Cluneal Neuralgia

Etiology

An understanding of basic SCN anatomy is critical for the diagnosis and treatment of CN. The SCN receives a contribution from the dorsal rami of the lower thoracic and lumbar nerve roots by their lateral cutaneous branches. Historically, it was thought the L1-L3 nerve roots were supplying the SCN.[6][7] However, several anatomical studies have shown there is significant variation, and neural contribution can range to include T11-L5 nerve roots.[8][9] As the spinal nerve roots exit, they pierce through the psoas major muscle and paraspinal muscles, traveling posterior to the quadratus lumborum muscle.

Next, the spinal nerve roots pass through the thoracolumbar fascia as they approach the iliac crest. As the SCN crosses the posterior iliac crest, the nerve may pass through an osteofibrous tunnel, which is formed by the thoracolumbar fascia anteriorly and the iliac crest posteriorly. Although there is some controversy regarding the existence of this structure, there are several anatomical studies validating its existence and its potential as a site of entrapment.[7][10]

Three SCN are described: medial, intermediate, and lateral. In one cadaveric study, Tubbs et al. described the distance of three superior cluneal nerves (medial, intermediate, and lateral) from the posterior superior iliac spine (PSIS). From the midpoint on the PSIS, these mean distances were measured at 5 cm, 6.5 cm, and 7.3 cm, respectively.[7]

The middle cluneal nerves (MCN) are a possible source of the low back, gluteal, and leg pain. However, most of the literature has focused on SCN entrapment. The MCN arises from the dorsal rami of S1-S3 and provides sensory innervation over the gluteal region.[11] The most common potential site of MCN entrapment is the long dorsal sacroiliac ligament.[12][13] Clinically, MCN entrapment is difficult to distinguish from sacroiliac joint dysfunction.

The inferior cluneal nerves (ICN) are included in a group of pelvic nerves that may contribute to chronic pelvic pain syndromes. The ICN course around the inferior margin of the gluteus maximus and innervate the inferolateral gluteal region.[14]

Epidemiology

The overall incidence of SCN entrapment in patients with low back pain is reported to be 14%, as shown in one study.[9] In a Japanese study of 34 patients with suspected SCN entrapment neuropathy who underwent surgical release, the entrapment was unilateral in 13 patients and bilateral in 21.[15] The incidence can be difficult to ascertain for MCN and ICN entrapment syndromes leading to CN.[16]

Other studies place the incidence of SCN entrapment at 1.6 to 14%, the incidence of MCN entrapment in patients with low back pain at 13%, and the incidence of generalized transient cluneal neuralgia in post-partum patients at 0.73%.[5]  The prevalence of SCN entrapment is significantly higher among patients with vertebral fractures.  Generally, there are more women and older patients with SCN entrapment, with an average age of around 64.[17]

Pathophysiology

The pathophysiology of SCN, MCN, and ICN entrapment is similar in so far as they all essentially involve the mechanical irritation of the nerves when they are compressed locally or direct damage to the nerves. This compression could be due to edema around the nerve from direct damage, traction, friction, or repetitive compression, and/or it could be due to anatomical, traumatic, or inflammatory changes or differences preventing the appropriate course or movement of the nerves with body posture, locomotion, positioning, or abnormalities of muscle tone or contraction.[18][5] 

The ICN runs at the inferior border of the ischial tuberosity, passing under a fibrous expansion between the deep gluteal fascia and the ischial tuberosity, which is a common location where entrapment may occur, ICN entrapment may also occur between the insertions for the gluteus maximus and hamstring muscles, and additionally, it may occur from sitting on a hard seat from compression against the ischium and hamstring muscles.[17]

Histopathology

Relative nerve enlargement due to increased thinly myelinated fibers, sub-perineural edema, thickened peri-neurieum, and Renaut bodies are likely histopathological findings in entrapment cases. There is likely Wallerian centrofascicular degeneration in large fascicles with sub-perineural lesions in smaller fascicles and denervated Schwann cells adjacent to some of the Renaut bodies suggesting some degree of axonal drop-out. There will be foci of endoneurial edema with the accumulation of fibroblasts, fibrillar substance, and Renaut bodies.[19][5]

History and Physical

Patients presenting with low back and gluteal region pain should be evaluated for CN. In one study, 49% of patients who met a specific diagnostic criterion for SCN entrapment experienced leg symptoms, which can mimic symptoms of radiculopathy due to lumbar pathology.[9] No specific physical exam maneuver has been deemed sensitive or specific for CN. However, it is generally accepted that the pain is exacerbated with lumbar movement in multiple planes. It is critical to perform a Tinel-like sign at the site of the SCN course over the iliac crest.

Based on previous anatomical studies, it is suggested the medial SCN passes through the osteofibrous tunnel at about 6.6 cm lateral from the midline over the iliac crest. Palpation at this region may produce pain radiating from the low back to the gluteal region and posterior thigh in the distribution of the SCN and down the posterior leg in the form of "pseudo-sciatica." [8] As with any evaluation of low back pain, a complete neurological exam should be performed, including strength and sensation testing and assessing deep tendon reflexes and upper tract signs.

CN is a diagnosis of exclusion typically, as it is most important to rule out alternative diagnoses first. However, as is common in patients with chronic pain, most patients have had some evaluation of other possible etiologies, although that does not guarantee that previous evaluations were either appropriate, sufficiently robust, or consistent with the patient's contemporaneous presentation at the time of assessment. Nevertheless, some aspects of history and physical examination are decidedly useful in managing the differential diagnosis of low back pain with or without leg pain.

Localized Low back pain between the inferior gluteal folds and posterior costal arch, tenderness at the iliac crest rim, or reduced sensation below the iliac crest at the buttocks are findings consistent with CN. In the SCN and MCN, if involved, leg pain is usually found present with radiation to the ipsilateral leg, such that leg complaints are found in 47 to 84% of CN patients. CN pain may be elicited or worsened with walking, prolonged sitting, extending, bending, or rotating the lumbar region or any movement of the lumbar region.[5]

CN involving the ICN typically presents with painful and/or burning sensations near the medial and caudal part of the buttocks, proximal/dorsal thigh, skin of the labia major or scrotum, and/or lateral anal margin. Pain should worsen with sitting, especially on hard surfaces, and should be relieved with standing or lying on the patient's side. Pain should classically be significantly worsened with activities such as bicycle, motorcycle, or horseback riding.[17][20]

Evaluation

After completing a thorough history and physical examination, diagnostic nerve blocks serve as the next step in evaluating CN. Isu et al. outlined five diagnostic criteria for SCN entrapment: low back pain with involvement of the iliac crest and gluteal region, worsening pain with movement of the lumbar spine, tenderness over the posterior iliac crest at the location of nerve entrapment, reproduction of pain and/or numbness with compression at site of nerve entrapment, and symptom relief with a diagnostic block.[16] A similar diagnostic criterion is described for MCN entrapment with corresponding MCN anatomy. 

ICN entrapment can be evaluated per the Nantes criteria, with the understanding of the almost 25% coexistence of pudendal neuralgia with ICN entrapment to determine the likelihood of ICN involvement.[20] 

As with SCN, and MCN involvement, it is critical to ensure other etiologies for the patient's pain have been thoroughly evaluated and considered, as well as to consider SCN/MCN/ICN involvement in the patient's pain complaints to avoid treatment courses for causes better explained by SCN/MCN/ICN involvement.

Treatment / Management

The treatment of CN is often multimodal, starting with conservative measures such as physical therapy and progressing to pharmacotherapy and minimally invasive procedures. If symptoms are refractory, surgical decompression can also be performed.

Similar to other compressive neuropathies, anticonvulsant neuropathic pain agents such as gabapentin and pregabalin are often used to try and achieve analgesia. Additionally, other classes of medications, including serotonin-norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs), and non-steroidal anti-inflammatory drugs (NSAIDs), may also be used.[21] These medications can result in varying side effects and may not be tolerated.(B2)

Given the focal nature of CN, targeted interventional procedures are increasing in popularity as an integral part of the treatment paradigm. There are several different approaches to these procedures, as outlined below.[17]

• Nerve blocks can serve as both diagnostic and therapeutic. The technique is often provider-specific and usually consists of a local anesthetic with or without the addition of corticosteroids. As with other peripheral nerve blocks, the therapeutic benefit can vary greatly. Often, the patient will experience the analgesic effects of the local anesthetic for at least its duration of action, and with the addition of corticosteroids, the pain relief can last for months. The SCN can be targeted via palpation guidance using anatomical landmarks, ultrasound guidance, or under fluoroscopy. Nielsen et al. recently described a novel SCN block technique under ultrasound guidance that resulted in successful anesthesia in 90% of blocks.[22]
• Radiofrequency ablation (RFA) can be performed after successful nerve blocks for potentially longer-lasting symptom relief. Ablation of CN via conventional thermal radiofrequency and pulsed radiofrequency resulting in significant analgesia have been described in multiple case reports.[11][23] The risks of thermal RFA include hypesthesia, dysesthesia, and neuroma formation, which theoretically are muted with pulsed RFA. However, the efficacy of pulsed RFA compared to thermal RFA is still under investigation.
• Neuromodulation using peripheral nerve stimulation (PNS) has gained popularity for targeted therapy in a variety of pain conditions of peripheral nerve origin.[24] After a successful diagnostic nerve block, PNS has been shown to decrease Numerical Rating Scale (NRS) pain scores by almost 85% in a variety of peripheral neuralgias. In a recently published case series, Abd-Elsayed demonstrated 100% pain relief for greater than one year after the placement of two contact leads over the right SCN.[25]
• Surgical decompression is last in the line of treatment options and only appropriate when all other modalities have been exhausted. As our anatomical understanding of the cluneal nerves has evolved and treatment options such as RFA and PNS have progressed, the need for surgical intervention has been sparse. Results from surgical decompression have also been shown to be inconsistent, with one report demonstrating excellent pain relief in only 13 out of 19 cases.[26]
• The ICN, if involved, can be addressed by many of the modalities above; additionally, nerve blockade injections of the ICN with >50% relief in pain while sitting compared to previous pain with sitting before the injection is considered a strong indication of ICN nerve involvement and the likelihood of success with further therapies directed at the ICN by the Nantes criteria.[20]
(A1)

Differential Diagnosis

The differential diagnosis for CN is broad, including both common axial lumbar spine pathology and peripheral nerve and muscle etiologies. Lumbar radiculopathy secondary to intervertebral disc herniation must be considered. Other causes of lumbar radiculopathies, such as foraminal stenosis due to lumbar spondylosis, also remain high on the differential. These causes can mimic symptoms of CN like lower extremity pain and paresthesia. Furthermore, extra-spinal disorders like piriformis syndrome, which can lead to compression of the sciatic nerve, and myofascial pain syndromes involving the quadratus lumborum and gluteal musculature can present similarly to CN.[21] 

Lastly, sacroiliac joint dysfunction is a prevalent musculoskeletal issue that produces symptoms in the low back, gluteal, and hip regions that must be considered when evaluating CN. A cluneal nerve block is an important diagnostic step in differentiating CN from these other clinical disorders.

Prognosis

Limited data exist on the prognosis of CN. Diagnostic nerve blocks with local anesthetics can provide short-term pain relief, and the addition of corticosteroids may provide longer-term analgesia. Neuromodulation offers the potential of symptom relief for greater than one year, with long-term data still being collected for this emerging technology.

Complications

Although rare, procedural complications are a known risk of treating CN. Risks of more involved procedures, including RFA, PNS, and surgical decompression, are theoretically greater than for nerve blocks. One study assessing the safety and efficacy of PNS in the treatment and management of patients with chronic pain of likely peripheral nerve origin found no device-related severe adverse events were reported in 94 participants followed for up to one year.[24] 

Pharmacologic intervention with common neuropathic pain agents such as gabapentin and pregabalin can lead to drug-associated adverse events. The most common adverse effects of these medications include dizziness, drowsiness, and nausea.[27]