Failed Back Surgery Syndrome

Etiology

Failed back surgery syndrome is a condition with a complex etiology and many factors that predispose patients towards chronic pain. These predictive factors generally divide into preoperative (patient) factors, intraoperative factors, and postoperative factors.[1][3][4][5]

Preoperative patient factors associated with failed back surgery syndrome include anxiety, depression, or other psychiatric comorbidities, obesity, smoking, the presence of litigation or worker’s compensation claims, and physical or radiologic findings such as stenosis, fibrosis, and disc herniation.[6][7] Patient psychosocial factors have been shown to have the strongest association with the development of FBSS.[8] Additionally, the choice of an inappropriate surgical candidate or surgical approach correlates with a higher risk of developing FBSS.[3] Patients who have undergone multiple prior back surgeries have a higher chance of developing FBSS and a lower chance of achieving successful pain resolution with surgery.[5]

Intraoperative risks for developing failed back surgery syndrome include operating at either the wrong vertebral level or operating at a single level while the origin of pain spans several levels, thus providing insufficient pain relief.[3][5] The wrong vertebral level may pertain to factors such as lumbarization, sacralization, or even more important when the etiology gets attributed to a non-attributable pathology. These factors represent the ambiguity in clearly identifying a locus in patients with multi-segment changes. Improper technique during surgery may also lead to failure to relieve pain or the onset of new pain.[5]

Long-term complications of the surgery, either avoidable or inevitable, may also lead to the development of failed back surgery syndrome in some patients. Surgery may exacerbate existing symptoms or cause new symptoms by inducing spinal stenosis, spinal instability, epidural fibrosis, or disruption of adjacent discs.[9][10]

Epidemiology

Estimates of the percentage of adults who experience chronic lower back pain during their lifetime range from 51% to 84%.[11] The incidence of lower back pain increases with age and in females.[12] As the population ages, the incidence of surgery for lower back pain increases dramatically; the number of primary lumbar fusions, for instance, increased by upwards of 170% from 1998 to 2008.[1] Failed back surgery syndrome is reported to affect between 10 to 40% of patients following back surgery, but estimating the incidence of FBSS is difficult due to the wide scope of its definition and its heterogeneous etiology.[4][13] Increased complexity of back surgery increases the rate of FBSS; failure rates range from 30% to 46% for lumbar fusion and 19% to 25% for microdiscectomy.[3][14]

Pathophysiology

The pathophysiology of failed back surgery syndrome is unclear and is attributable to several factors. Lateral stenosis of the foramina has been identified as the most common structural abnormality identified in patients with FBSS. However, other common pathologies in failed back surgery syndrome patients include painful disc degeneration, disc herniation, neuropathic pain, and pseudoarthrosis.[15] Pathophysiological findings associated with FBSS and directly attributable to surgery include epidural fibrosis, increased spinal instability due to discectomy or laminectomy, and redistribution of load to adjacent disc tissue, a phenomenon known as “transition syndrome.”[3]

History and Physical

An accurate and thorough history and physical examination of patients with persistent pain after lower back surgery are crucial for correct diagnosis. The character and location of the patient’s pain should be identified and compared to presurgical pain; lack of immediate pain relief may indicate operation at the wrong level whereas new-onset pain may indicate surgically-induced nerve damage.[1] Pain in the leg likely indicates nerve compression from stenosis, epidural fibrosis, or disc herniation, while low back pain is more common in facet joint arthropathy, sacroiliac joint issues, or myofascial etiologies.[3] Patients should be asked about “red flag” symptoms that may indicate life-threatening conditions; these include, but are not limited to: saddle anesthesia or bowel/bladder incontinence, indicative of cauda equina syndrome; fever, chills, or weight loss indicating infection; and signs of malignancy.[3][5] Patients should also have an evaluation for anxiety, depression, and other psychiatric conditions due to their high comorbidity with FBSS.[16]

Physical exam for the failed back surgery syndrome patient is generally not useful for identifying a specific etiology of pain, although the practitioner may be able to elicit several suggestive findings.[5]. Symptoms due to spinal stenosis are usually exacerbated upon spinal extension and relieved by flexion. On the other hand, pain from a disc herniation may present with a positive sign on straight leg rais.[3] Focal neurological deficits in FBSS patients warrant further testing.[5] Deficits in strength or sensation in the lower extremities may help narrow down which nerve roots are affected.[3] Waddell signs can be used to evaluate for psychogenic etiology of lower back pain; while the interpretation of these tests is controversial, they may be useful especially if there is a suspicion of secondary gain.[3][16]

Evaluation

X-rays are a simple and inexpensive first imaging evaluation for suspected failed back surgery syndrome. X-rays are useful for detecting vertebral and sacroiliac defects and/or misalignment and are superior to MRIs for the detection of spondylolisthesis.[17] Adjacent segment degeneration and loss of lordosis are common abnormalities found on radiography.[5] However, X-rays are unable to detect spinal stenosis, the most common pathological finding in FBSS, and are also unable to evaluate soft tissue, such as intervertebral discs, epidural scarring, or fibrosis.[1][3]

MRI with and without gadolinium contrast continues to be the gold standard imaging modality for failed back surgery syndrome due to its excellent ability to detect soft tissue abnormalities such as epidural fibrosis and disc herniation.[5][16] Contrast is especially indicated in patients with a history of disc herniation surgery.[15] In patients with ferromagnetic implants, a CT myelogram is used to avoid implant artifacts created on MRI.[3][10]

Other tests may be used to elucidate the etiology of failed back surgery syndrome and differentiate it from other causes of lower back pain. Erythrocyte sedimentation rate and C-reactive protein may be used to evaluate for possible infection, especially in patients with constitutional symptoms or a predisposition towards infection.[16] Diagnostic nerve blocks can diagnose specific etiologies of FBSS such as facet joint arthropathy (dual medial branch blocks), sacroiliac joint pain (lateral branch blocks and intra-articular injection), and foraminal stenosis (transforaminal epidural and selective single level blocks), and can identify specific roots associated with a patient’s symptoms. When combined with steroids, these diagnostic procedures can also provide therapeutic relief.[1] Adherence to strict standards of accurate needle placement, contrast injection, as well as a limited active agent is essential in improving the sensitivity and specificity of these blocks. The emphasis on the diagnostic nature must is critical for the patient, and steroid use minimized or avoided to reduce false-positive results.

Treatment / Management

Therapies for the treatment of failed back surgery syndrome broadly split into conservative (physical therapy or medication) and aggressive (interventional or surgical) management. Conservative management should always be the first option before invasive techniques in patients without indications for emergency surgery.[5]

Studies of conservative treatment specifically for failed back surgery syndrome are rare, and the studies that do exist are often contradictory.[18] Common pharmacologic treatments include non-steroidal anti-inflammatory drugs (NSAIDs), opioids, anticonvulsants, and antidepressants. NSAIDs are commonly prescribed drugs for many different etiologies of lower back pain, for which they have been shown to have an advantage over placebo.[19] While opioids are commonly used to treat chronic pain, evidence for their use in FBSS is generally weak, and the risk of dependence and resultant substance use disorder must be strong consideration before their use. As such these should be considered a treatment of last resort and undertaken in a multidisciplinary program. Small trials using anticonvulsants such as gabapentinoids or antidepressants in FBSS therapy have shown promising initial results, but long-term efficacy is undetermined.[3][18][20](A1)

Strong Level II (small randomized controlled trial [RCT]) evidence exists for the efficacy of active physical therapy in patients with gabapentinoids.[20] Physical therapy must focus on developing tolerance, neuroadaptation, conditioning, and the exercises conducted in a quota-based, non-pain contingent fashion under the direction of an experienced physical therapist who understands the patient's pathology, strengths, and limitations. Cognitive behavior therapy or other psychiatric therapy can lead to better outcomes in patients with FBSS and may enhance the efficacy of interventional treatments for patients’ pain.[21](A1)

Interventional treatment for failed back surgery syndrome is highly dependent on the sub-etiology of the patient’s pain and the detection of specific deformities on imaging. If facet joint arthropathy is identified, procedures such as medial branch blocks or radiofrequency ablation can be performed to relieve the pathology. Radiofrequency ablation must be performed in a continuous mode with a large-bore needle placed parallel to the nerve to maximize the chances of interrupting a significant segment of the nerve and thus achieve prolonged pain relief. Likewise, adhesiolysis has been shown to be an effective treatment for FBSS where epidural fibrosis is a major contributing factor.[10] More generally, an epidural steroid injection can be an effective treatment for several pathologies that result in FBSS, such as epidural fibrosis, disc disruption or herniation, and spinal stenosis.[3] However, the quality of these studies is generally of moderate or weak quality, leaving some controversy as to how effective epidural injections actually are.[10]

Neuromodulatory therapies for failed back surgery syndrome, such as spinal cord stimulation (SCS), involve the implantation of a stimulatory device to mediate the patient’s pain. Strong evidence from large randomized trials exists for the superiority of SCS over conservative management and repeated surgery for FBSS.[22][23] Also, SCS efficacy is easy to trial with a percutaneous device before permanent implantation. However, spinal cord stimulation is not a panacea; permanent implantation fails to provide reliable long-term pain relief in about 40% of patients.[24] SCS also results in high upfront costs at the time of implantation, although long-term cost-benefit analyses favor SCS over other therapies.[3](A1)

Lastly, reoperation may be an option for patients with clearly identified pathologies that may be relieved with surgery. As noted above, reoperation generally correlates with inferior outcomes and higher morbidity compared to implantable technologies such as SCS. However, the presence of certain symptoms, such as bowel/bladder impairment, motor weakness, or neural deficit warrants immediate surgical intervention.[1][10](B3)

Differential Diagnosis

In a non-operated back the most common cause of pain is discogenic pain syndrome followed by facet and sacroiliac joint related pain, and with careful testing, the etiology of low back pain can be identified or attributed in the majority of patients.[25] However, in failed back surgery syndrome there are additional diagnostic considerations most important being epidural fibrosis leading to neuropathic back pain. Definition of FBSS relies more on clinical history (i.e., history of back surgery) than on a specifically identifiable pathology, so the differential diagnosis of failed back surgery syndrome is essentially equivalent to determining a more specific causative abnormality which can guide treatment. In 95% of FBSS patients are eventually provided with a known diagnosis.[7] Evaluation of lower back pain should immediately attempt to rule out serious conditions such as cauda equina syndrome, infection, or malignancy.[5]

Prognosis

Failed back surgery syndrome has proven to be both a difficult condition to experience for patients and a challenging condition to treat for clinicians. Compared to patients with other chronic pain syndromes, including rheumatoid arthritis, osteoarthritis, and fibromyalgia, FBSS patients have been shown to exhibit a lower quality of life scores and higher amounts of pain, unemployment, opioid use, and disability.[26]

Unsurprisingly, treatment choice has been found to affect outcomes in patients with failed back surgery syndrome. Stronger evidence exists for favorable results in patients treated with SCS.[27] Increasing numbers of revision surgeries are associated with a progressively lower chance of successful pain relief.[5][28] Spinal cord stimulation is now the treatment of choice, once psychosocial factors are accounted for, and after the failure of reasonable measures, including aggressive PT, diagnostic blocks followed by therapeutic modalities as indicated. It must be an option where appropriate, and before considering chronic opiate therapy.

Complications

In addition to intractable low back pain, patients with failed back surgery syndrome often suffer from associated disorders. Psychiatric comorbidities are common in FBSS patients and correlate with worse pain outcomes. A survey of failed back surgery syndrome patients by Long and colleagues found that 67 of 78 patients with FBSS suffered from depression, with the vast majority experiencing an onset of depressive symptoms following the initial onset of pain.[29]

Patients with failed back surgery syndrome may also experience complications from attempted treatment of their pain. Prolonged NSAID use has significant adverse effects on gastrointestinal and renal health. Opioids also have significant addictive potential; one study found that opioid overdose was the most common cause of death following lumbar fusion surgery.[30] Back surgery itself also has several significant complications, including anesthesia-related complications and infection.