking nerve root decompression is completed, the working cannula should be rotated upward and outward to the outlet root position, and then the outlet nerve root should be decompressed until the dural sac pulsation is satisfactory. According to the responsible nerve roots determined by the patient's preoperative SNRB, the corresponding nerve roots should be fully decompressed. The notable drawback of PTED is that the operation is difficult and the learning curve is steep[27]. The surgeon must undergo rigorous training, possess a good understanding of local anatomy and three-dimensional analysis capabilities related to FLLDH, and master percutaneous puncture and endoscopic operation techniques. Type II and type III FLLDH require careful decompression of the outlet nerve root, making the operation more challenging and increasing the risk of nerve damage. Beginners should use this technique carefully to prevent serious complications.

The use of SNRB not only enhances the effectiveness of PTED in treating FLLDH, but also decreases the learning curve associated with PTED. Although SNRB's experience did not have a significant impact on symptom recurrence and reoperation following PTED surgery, it did enhance the performance of PTED, leading to shorter operating times, reduced radiation exposure, and lower rates of surgical complications[28]. In our clinical process of combining SNRB with PTED for treating FLLDH, the SNRB procedure was performed first. If the SNRB result is negative, the puncture needle is removed immediately. On the other hand, if it is positive, the needle is repositioned to the superior articular process. Subsequently, the working channel for PTED was established, and the PTED procedure was performed. This method helps the surgeon become familiar with the patient's anatomy and reduces the surgical time and risks.

Because PTED for FLLDH operation has less interference with the nerves in the spinal canal, fewer surgical complications occur[15,29,30]. Transient nerve root injury is the most common complication that manifests as numbness and decreased muscle strength in the innervated skin area. Radical hyperalgesia and causalgia are the most frequent complications post-surgery, with an incidence ranging from 7% to 25%[31]. The incidence of postoperative burning nerve root pain is 8%-17%[32,33]. Radical hyperalgesia and causalgia after FLLDH surgery may be related to excessive stimulation or damage to nerve roots and DRG during the operation. In particular, repeated intervertebral foraminal punctures and working cannula are placed close to the proximal end of the intervertebral foramen, resulting in extrusion of the outlet nerve root and DRG. Excessive use of bipolar radiofrequency electrocautery during surgery is the main cause of nerve edema and injury after surgery. Overall, the total complication rate of this group of patients was 6.7%, and no recurrence or reoperation was found during the follow-up period. The recurrence rate of PTED surgery reported in the literature is 7.5%-18.1%[34,35]. However, no recurrence cases were observed during the follow-up period in this group. This may be related to the following factors: First, accurate SNRB was performed before the operation to determine the responsible nerve root, followed by precise nerve root decompression; second, for type II FLLDH, entry into the spinal canal during the operation is unnecessary. For type I and III FLLDH, damage to the posterior longitudinal ligament and fibrous annulus is minimized during the operation, reducing the recurrence and reoperation rate. Third, patients were instructed to adhere to strict and standardized low back muscle function exercises to further reduce the likelihood of recurrence postoperatively. Forth, SNRB enables the surgeon to gain a thorough understanding of the patient's anatomy, facilitating precise PTED surgery which effectively reduced surgical time and risks.

CONCLUSION

In summary, SNRB provides an effective method for FLLDH to define the responsible segment and nerve root. This method significantly enhances the diagnosis rate of FLLDH and aids in formulate preoperative decompression strategies for PTED and adequate intraoperative decompression, predicting the postoperative effect and improving the postoperative curative effect. SNRB combined with PTED for FLLDH is effective and warrants further study regarding the diagnosis, treatment and management of FLLDH.