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Popliteal Nerve Blocks: Risks and Benefits in Foot and Ankle Surgery
Popliteal nerve blocks commonly serve as the regional anesthesia of choice to provide perioperative pain control in many types of foot and ankle surgery. In general, peripheral nerve blocks may limit the side effects of general anesthesia by decreasing the amount of intraoperative sedation needed. Peripheral nerve blocks also decrease postoperative narcotic use and allow earlier time to discharge.1-3 Patients undergoing foot and ankle surgery augmented with popliteal nerve blocks historically report improved satisfaction scores, reduced pain, and faster recovery.2,3 For these reasons, the authors note popliteal nerve blocks have gained popularity and remain relatively routine in foot and ankle surgery.
Labat first decribed popliteal nerve blocks in 1922.4 There are currently several techniques for administering this type of regional anesthesia, each with advantages and disadvantages. Ultimately, the goal of its administration is to achieve intraoperative and postoperative anesthesia and analgesia. Unfortunately, this is not always achieved and there is notable risk for neurologic morbidity. Popliteal blocks have historically appeared to be a safe and reliable option for decreasing postsurgical pain after foot and ankle surgery, but recent data has revealed that this procedure is not without complications.5
Reviewing Popliteal Nerve Block Techniques
A popliteal nerve block involves the administration of a local anesthetic to the perineural region along the sciatic nerve distribution at the level of the popliteal fossa, with the option for an adjunctive saphenous nerve block near this location. The injected local anesthetic acts on the nerve by inhibiting depolarization and thus preventing signal transmission.6 Ropivacaine is often the local anesthetic of choice, as it is less lipophilic than bupivacaine and theoretically should produce less motor blockade. Otherwise, ropivacaine and bupivacaine have similar onset time, duration, and efficacy when utilized for popliteal blocks.5
Ultrasound guidance and nerve stimulation techniques are the main techniques employed to identify the sciatic nerve during popliteal blocks. These techniques may be used in isolation, or in combination to confirm injection placement within the popliteal fossa. Nerve stimulation involves insertion of a needle that connects to a stimulator delivering electrical current in a pulsating fashion. As the needle advances and approaches the nerve, one will observe a motor response of the foot. The operator will then decrease the electrical current to approximately 0.5mA while still maintaining motor function. This lower current ensures proximity to the nerve, which serves to target the anesthetic to the epineural location.7
Ultrasound-guided popliteal blocks offer direct visualization of the nerve, unlike the nerve stimulation technique. Ultrasound can help confirm anatomic variation of the nerve course, as well as provide visualization of the anesthetic during injection and diffusion adjacent to the nerve. Evidence supports ultrasound guidance and nerve stimulation as successful when used in isolation, or in combination.8-10 Previous randomized controlled trials reported better success rates of popliteal blocks when performed by ultrasound guidance compared to nerve stimulation, while others have shown no difference in block performance or complication rates.8-10
Some feel ultrasound guidance is advantageous over nerve stimulation because of the ability to visualize the nerve rather than having to use palpation, landmarks, and continual testing of motor response to identify the correct anatomic location.11 Danelli and colleagues observed that ultrasound guided techniques may be superior to nerve stimulation for popliteal sciatic nerve blocks.11 They observed ultrasound guidance to lead to more successful analgesia/anesthesia with less procedural pain and requiring less time to perform compared to the nerve stimulation technique. Technique variation, anatomic variants, and physician experience are all variables that continue to influence outcomes of each nerve block technique.11
Exploring the Pathophysiology of Peripheral Nerve Injury
It is often challenging to decipher the exact cause of the often multifactorial etiology of peripheral nerve injury following popliteal nerve blocks. Many perioperative factors such as local anesthetic choice, surgical positioning, tourniquet use, and patient-specific comorbidities are responsible for the degree of injury.12
The postsurgical inflammatory response may also induce a transient peripheral neuropathy and manifest clinically with neuritic-type symptoms. Similarly, the “double crush” theory suggests that patients with preexisting neurological deficits are at higher risk for subsequent nerve injury. Such a phenomenon may occur even if secondary nerve insult occurs at a site away from the site of original neurologic injury. Causes of these preexisting neurological deficits include nerve entrapment, metabolic, toxic, or hereditary sources, demyelination, or ischemic neuropathies. Diabetic neuropathy is of particular concern, as it may increase the risk of postoperative nerve damage by 10-fold.14
The majority of neurological injuries related to peripheral nerve blocks involve prolonged paresthesia distal to the knee or infiltration site. Although the exact mechanism of neurologic injury following popliteal nerve blocks remains unclear, detailed descriptions of the physiologic mechanism of peripheral nerve injury do exist.6
Nerve injuries occur during the injection process by 3 primary mechanisms: mechanical injury, chemical injury, and/or ischemic injury. In regard to the injection itself, the location of the needle tip is important to determine the likelihood of injury and one must understand peripheral nerve anatomy. An injection that takes place outside the epineurium is considered perineural and any injection inside the epineurium is considered intraneural.13,15 An intraneural injection can be either extrafascicular (without breaching the perineurium) or intrafascicular (breaching the perineurium). Even small amounts of injected intrafascicular solution can lead to axonal degeneration and permanent nerve damage, leading us to believe that damage to the perineurium is a likely cause of peripheral nerve block-related nerve injury.13,15
Other studies have demonstrated that nerve trauma may not result in direct nerve damage, but rather a neurotoxicity component resulting from intrafascicular placement of local anesthetic during the injection.16,17 Jeng and colleagues described how intrafascicular injections along with high injection pressures can result in neuronal injury.17 Additionally, a long-beveled needle can potentially cause more mechanical injury to a nerve compared to a short-beveled needle. Fortunately, inadvertent intraneural injections often do not result in intrafascicular needle placement.17 At the cellular level, nerve damage may be related to prolonged action potentials caused by direct pressure from the injected fluid bolus or a hematoma secondary to adjacent vascular injury. It is thought that this pressure injury cascade may also induce ischemic injury to the nerve by compromising capillary perfusion and decreasing vascularity to the nerve.13 The exact reason for nerve injury is often multifactorial and difficult to ascertain and understanding of the pathophysiology of block-related nerve injury may help clinicians to minimize adverse events.17
Understanding Other Potential Complications of Peripheral Nerve Blocks
Current literature reports minimal risk and low complication rates of peripheral nerve blocks,18,19 but we know this procedure does not come without risks. Reports of the overall rate of neurological injury after administration of popliteal blocks range from 0% to 10%.18,19 Provenzano and colleagues reported no complications affiliated with popliteal fossa nerve blocks in a group of 467 patients who underwent foot and ankle surgery.18 Conversely, Lauf and colleagues discovered a short-term complication rate of 10.1% and the long-term complication rate of 4.3% in 855 popliteal blocks for foot and ankle surgery.19
Complications of popliteal nerve blocks related to peripheral nerve damage may include motor and/or sensory deficits to the lower extremity at or distal to the site of insult. Clinical manifestations commonly include prolonged numbness, burning/tingling, hyperalgesia, diffuse paresthesias, weakness, paralysis, or chronic pain. Fortunately, most recorded neurologic complications following popliteal blocks are considered minor and limited to sensory deficits. Isolated sensory deficits occurring within the territory of the peripheral block can be closely monitored and managed with symptom control as symptoms typically often resolve within 4-6 weeks.3,13,20 Major complications including prolonged sensory or motor deficits have been documented in less than 1% of patients undergoing popliteal nerve blocks. Unfortunately, prolonged motor deficits are generally a more ominous sign and recovery potential is difficult to predict.13,18-21
Increased fall risk has been another cited potential complication following popliteal nerve blocks in foot and ankle surgery. Johnson and colleagues performed a retrospective analysis of falls in patients receiving popliteal nerve blocks following foot and ankle surgery.22 The authors observed no increase in fall rate attributable to single-injection popliteal nerve block and statistically, the risk of falls after popliteal nerve blocks was not significantly greater than the fall rate of all patients undergoing major orthopedic surgeries. The authors concluded that one cannot attribute popliteal nerve blocks alone to an increased postoperative fall risk. Rather, they thought quadriceps weakness played a larger role in postoperative falls.22
Diagnosing and Managing Peripheral Nerve Block Injuries
Early recognition of postoperative nerve injury and proper subsequent management is essential to allow for the best chance of neurological recovery. Diagnosis of suspected peripheral nerve injury is based on history, symptoms, and physical examination. Upon initial presentation, it is important to determine if there is single or multiple nerve involvement and if there is motor and/or sensory impairment. One should rule out potentially reversible factors such as extrinsic compression (tight cast or dressing) or intrinsic compression (hematoma, compartment syndrome), as these are other possible sources of nerve injury in the postoperative period.
Management of postop nerve injury is dictated by the degree and type of nerve injury (motor versus sensory). If physical examination only reveals mild sensory deficits, reassurance that sensation is likely to return is the key to alleviating patient anxiety. Close follow-up within 4 weeks is recommended to monitor patient symptoms and review the need for further diagnostic tests.13 Electrodiagnostic studies consisting of electromyography (EMG), nerve conduction studies (NCS), or both should be obtained when peripheral nerve injury is suspected. However, it is often recommended that these studies are delayed for 2–3 weeks, as this allows for sufficient signs of Wallerian degeneration to appear.13 At our institution, surgeons will commonly refer patients for high-frequency ultrasound imaging to diagnose the location and degree of nerve pathology when suspecting peripheral nerve injury.
Consultation with a neurologist is advisable when suspecting any nerve block complication. Early neurology consultation is especially important in the setting of motor nerve involvement or progressive neurologic deficit. When the timeline for obtaining electrodiagnostic studies is in question, the authors ultimately recommend deferring to the consultant neurologist for imaging recommendations, as some may want early imaging to exclude preexisting peripheral nerve injury. Formal physical therapy is typically initiated shortly after detection of the nerve injury, with the intent of reducing inflammation, mitigating disuse atrophy, and preventing flexion contractures. No pharmacological therapy has been demonstrated to enhance neuroregeneration, but the use of neuropathic agents and non-narcotic analgesics to provide symptom relief in acute peripheral nerve injury is indicated. When incomplete or no improvement has taken place by 5 months, a referral to a specialist peripheral nerve surgeon should be considered.13,20
Weighing the Risks and Benefits
Popliteal nerve blocks are indicated in patients undergoing surgery to the lower leg, foot and/or ankle, with the goal of reducing perioperative pain and improving patient satisfaction. Contraindications include infection at site of injection, local anesthetic allergy, coagulopathies or pre-existing peripheral neuropathies.3,6,12 Currently, the existing literature lacks sufficient evidence as to the best indications for popliteal nerve blocks in lower extremity procedures. This absence of clinical guidelines ultimately makes it the responsibility of both the surgeon and anesthesia team to evaluate the risk-benefit profile of each individual patient to determine if a perioperative popliteal nerve block is appropriate. One should consider the use of a peripheral nerve block based on the patient’s relative pain tolerance, anatomic location of the surgical procedures, and the amount of surgical trauma the patient will endure.2,12-14
Based on available data, popliteal nerve blocks have a relatively low complication rate ranging from 0–10% with long term complications being less than 1%.6,19 Lauf and colleagues identified specific risk factors for complications following popliteal nerve blocks. Those risk factors included patients between 40–65 years of age, normal or underweight body mass index (BMI), current smoking, and surgery performed at an outpatient surgery center.19
At the authors’ institution, the risks of regional blocks are encompassed within the anesthesia consent form, and patients are expected to review the risks prior to consenting to the popliteal block by the anesthesia team on the day of surgery. Depending on the surgeon and anesthesia staff, the explanation of risks and benefits may vary. After anecdotally observing a slight increase in nerve-related popliteal block complications, we actively counsel patients on the risks and benefits of these blocks. We find evidence-based recommendations most effective in helping patients decide if they wish to electively undergo a popliteal nerve block in the perioperative period, or if a regional block closer to the surgical site is better suited for that particular patient.
Final Thoughts
At our institution, procedural indications for popliteal blocks remain variable depending on anesthesia staff, location, and surgeon preference. Most surgeons at our institution do not utilize popliteal blocks in forefoot surgery, and rarely utilizing them in midfoot surgery. We will traditionally recommend popliteal nerve blocks in sensate patients undergoing elective osseous reconstruction of the hindfoot or ankle, pending they have none of the above-mentioned risk factors. We refrain from employing popliteal blocks in patients with peripheral neuropathy or demyelinating neuropathies. In cases of hindfoot or ankle trauma, the decision to proceed with a popliteal block is often guided by the acuity and severity of injury while also considering patient-specific risk factors.
The authors are currently collaborating with anesthesia staff and other foot and ankle surgeons within the institution to standardize the preoperative education process for popliteal block consent. We hope to ultimately establish a risk stratification scale to identify which patients are at risk for popliteal block–related nerve injury, and which foot and ankle procedures can successfully be performed without the use of a popliteal block. We look forward to future high-level studies aimed at defining the risks associated with popliteal nerve blocks and subsequent management of such injuries. We hope this review will raise awareness to the potential risks of popliteal nerve blocks, while encouraging surgeons to educate their patients on the associated risks before consenting.
Dr. Judickas is a first-year resident at the Grant Medical Center Foot and Ankle Residency Program.
Dr. Stone is a third-year resident at the Grant Medical Center Foot and Ankle Residency Program.
Dr. Barron is a board-certified foot and reconstructive rearfoot/ankle surgeon. He practices at Gentle Foot Care—A Division of Ohio Foot & Ankle Specialists. He is the Podiatry Division Chief at Doctors Hospital in Columbus, OH, and is the Director of Research to the Grant Medical Center Foot and Ankle Residency Program.
References
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