Full-endoscopic Spine Surgery for the Treatment of Far-out Syndrome: A Case Series

Abstract

Far-out syndrome is a pathological condition characterized by entrapment of the L5 nerve root by the L5 transverse process, sacral ala, vertebral body, and adjacent soft tissue. Although many cases of far-out syndrome have been reported, a standardized surgical approach has not yet been fully established. As older adults are often affected by this condition, full-endoscopic spine surgery may be a preferable option due to its minimally invasive nature.

This case series included six far-out syndrome cases treated with full-endoscopic spine surgery at our hospital. Patient background and operative data were collected from medical records. The numerical rating scale scores for pain and numbness, and the modified MacNab criteria, were recorded for 1 year postoperatively.

The mean age was 73.5 years, and the mean duration of symptoms was 46.5 months. The mean operative time was 62.7 min without complications. In five of six cases, the amount of intraoperative bleeding was lower than the measuring limit in our hospital. The numerical rating scale score for pain improved at 1 year after surgery in all cases except case 6, who sustained a vertebral compression fracture 11 months after full-endoscopic spine surgery. According to the modified MacNab criteria, excellent or good outcomes were observed in five cases (83%) except for case 6.

Full-endoscopic spine surgery for far-out syndrome appears to offer not only minimal invasiveness but also satisfactory surgical outcomes.

Introduction

Far-out syndrome (FOS) is a rare pathology of L5 nerve root entrapment in the extraforaminal region, proposed by Wiltse et al.¹⁾ in 1984. The surrounding anatomical structures, such as the transverse process of the L5 vertebra, sacral ala, L5/S1 vertebral disc, and soft tissues connecting former structures (ligaments, muscles, and granulation), are involved in this pathology. Although early reports of FOS may have included extraforaminal dis herniation, simple extraforaminal disc herniation must be excluded from FOS after the introduction of magnetic resonance imaging (MRI) for diagnosis. Although Wiltse et al.¹⁾ proposed two types of patients: (1) the elderly person with degeneration and (2) a somewhat younger population with isthmic spondylolisthesis, the transverse process of the L5 vertebra dipped downward to the sacral ala and settled down against the ala in both types (pseudarthrosis formation).¹⁾ The characteristic feature is L5 radiculopathy in older adults, but the exact incidence of FOS is not well known because most studies are case reports.

The initial surgical approach used by Wiltse et al.¹⁾ was a microscopic decompression. Subsequently, several approaches have been developed for the treatment of FOS, such as microendoscopic,^2-4) full-endoscopic,⁵⁾ and unilateral biportal endoscopic^6-9) decompression. Among these approaches, decompression by full-endoscopic spine surgery (FESS), which was initially developed for the treatment of lumbar disc herniation, seems to be the least invasive because of its small skin incision, less destruction of surrounding tissues, less intraoperative bleeding, and rapid recovery.

In this case series, we summarize our experiences with FOS treated by FESS and present the detailed operative approach to FOS and 1-year operative outcomes of FESS.

Materials and Methods

Patient selection

This study was a retrospective, single-institution case series in which six consecutive patients with FOS underwent decompression using a 5.2-mm working channel (WC) endoscope (Tokibo Co. Ltd., Tokyo, Japan) between August 2023 and April 2024. All patients had apparent L5 radiculopathy that was resistant to medical treatment and epidural steroid injection, and the L5 nerve block could only tentatively relieve the pain. We excluded patients having severe spondylolisthesis (Meyerding classification: grade >I) and degenerative scoliosis (coronal Cobb angle >15°).

All procedures involving human participants were performed in accordance with the ethical standards of the research committee of the Iwai Medical Foundation (Institutional Review Board approval no. 20200507) and the 1964 Declaration of Helsinki. Informed consent was obtained from the disclaimer documents of the surgical procedure handed over to the patients with explanations and signed. The study has been conducted and adhered to the guidelines outlined in the preferred reporting of CasESeries in surgery (PROCESS) criteria.¹⁰⁾

Data collection

Patient characteristics such as age, sex, affected side, symptom duration, and presence or absence of spondylolytic spondylolisthesis were collected (Table 1). Preoperative MRI (T2-weighted, Fast Imaging Employing Steady-state Acquisition (FIESTA), and diffusion tensor imaging tractography¹¹⁾) was used to identify extraforaminal stenosis and the side of the FOS. MRI was also used to exclude L5 radiculopathy caused by L4/5 intracanal and L5/S1 foraminal stenoses. Computed tomography (CT) was performed to determine hypertrophic changes and osteophytes of the surrounding bony structures. Postoperative MRI and CT were used to confirm the decompression of the corresponding nerve root.

Table 1.

Demographic data and operative findings of 6 patients

Case No.	Age	Sex	Affected side	Symptons duration (months)	spondylolytic spondylo- listhesis	Coronal Cobb angle (°)	Operation time (min)	Blood loss (mL)	Postoperative hospital stay (day)	Complications
1	73	M	R	84	+	7	64	10	1	-
2	68	M	L	27	-	8	85	10	1	-
3	85	M	L	72	-	11	64	10	1	-
4	72	M	R	36	+	9	56	10	1	-
5	77	F	R	24	-	11	49	10	1	-
6	66	F	L	36	-	3	58	20	1	-

F: female; L: left; M: male; R: right

Operative time, blood loss, length of postoperative hospital stay, and operation-related complications were obtained from the patients' medical records (Table 1). Pain and numbness of the corresponding L5 nerve root were evaluated using the numerical rating scale (NRS) at hospital admission and 1 year postoperatively.¹²⁾ Patient satisfaction was evaluated using the modified MacNab criteria¹³⁾ at 1 year postoperatively. The resumption of activities that had been suspended due to pain or numbness was also obtained from the interviews (Table 2).

Table 2.

Operative outcomes for FOS treated with FESS

Case No.	NRS Pain		NRS Numbness		modified MacNab criteria	Resumption of activities
	Preope	1-year	Preope	1-year	1-year
1	9	1	8	1	excellent	Kendo
2	7	5	7	3.5	good	golf
3	10	0	8	3	good	running
4	5	0	0	0	excellent	swiming
5	7	0	6	0	excellent	gymnastics
6	8	8	0	0	fair	-

NRS: numerical rating scale

Surgical technique

Patients were carefully log-rolled in the prone position. The FESS was performed under general anesthesia combined with motor-evoked potential (MEP) monitoring by a single skilled surgeon (H. Koga). During the operation, a fluoroscope was placed across the center of the operating table to ensure appropriate timing. We used the posterior approach just above the pseudarthrosis, consisting of the L5 transverse process and sacral ala, and the basic operative procedure has been previously described.^14,15) The center of the pseudarthrosis was located 25-35 mm lateral from midline, and the position was also confirmed by the fluoroscope (anterior-posterior view). Under fluoroscopic guidance (lateral view), the endoscope was inserted vertically toward the center. We first removed the caudal part of the transverse process and the cranial part of the sacral ala using a high-speed drill (Fig. 1A, 1-3). Subsequently, the exposed lumbosacral ligament was detached from the residual bone edges, and the L5 nerve root was exposed (Fig. 1A, 4). In cases of spondylolytic spondylolisthesis, it is necessary to detach the granular tissue from the spondylolysis. The endpoint of this procedure was confirmation of the nerve root, improvement of intraoperative MEP monitoring, and free passage of the dissector (3 mm width) anteriorly beyond the dorsal border of the L5 vertebral body (Fig. 1C). After exposing the dorsal surface of the L5 nerve root, we checked for the presence of anterior components involved in entrapment using the dissector. The skin incision is closed without drainage.

Fig. 1.

Chronological intraoperative findings (A), schematic diagram of Fig. 1A (B), and lateral view of intraoperative fluoroscopic image (C).

A After exposing the bone surface (pseudarthrosis consisting of the L5 transverse process and sacral ala) using a bipolar coagulator, the L5 transverse process and sacral ala were removed using a high-speed drill (2, 3). After sufficient exposure of the lumbosacral ligament, the lumbosacral ligament was removed using several forceps. Finally, the L5 nerve root and the intravertebral disc were visualized in the endoscopic operative field (4).

B Schematic diagram of Fig. 1A.

L5TP, L5 transverse process; SA, sacral ala; LSL, lumbosacral ligament; IVD, intravertebral disc; L5NR, L5 nerve root

C One of the endpoints of this procedure is confirmation that the dissector (3 mm width, arrowheads) could freely pass anteriorly beyond the dorsal border of the L5 vertebral body (dotted line).

L4: L4 vertebral body; L5: L5 vertebral body; S: sacrum

Statistical analysis

The preoperative and postoperative NRS scores were compared using the Wilcoxon signed-rank test. All statistical analyses were performed using the STATA software (version 16.0; StataCorp LLC, College Station, TX, USA). A two-sided p-value of less than 0.05 was considered statistically significant.

Results

Table 1 summarizes the baseline characteristics and surgical details of the patients. Six patients―consisting of four men and two women―were included in this case series. Their mean age was 73.5 years (range, 66-85 years). Four patients complained of both pain and numbness in the corresponding L5 nerve area, and two patients complained of only leg pain. The mean symptom duration was 46.5 months (range, 24-84 months). Although there was no apparent muscle weakness in the L5 nerve area, the patient desired to receive surgical treatment for the resumption of daily activities. The mean operative time was 62.7 mins (range, 49-85 mins). Except for one case (20 mL), the amount of intraoperative bleeding was lower than the measuring limit in our hospital (10 mL). The postoperative hospital stay was 1.0 day for all patients. No complications were associated with decompression surgery (Table 1). Intraoperative endoscopic findings showed hypertrophy of the bony structures consisting of the lumbosacral tunnel (sacral ala, L5 transverse process, and superior articular process), lumbosacral ligament, and granular tissue protruding from spondylolysis, causing entrapment of the L5 nerve root. We could not find the anterior components involved in entrapment, such as osteophytes of the L5-S1 vertebral bodies¹⁶⁾ in this case series.

Table 2 summarizes the operative outcomes. The NRS scores were used to evaluate pain and numbness, and the NRS scores improved, except in one case (Case No. 6).

Pain scores improved significantly from a preoperative mean of 7.7 ± 1.6 to 2.3 ± 3.4 at 1-year postoperatively (p = 0.028). In contrast, numbness scores decreased from 4.8 ± 3.8 to 1.3 ± 1.6, but this change did not reach statistical significance (p = 0.068). A subgroup analysis limited to the four patients who reported preoperative numbness showed a similar reduction (7.3 ± 0.96 to 1.9 ± 1.5), though this was also not statistically significant (p = 0.125). Case 6 had a compressed vertebral fracture at T11 11 months after the operation. Subsequently, her leg pain reappeared, and her activities of daily living deteriorated.

Satisfaction was evaluated using the modified MacNab criteria. Except for Case 6, the outcomes of the remaining five cases were excellent or good at 1 year after the operation (83%).

Representative case

A 68-year-old male patient complained of severe left leg pain and numbness (outer side of the lower leg and dorsum of the foot) that persisted for more than 2 years. The symptoms were resistant to conventional conservative treatment. Axial MRI revealed an indistinct left L5 nerve root in the L5/S1 extraforaminal region (Fig. 2A). Hypertrophic sacral ala was confirmed on axial CT image (Fig. 2B), and the sacral ala formed pseudarthrosis with a left transverse process on 3D-CT image (Fig. 2C). The diffusion tensor imaging tractography revealed entrapment of the left L5 nerve root at the extraforaminal region (Fig. 2D arrow heads). The sensory nerve action potential of the left peroneal nerve disappeared,¹⁷⁾ we diagnosed FOS and full-endoscopically decompressed the region. Postoperative MRI and CT images revealed thickening of the L5 nerve root and decompression of the hypertrophic bone structures (Fig. 2D-F). He still had slight radicular symptoms, and the extent of these symptoms had diminished to the dorsum of his left foot. The patient exhibited satisfactory postoperative recovery and resumed golf play at the 1-year follow-up.

Fig. 2.

Preoperative and postoperative images of T2-weighted magnetic resonance, CT, and diffusion tensor imaging tractography.

Preoperative (A-D) and postoperative (E-H) images. A and E indicate axial MRI, and B and F indicate the same plane as the axial CT images. C and G indicate 3D-CT images. D and H indicate diffusion tensor imaging tractography. Arrowheads indicate the thickening of the L5 nerve root (E), decompression of the hypertrophic bone structures (F), disappearance of pseudarthrosis (G), and entrapment of the left L5 nerve root (D).

Discussion

FOS is one of the causes of failed back surgery syndrome (FBSS), a condition characterized by unchanged or worsening preoperative symptoms following back surgeries. Performing decompression at the wrong level is a significant risk factor for FBSS.¹⁸⁾ Although an accurate preoperative diagnosis of extraforaminal entrapment is relatively difficult, diffusion tensor imaging tractography¹¹⁾ and electrophysiological examinations, such as sensory nerve action potential,¹⁷⁾ are useful for distinguishing between L5 radiculopathy caused by other areas. Double-crushed L5 nerve root (e.g., combination of FOS and L4/5 intracanal stenosis) has also been reported.¹⁹⁾ We tried to exclude such conditions from this case series using the above-mentioned diagnostic techniques.

Several surgical approaches have been proposed for treating FOS. Microsurgical decompression was first reported by Wiltse et al.¹⁾ in 1984. Because the surgical field of FOS becomes deep, the use of a surgical microscope has the advantage of obtaining good surgical vision. Since this report, decompression and fusion surgeries have also been reported by several investigators.^20,21) The merits of decompression and fusion surgeries include improvement of instability, gain of disc height, and correction of bone abnormalities. However, fusion surgeries are more invasive than microsurgical decompression and have characteristic complications (adjacent segmental disease, subsidence, non-fusion, and a higher infection rate). Considering that older patients have FOS, less invasive surgeries should be performed to avoid such complications.

FESS was originally developed for the treatment of lumbar disc herniation, and the indication has been expanding to decompression and other areas of the spine (cervical and thoracic).^14,15,22-25) Because of its good operability in narrow anatomical areas, successful treatment for L5/S1 extraforaminal disc herniation has been reported since 2012.²⁶⁾ Although FESS has already been one of the standard treatments for L5/S1 extraforaminal disc herniation, FOS (excluding extraforaminal disc herniation) has not been well studied. At the moment, only one study is available regarding FOS treated by FESS.⁵⁾ In this study, 16 patients (the mean age 70.94 ± 4.04 years, the average duration of symptoms 10.81 ± 5.13 months, the mean follow-up period 16.69 ± 5.84 months) were treated, and excellent or good outcomes of the modified MacNab criteria were observed in 15 cases (93.70%). Both our results and those of this report demonstrate that FESS is a minimally invasive, safe, and effective treatment for FOS.

FOS includes several different pathologies. This case series does not include FOS with severe degenerative scoliosis or FOS caused by ventral components such as a bony spur of the L5 vertebral body or sacrum. Therefore, we do not have a definitive answer regarding the effectiveness of FESS for these conditions. In particular, when addressing ventral components, it is essential to understand the anatomical variations of surrounding structures, including arteries and veins, to ensure safety. Fortunately, we have not encountered unexpected bleeding from the deep operative field, but further surgical experience is required to draw firm conclusions. Some surgeons have chosen an anterior (extraperitoneal) approach for ventral components such as large osteophytes.^27,28) We might gain useful insights for improving FESS techniques by studying these fundamentally different approaches.

Bertolotti syndrome is another lumbosacral pathology characterized by congenital enlargement of the L5 transverse process with pseudarthrosis or fusion with the sacrum.²⁹⁾ The chief complaint is low back pain, and the most common age for this disease is middle age. The most recent systematic review indicated that the mean age of the patients was 47.7 years, and 96.4% of the patients had low back pain.³⁰⁾ However, this condition is apparently different from FOS based on the degenerative changes of the spine. Because surgical treatment requires decompression of the pseudarthrosis or fusion, the operative approach for Bertolotti syndrome is similar to that for FOS. Successful removal of the enlarged L5 transverse process and sacrum, consisting of pseudarthrosis or fusion, has been performed endoscopically in previous studies.^31-34) FESS seems to have great advantages in very deep surgical fields, such as FOS or Bertolotti's syndrome.

In this study, we used a 5.2-mm WC endoscope, which is larger than that of a conventional endoscope for discectomy (approximately 4-mm WC). For the treatment of FOS, decompression of the pseudarthrosis using a high-speed drill is followed by removal of the lumbosacral ligament, which is an important subsequent operative step. A large WC endoscope allowed us to use a solid curette and large Kerrison rongeur, thereby achieving appropriate and quick removal of the lumbosacral ligament (Supplementary Video 1). Especially for spondylolytic spondylolisthesis cases, detachment of granulation tissue from spondylolysis is necessary, in addition to lumbosacral ligament detachment. In such situations, a large WC endoscope demonstrates this value. The 5.2-mm WC endoscope requires a larger skin incision (10 mm); however, the manipulation will not be disturbed, even in patients with a high iliac crest. Therefore, we recommend using an endoscope with a larger WC for the treatment of FOS.

Our study was not prospective, randomized, or multi-institutional, which is a limitation. The lack of FOS patients with severe degenerative scoliosis is another limitation, because degenerative scoliosis is one of the important factors for FOS. Furthermore, a larger number of patients, longer follow-up periods, and a comparative study with conventional treatments are required to establish FESS as the standard treatment for FOS. To develop methods confirming the completion of decompression while ensuring safety is also important for the establishment.

Conclusions

Herein, we report six cases of FOS treated with FESS. In addition to the removal of pseudarthrosis consisting of the L5 transverse process and sacral ala, detachment and removal of the lumbosacral ligament is a critical surgical step for decompression.

Conflicts of Interest Disclosure

All authors have no conflict of interest.

Patient Consent

The patient provided consent for the submission of this case series to the journal.

Supplementary Material

Supplementary Video

Intraoperative endoscopic findings.

After removing the pseudarthrosis, the lumbosacral ligament covering the L5 nerve root was removed using a curette and Kerrison rongeur. This case is the same as that shown in Figure 1.