Ablative Nerve Block

In 1975, ablation was initially used to treat chronic back pain and sciatica of unknown etiology as an alternative to the prior treatment of severing the dorsal rami via the intertransverse ligaments. It is a minimally-invasive procedure that involves coagulation necrosis of afferent nociceptive signals via high-frequency waves (300 to 500 Hz). Currently, ablation is most commonly used to destroy the medial branches of the dorsal rami which are responsible for facet-joint-mediated back pain. However, ablation of peripheral nerves that have a primary role in nociception for the treatment of pain is supported by the recent literature, particularly when symptomatology has been refractory to conservative management but is relieved with targeted blocks using short-acting anesthetic agents.

Facet-mediated pain is due to facet arthropathy or facet arthritis which arise during the degenerative process of the spinal column. A single facet joint is composed of the inferior articulating process of one vertebra and the superior articulating process of the vertebra directly inferior. Medial branches from the dorsal rami of spinal nerve roots at the same level and one level above provide sensory innervation to the facet (for example, the L4-L5 facet joint is innervated by the medial branches of L3 and L4). The medial branches typically course over the lateral border of the superior articulating process.

An insulated electrode with a non-insulated tip is advanced toward the concavity that is formed between the superior articulating process and the adjacent transverse process, which is in proximity to the nerve that is suspected of causing the symptoms. It is in this region where the use of high-frequency or radiofrequency energy is generated to produce a lesion via coagulative necrosis, thereby disrupting afferent pain signals. In pain medicine, the use of radiofrequency ablation most commonly involves targeting the specific medial branches of the dorsal rami that innervate pain-producing facet joints under fluoroscopic guidance. However, further investigations regarding the efficacy of ablation in other common pain syndromes continue to diversify its use.

Nerve ablation has typically been used in the treatment of facet-mediated axial back pain of the cervical and lumbar spine that has failed conservative therapy. Additionally, it has other uses such reflief for chronic neck pain after whiplash and chronic headache syndromes due to occipital and trigeminal neuralgia. Ablation is performed after successful analgesia of the suspected trouble-causing nerve with a local anesthetic. There continue to be studies investigating the role of ablation in peripheral nerve-mediated pain outside of the spinal column, particularly in knee osteoarthritis and plantar fasciitis.

Absolute contraindications for ablative nerve blocks are few, but these include active local infection at the site of needle insertion and elevated intracranial pressure. The use of anti-coagulants provides a unique challenge for practitioners, as clinical judgment must be used following accepted guidelines. Current guidelines put forth by ASRA (American Society of Regional Anesthesia and Pain Medicine) recommend that:

• Aspirin to be stopped 6 days before the procedure
• Clopidogrel to be stopped 7 days before the procedure
• Apixaban to be stopped 3 to 5 days before the procedure
• Rivaroxaban to be stopped 3 days before the procedure
• Warfarin to be stopped 5 days before the procedure
• Intravenous heparin to be stopped 4 hours before the procedure

Other relative contraindications that the clinician must consider before the procedure include:

• Neurologic abnormalities
• Concerning clinical or imaging finding
• Definitive causes of low back pain (for example, disc herniation, spondylolisthesis, spondylosis, spinal stenosis, malignancy, infection or trauma)
• Lack of pain relief from prior diagnostic nerve blocks

The procedure is routinely done in a sterile procedure suite, with the patient lying prone on a procedure table.

 Key components of the procedure include:

• C-arm mobile fluoroscopic unit
• Local anesthetic
• continuous high-frequency generator with built-in thermocouple, impedance, voltage and amperage monitor
• Introducer needles (50 to 150 mm in length, 18 to 22 gauge in diameter)
• Active and ground electrodes

The active electrode delivers the high-frequency current which is dissipated through a ground electrode that is attached to the patient.

As with other interventional spinal procedures, only physicians specifically trained in fluoroscopically-guidance procedures should perform radiofrequency ablation. Qualified physicians typically undergo residency training in the fields of anesthesiology, physiatry, neurology, psychiatry or neurosurgery. This is followed by an interventional pain or spine fellowship that allows adequate training under an experienced interventionalist prior to performing the procedure independently. Support staff for the procedure can include an assistant to draw up medications and operate the radiofrequency generator and radiology technician to operate the C-arm, under the guidance of the practicing physician.

The destruction of tissue via radiofrequency must occur after successful diagnostic anesthetic nerve blocks have located the target nerve. During the procedure, the patient should receive little to no sedation, as they must define what they are experiencing during stimulation and lesioning of the nerve.

1. The patient is placed in a comfortable position with adequate exposure of the region overlying the target nerve. The skin is cleaned and prepped, and the target is found using fluoroscopy. A small amount of local anesthetic is injected subcutaneously at the point of needle insertion.
2. The introducer needle is then inserted through the skin, subcutaneous tissue, and muscle toward the target using fluoroscopy to guide the trajectory.
3. Once the tip of the needle is placed satisfactorily close to the target, the active electrode is inserted through the needle.
4. Sensory stimulation is used at first, to recreate the painful symptoms that the patient experiences, thus locating the target. 
5. This is followed by motor stimulation to ensure that the active tip is not close to the motor nerves.
6. Once confirmed, the generator produces a continuous output of voltage at the tip of the electrode, where it will be kept at 80oC for approximately one minute, creating a zone of thermally induced coagulation.

Nerve ablation is a minimally invasive, relatively low-risk procedure. However, adverse events may occur during placement of the introducer needle or during the ablative process. Advancement of the introducer needle has the potential to cause vascular or neural insult along the trajectory that it is traveling, while the process of thermal ablation may lead to burns (due to errors in ground pad placement), worsened pain, sensory loss or new onset neuropathic pain.

Complications are most common after intracranial ablation of the trigeminal ganglion, which may manifest as facial numbness, dysesthesia, anesthesia dolorosa, corneal anesthesia, keratitis and trigeminal motor dysfunction.

Adverse events from ablation of lumbar medial branches are far and few between with transient postoperative pain dominating as the premier adverse event.

As with any invasive technique, the risk of allergy to materials or anesthetic, hematoma formation, and infection must be considered.

One hundred million Americans suffer from some form of chronic pain that results in more than $100 billion in expenses annually. Furthermore, 84% of American adults will suffer from chronic, low-back pain at some point in their lifetime. While the causes of low-back pain are often multifactorial and can be due to many causes, the degenerative cascade that affects the spine plays a significant role. Destruction of cartilage that comprises the spinal facet joints results in severe discomfort when placed under stress, typically in extension or rotation of the spine. The sensory nerves responsible are the medial branches of the dorsal rami, which are the most common target in ablative procedures. Radiofrequency ablation has been shown to be efficacious in reducing the severity of back pain for extended periods of time (ranging from 6 months to 24 months in duration) but also has utility in the treatment of trigeminal and occipital neuralgia, complex regional pain syndrome, hip and knee osteoarthritis, and plantar fasciitis. As with other interventional procedures that provide pain relief, ablation aims to play a significant role in the reduction of opiates and other habit-forming pain medications.

This review focuses on the use of conventional radiofrequency ablation, which utilizes a continuous energy source causing coagulative thermal necrosis. Other forms of ablation used in pain management include:

• Pulsed radiofrequency: Applies heat in a pulsatile manner, but uses lower temperatures in a than conventional radiofrequency to avoid neurolysis
• Cooled radiofrequency: Utilizes specialized electrodes that are cooled with water-flowing along its shaft, but not at the active tip. This allows higher temperatures and more spherical lesion sizes to be achieved at the target site and less risk of tissue damage superficial to the target.
• Cryoablation: Causes destruction of axons by disrupting the vasa nervorum. It is increasing in popularity due to a decreased incidence of post-procedural hyperalgesia and neuroma formation compared to conventional radiofrequency.