Incidence of Neurological Deficits

The incidence of neurological complications following regional anesthesia varies significantly based on the type of nerve block, patient characters, and the use of imaging tools. Determining the exact incidence of perioperative nerve injury presents significant challenges. Generally, the incidence of nerve injury following neuraxial blockade is extremely low, though these injuries can often be permanent. In contrast, injuries resulting from peripheral nerve blockades are more common but rarely lead to long-term or permanent damage [5, 9].

Despite advancements in technology, such as US and fluoroscopy guidance, computed tomography (CT) and magnetic resonance imaging (MRI) guidance, the rates of peripheral nerve injuries have remained consistent over the years. Overall, the data indicate that the incidence of neurological complications is low, typically reported at less than 1%, while permanent neurological deficits occur in approximately 0.01–0.03% of cases. Long-term injuries following peripheral nerve blocks are rare, with an incidence of 2–4 per 10,000 blocks [5, 9]. Neurological complications after neuraxial anesthesia are also very rare, with transient neurological symptoms occurring in 0.1–0.4% of cases, and long-lasting neurological deficits ranging from 0.3 to 10 per 10,000 procedures. However, these rates have been observed to rise, particularly among high-risk patients [5, 10].

For neuraxial techniques, the incidence of specific neurological complications includes epidural hematoma, which occurs in 2 per 100,000 to 1 per 140,000–220,000 neuraxial procedures, and spinal epidural abscess, with an incidence of 1 in 40,000 to 1 in 100,000 neuraxial anesthetics. Although these are very rare, they can have devastating neurological consequences if unrecognized [11]. A Swedish study reported vastly different incidences of spinal hematoma, ranging from a risk of 1:200,000 in young women undergoing obstetric epidural blockade to a risk of 1:22,000 in elderly women undergoing hip fracture repair, and as high as 1 in 3600 for those undergoing knee arthroplasty [12].

The incidence after pain interventions revealed that transforaminal epidural procedures for pain management can be associated with catastrophic neurological injuries. Several case reports have described infarctions of the spinal cord, brain stem, cerebrum, or cerebellum following both lumbar [13, 14] and cervical [15] transforaminal injections. Recent evidence has highlighted the role of particulate steroids in these injuries, suggesting that the effectiveness of non-particulate steroidal preparations, such as dexamethasone, may be comparable to that of particulate preparations [14, 16]. It is recommended not to use particulate steroids for cervical transforaminal epidural injections. To avoid direct injection into critical structures, the final position of an immobile needle during transforaminal injection should be confirmed by injecting a contrast medium under real-time fluoroscopy [5].

Mechanism of Neural Injury

Neural injury following regional anesthesia or pain interventions can manifest as transient neurological symptoms (TNS), including burning pain, hypoesthesia, tingling or paresthesia, and motor weakness. These symptoms may develop following regional techniques or pain interventions. TNS is generally a self-limiting condition that resolves spontaneously within a few hours to a few days post-surgery. However, persistent nerve damage can occur due to direct trauma, prolonged ischemia, predisposing neuropathy or neurotoxicity, potentially leading to long-term sensory or motor deficits [11, 17]. Predisposing diseases may decrease the threshold of nerve injury, increasing the risk of postoperative nerve injury. This concept aligns with the “double crush” hypothesis, which suggests that neural function is compromised when axons compressed at one site, become more vulnerable to damage at another site. Additionally, proximal neural injuries may heighten the likelihood of distal nerve impairment [9]. Although the underlying mechanisms of neurological deficit following regional anesthesia are not fully understood, several contributing factors have been identified.

Mechanical or Traumatic Injury

Nerve damage following LA injection may result from direct trauma, hematoma formation, or secondary to local inflammation. Injury can occur from different types of trauma to the spinal cord or peripheral nerves during the procedure due to needle or catheter trauma, intraoperatively from surgical trauma and manipulations, or postoperatively due to nerve compression caused by post-operative positioning, tight dressings, hematomas, inflammation, and infection [9, 11]. The main source of direct peripheral nerve injury is likely attributed to the injection of LA into a fascicle, causing myelin and axonal degeneration [11]. To reduce the risk of nerve injury, regional techniques should ideally be performed under imaging guidance, such as US or fluoroscopic, to improve needle placement accuracy and enhance needle visibility [5].

Chemical Neurotoxicity

Local anesthetics can cause neurotoxicity, particularly when the nerves are exposed to high concentrations or prolonged infusions of the LAs [11]. Establishing maximum allowable doses for LAs is essential to prevent neurotoxicity. Nerve injury can also result from the injection of toxic substances, such as alcohol, phenol, or high concentrations of LAs [5, 11].

All commonly used LAs in clinically used concentrations produce neuroapoptosis and neurotoxicity in a concentration-dependent manner. The toxicity correlates with the lipophilicity and therefore with the potency of the LA, but is independent of the chemical class (ester/amide). Ester- or amide-type LAs are equally neurotoxic.

The effect of LAs on neurons includes calcium homeostasis, reducing mitochondrial metabolism thereby increasing oxidative stress, and directly increasing the rate of cell death [18, 19]. The net result of these processes results in a dose-dependent neuronal death, with low doses of anesthetic inducing apoptosis and a high dose inducing necrosis [9, 19].

Studies on the toxicity of LAs have shown that the degree of neurotoxicity depends primarily on the concentration of the LA. Over time, the concentrations of the LAs used have decreased. The high concentration of lidocaine 2% is more toxic than equipotent doses of bupivacaine 0.5% [18, 20]. A comparison of LD50 values for different LAs indicated the following order of neuroapoptotic potency, listed from highest to lowest toxicity: tetracaine > bupivacaine > prilocaine = mepivacaine = ropivacaine > lidocaine > procaine = articaine [21].

Cases of cauda equina syndrome (CES) were initially reported after the introduction of the microcatheter technique for continuous spinal anesthesia using hyperbaric 5% lidocaine. Subsequently, transient neurologic symptoms characterized by radicular pain without motor deficit were observed after a single dose of spinal lidocaine injection. The development of CES strongly implicated neurotoxic effects of lidocaine, which were exacerbated by the accumulation of hyperbaric lidocaine in the caudal dural sac when repeated dosing through spinal catheters [18].

Vascular Cause

Nerve ischemia can result from hypoperfusion, prolonged use of a tourniquet, vascular compromise during nerve blocks, or vascular abnormalities. To reduce the risk of complications, it is important to avoid prolonged tourniquet times or excessive pressure [6, 11]. Vascular injuries are among the most catastrophic potential complications following regional anesthesia. Ischemic vascular injuries may arise from embolic phenomenon, direct trauma, or vasoconstriction of the artery of Adamkiewicz, leading to anterior spinal artery syndrome or watershed ischemia due to hypotension or vasoconstriction [11].

Inflammation and Infection

Infection, often accompanied by inflammation, can lead to nerve damage. Strict adherence to aseptic technique is crucial, particularly in emergencies or immune-compromised patients. Non-specific inflammation of the peripheral nerves is increasingly recognized as a significant mechanism contributing to neurological deficits following peripheral nerve blocks [11, 22].

Pre-Existing Pathology

Patients with underlying nerve pathology are more susceptible to peripheral nerve complications, although the exact mechanisms remain unclear. Risk factors include prolonged duration of nerve block and increased sensitivity to LA neurotoxicity. Pre-existing conditions, such as diabetic neuropathy or post-chemotherapy-induced neuropathy, increase the risk of already compromised nerves [2, 22, 23].

Other Causes

Direct neurogenic injury to the spinal cord or peripheral nerve caused by needle or catheter trauma, LA toxicity, or surgical trauma varies in severity and prognosis [11]. Some nerve injuries may occur independently of the anesthetic or surgical intervention. For example, a patient who underwent total knee replacement under neuraxial anesthesia with a continuous epidural catheter may develop foot drop due to a peroneal compressive neuropathy at the fibular head, unrelated to the epidural catheter. Such injuries can often be treated and may lead to complete recovery [2, 5].

Specific Injuries Following Neuraxial Procedures (Spinal or Epidural)Epidural Hematoma

This serious complication typically arises from unrecognized coagulation disorders or anticoagulation therapy. Early symptoms may include vague sensory changes below the site of the hematoma, followed by flaccid paralysis of the lower extremities. Late complications can involve neurogenic bowel and bladder dysfunction [24]. If not promptly diagnosed and treated, an epidural hematoma can compress the spinal cord, potentially resulting in irreversible neurological damage.

Epidural Abscess

This is the second most severe complication following neuraxial procedures, primarily resulting due to failure to ensure an aseptic technique, particularly in emergency situations or among immune-compromised patients. Symptoms include pain, fever, and elevated inflammatory markers, along with specific signs such as sensory deficits, paraparesis, and, later, neurogenic bowel and bladder dysfunction [25]. An epidural abscess can lead to spinal cord compression and irreversible neurological damage.

Spinal Cord Injury

Although rare, spinal cord injuries can be severe complications from needle misplacement, accidental intramedullary injections, spinal deformities, or unknown anatomical variations, as well as secondary to compressive lesions like hematoma or abscess. Utilizing imaging tools such as fluoroscopy or US is highly recommended, particularly for patients with complex anatomical challenges (e.g., scoliosis, previous spine surgeries) [2, 22]. Diagnosis of a compressive lesion, such as epidural hematoma or abscess, demands emergent neurosurgical consultation for consideration of decompression.

Post-Dural Puncture Headache (PDPH)

PDPH is an uncommon complication associated with intrathecal blocks, particularly when small pencil-point spinal needles are used. It occurs more frequently following an unintentional dural puncture with a large-bore epidural needle. PDPH remains a serious complication of labor epidural analgesia after accidental dural puncture. A persistent cerebrospinal fluid (CSF) leak resulting from a dural puncture can lead to a severe headache that worsens in the sitting or upright position [26]. However, the exact underlying mechanisms of PDPH are still incompletely understood, and longstanding beliefs of dysregulation of CSF homeostasis due to CSF fluid loss are currently being challenged. Other mechanisms of PDPH are suggested, related to the autonomic nervous system or the release of calcitonin gene-related peptide associated with activation of the meningeal and cerebral arteries. If not properly managed, PDPH can result in additional complications, such as subdural hematoma and cranial nerve palsies [27].

Factors Associated with Deviation from Standard of Care

Deviations from the standards of care have been identified as significant risk factors for the development of neurological complications following regional anesthesia.

Inadequate Patient Assessments

A key contribution to these deviations is inadequate patient assessments, in particular the failure to conduct a comprehensive preoperative evaluation. For instance, failing to perform a thorough preoperative evaluation may overlook anatomical abnormalities, previous neurological deficits, or musculoskeletal deformities. The primary goal of such evaluation for patients with neuromuscular disorders is to predict and minimize potential neurologic complications while improving surgical outcomes [28]. Patients with preexisting neurological conditions, such as inflammatory neuropathies, spinal canal stenosis, and vertebral pathologies, are at a higher risk of neurological complications following regional anesthesia [29].

Not reviewing the patient’s anticoagulation profile further exacerbates these risks, especially during neuraxial procedures. Similarly, inadequate history taking, such as neglecting to inquire about significant systemic diseases, ongoing neurological disorders, or past adverse effects related to regional anesthesia, can increase the likelihood of complications.

There is increasing evidence of the connection between preoperative psychological factors and psychological interventions to improvements in postoperative pain outcomes [30]. Poor communication with patients during and after the procedure may also contribute to a higher risk of neurological complications. Effective communication is critical to ensure patient understanding and to mitigate risks associated with regional anesthesia [31,32,33].

Technical Factors

Technical errors are another crucial aspect of deviations from the standard of care. Issues such as incorrect needle positioning, using excessive force, or improper angles during needle placement can result in direct nerve damage or technical failure [34]. Similarly, incorrect needle selection, such as using a cutting or large-gauge spinal needle, may increase the likelihood of adverse outcomes like PDPH [27]. Anatomical variations in nerve and vascular structures may further complicate procedures, particularly in patients with unique or atypical anatomy [33].

Limited Access to Imaging Tools

Failure to use proper imaging guidance, such as ultrasound or fluoroscopy, can result in incorrect insertion sites or needle misplacement, significantly increasing the risk of unintentional nerve damage. Ultrasound guidance during peripheral nerve blocks has been shown to improve success rates and to reduce the risks of complications [2, 22]. Similarly, the use of nerve stimulation during these procedures enhances needle placement accuracy, further minimizing the likelihood of complications [23,24,25]. A Cochrane review of 33 trials involving 2293 pediatric patients suggests that ultrasound guidance likely reduces the risk of failed regional blocks and moderately decreases postoperative pain 1 h after surgery, with a reduction of 1.3 points on the pain scale. It also significantly increases the block duration by approximately 42 min. There may be little or no difference in the risk of transient neurological injury [35].

Injection of the Correct Drug

Dosing errors involving LAs can pose significant risks. Improper use of the recommended doses may result in systemic or neurotoxicity. Higher concentrations, excessive volumes, or prolonged infusions of LAs administered via continuous catheter techniques can increase the risk of neurotoxicity. Certain LAs like lidocaine in high concentrations have been specifically associated with neurotoxicity. Furthermore, unintentional intravascular injection or the administration of large volumes of LAs can lead to systemic toxicity [22, 34].

Suboptimal Surgical Positioning

Improper patient positioning during surgery can significantly increase the risk of nerve injury.

Additionally, the positioning required for surgical procedures may contribute to neurologic complications. Several mechanisms are implicated in the nerve injury during surgery, including traction, transection, compression, contusion, ischemia, and stretching [2, 36]. These complications can range from transient peripheral neuropathies to more severe conditions such as compartment syndrome and rhabdomyolysis. Data from the American Society of Anesthesiologists Closed Claims highlights that peripheral nerve injury, a subset of positioning-related injuries, accounts for 15% of claims across surgical disciplines. Placing patients in non-standard positions during surgery, including excessive flexion, abduction, or tight fixations during general anesthesia, along with prolonged surgery, can lead to nerve compression, and highlighted risk of nerve injury, especially in predisposed patients [30, 33, 37]. Furthermore, the physical forces exerted during surgery, such as placement of prostheses, can be excessive, and may strain anatomical structures distal from the surgical site, including the vertebral column [5, 36]. Additionally, certain surgical procedures necessitate patient positioning that would be intolerable without anesthesia. Awareness of suboptimal surgical positioning should encourage a careful evaluation of the risk–benefit ratio when considering regional anesthetic techniques [2, 36].

Musculoskeletal Disorders

Musculoskeletal deformities can impact patient positioning during surgery, hinder access to regional anesthesia, and obscure anatomical landmarks. Poor positioning of the patient on the operating table may leave certain areas of the body inadequately supported, necessitating additional care during anesthetic management [2, 36]. Patients with longstanding rheumatoid arthritis which is characterized by the destruction of synovial joints, primarily affecting the small joints, as well as the temporomandibular joint and spine joints, pose unique challenges for anesthesiologists [2, 34, 37].

Orthopedic Surgery and Regional Anesthesia

Patients with musculoskeletal deformities may pose challenges for regional anesthesia due to difficulties with surgical positioning and limited access to regional techniques. Orthopedic surgeries are among the most common procedures performed in patients with rheumatoid arthritis or other musculoskeletal disorders, and most regional anesthetic methods may be utilized. The most common contraindications for regional anesthesia include patient refusal, anticoagulant therapy, infection at the puncture site, and hemodynamic instability [2, 35]. Additionally, patients with musculoskeletal deformities are more prone to undergo major orthopedic surgeries in suboptimal positions and for prolonged durations. All these risk factors are likely to contribute to increased adverse outcomes following neuraxial analgesia [35].

Regional Techniques in Anesthetized or Deeply Sedated Patients

The practice of initiating regional blocks in adults under general anesthesia remains controversial due to concerns about the inability of the anesthetized patients to report pain or paresthesia, and potentially increasing the risk of neurologic complications [