Investigating Nerve Block Failure: What Would Make a Nerve Block Not Work?

October 10, 2025 •

4 min read

While studies show an overall success rate of around 89% for peripheral nerve blocks (PNBs), about 1 in 10 may not be fully effective [1.3.2, 1.3.8]. Understanding what would make a nerve block not work involves looking at patient-specific, technical, and pharmacological factors.

Quick Summary

A nerve block can fail due to anatomical variations, technical errors in needle placement, or pharmacological issues like incorrect medication choice or local tissue inflammation. Patient factors such as obesity and anxiety can also contribute.

Key Points

Anatomical Variation: Nerves don't always follow a standard path, making them difficult to locate; this is a primary cause of failure [1.4.2, 1.4.5].
Technical Skill: The clinician's ability to use tools like ultrasound to visualize the nerve and guide the needle is crucial for success [1.2.1].
Patient Factors: Conditions like obesity can obscure landmarks, while rare genetic traits can cause resistance to anesthetics [1.2.5, 1.5.7].
Pharmacology: The effectiveness of local anesthetics is reduced in inflamed or infected (acidic) tissue [1.5.2, 1.5.5].
Type of Failure: Blocks can fail completely, be incomplete (not dense enough), or be 'patchy' (missing some areas) [1.2.3].
Ultrasound is Key: Ultrasound guidance has significantly reduced the incidence of complications like local anesthetic systemic toxicity (LAST) [1.6.5].
Management is Crucial: Having a backup plan, such as supplemental anesthesia or conversion to general anesthesia, is a key part of managing a failed block [1.2.5, 1.3.2].

Understanding Nerve Blocks and Their Function

A peripheral nerve block (PNB) is a type of regional anesthesia that involves injecting a local anesthetic medication near a specific nerve or bundle of nerves to block pain signals from a particular area of the body [1.4.4]. These procedures are a cornerstone of modern pain management, used for surgery and to treat chronic pain conditions [1.2.8]. The local anesthetic works by temporarily interfering with the nerve's ability to transmit signals to the brain, specifically by blocking sodium channels responsible for nerve impulses [1.4.4]. When effective, a nerve block provides targeted anesthesia and analgesia, often reducing the need for systemic medications like opioids [1.3.1]. However, despite high success rates, failures can occur, defined as the inability to achieve the desired level of pain control for a planned procedure without unplanned interventions [1.3.2].

Anatomical and Patient-Related Factors in Nerve Block Failure

One of the primary challenges in performing a successful nerve block is the inherent anatomical variability among individuals [1.5.7]. Nerves do not always follow a textbook path. For example, the brachial plexus, a network of nerves targeted for upper limb surgery, can have significant variations in its position relative to surrounding muscles in up to 40% of cases [1.4.2]. Similarly, the femoral nerve can sometimes be found in an unusual position deep within a muscle, making it difficult to locate even with ultrasound guidance [1.4.5].

Several patient-related factors also increase the risk of block failure:

Obesity: Excess adipose tissue can obscure anatomical landmarks and make it difficult for ultrasound waves to penetrate effectively, reducing image quality and making the nerve harder to identify [1.2.5].
Positioning Difficulties: Conditions like arthritis or contractures can prevent the patient from getting into the optimal position for the block, complicating the procedure [1.2.5].
Anxiety: High levels of patient anxiety can contribute to the perception of pain and may complicate the procedure [1.2.5].
Genetic Conditions: Rare conditions, such as certain types of Ehlers-Danlos syndrome, can involve mutations in sodium channels, making an individual less sensitive to the effects of local anesthetics [1.2.5, 1.5.7].

Technical Reasons for Failure

The success of a nerve block is highly dependent on the skill and experience of the clinician. Technical errors are a significant cause of failure [1.6.1]. With the advent of ultrasound guidance, the ability to visualize the nerve, needle, and anesthetic spread in real-time has improved success rates [1.6.5]. However, errors can still happen.

Common technical failures include:

Incorrect Needle Placement: The anesthetic must be deposited precisely around the nerve. Injecting too far from the nerve, into a blood vessel, or into a neighboring structure will result in a failed or misdirected block [1.2.3, 1.2.4].
Poor Visualization: Failure to correctly align the needle with the ultrasound beam or misinterpreting anatomical structures can lead to missing the target nerve [1.2.1].
Maldistribution of Anesthetic: Even if the needle tip is in the right area, the local anesthetic may not spread adequately to cover all parts of the nerve or plexus, resulting in a 'patchy' block where some areas remain sensitive [1.2.1, 1.2.3].
Wrong-Sided Block: In rare cases, the block may be performed on the wrong side of the body. Checklists and 'stop before you block' protocols are designed to prevent this serious error [1.6.2, 1.6.3].

Comparison of Failure Factors


Factor Type	Description	Common Examples	Mitigation Strategy
Anatomical	Variations in the patient's anatomy make nerve location difficult.	Nerves running through unexpected muscle tissue, unusual branching patterns [1.4.1, 1.4.5].	Pre-procedural scanning with ultrasound, use of nerve stimulation as an adjunct [1.2.5].
Technical	Errors made by the practitioner during the procedure.	Poor needle visualization, maldistribution of anesthetic, wrong-site block [1.2.1, 1.6.2].	Advanced training, use of ultrasound guidance, adherence to safety checklists [1.2.5, 1.6.1].
Pharmacological	Issues related to the anesthetic medication itself or its interaction with the body.	Inflammation at the injection site, rapid absorption into the bloodstream, incorrect drug choice [1.5.5].	Choosing appropriate anesthetic for duration, avoiding inflamed tissue, careful aspiration before injection [1.2.5].

Pharmacological Causes of Ineffectiveness

Beyond placement, the medication itself plays a critical role. The chemical environment of the tissue and the properties of the anesthetic can lead to failure.

Inflammation and Infection: Inflamed or infected tissue is acidic [1.5.5]. Local anesthetics are less effective in an acidic environment because it prevents the drug from changing into the form needed to cross the nerve membrane and block the sodium channels [1.5.2, 1.5.5]. Increased blood flow to an inflamed area can also 'wash out' the anesthetic more quickly [1.5.5].
Incorrect Anesthetic Choice: Anesthetics have different durations of action. If the chosen medication wears off before the surgery is complete, it results in a 'wear-off' block [1.2.3]. The choice must be appropriate for the expected length of the procedure [1.2.5].
Systemic Absorption: If the anesthetic is accidentally injected into a blood vessel, it is quickly carried away from the target nerve and can cause systemic toxicity, a rare but serious complication [1.2.4, 1.2.6].
True Resistance: Although extremely rare, some individuals have a true resistance to local anesthetics, often linked to genetic mutations in their sodium channels. These patients may fail to achieve numbness despite technically perfect procedures [1.5.4, 1.5.6].

Conclusion: A Multifactorial Challenge

Ultimately, what makes a nerve block not work is rarely a single issue. It is often a combination of patient-specific anatomy, the clinician's technique, and the pharmacology of the anesthetic agent [1.5.1]. The failure rate for peripheral nerve blocks, while relatively low at about 8.5% to 11%, underscores the complexity of these procedures [1.3.2, 1.3.3]. Advances like ultrasound guidance and a deeper understanding of anatomical variations have significantly improved success rates, but a structured approach, careful planning, and a backup plan remain essential for managing cases where a block is not completely effective [1.2.5].

For more information from an authoritative source, you can visit NYSORA (The New York School of Regional Anesthesia).

Frequently Asked Questions

If a nerve block is incomplete or has failed, you will experience pain or sensation in the area that was supposed to be numb, especially when a painful stimulus is applied [1.2.3]. You might feel a 'patchy' block where some spots are numb and others are not.

Yes, patient anxiety is considered a contributing factor to nerve block failure. High anxiety can heighten the perception of pain and may make it more difficult for the patient to remain still during the procedure, though it doesn't directly stop the medication from working [1.2.5, 1.5.7].

True resistance to local anesthetics is very rare but possible. It is often linked to genetic conditions like Ehlers-Danlos syndrome or specific mutations in sodium channels that prevent the anesthetic from blocking nerve signals effectively [1.2.5, 1.5.6].

If a needle touches a nerve, you may feel a brief sensation of 'pins and needles' or a jolt of pain. Clinicians use this feedback (if the patient is awake) to reposition the needle. While it doesn't always cause damage, direct needle injury is a potential risk [1.2.4, 1.6.9].

Inferior alveolar nerve blocks, common in dentistry, have a reported failure rate of up to 20-47% [1.3.6]. This is often due to anatomical variations, accessory nerve supply to the teeth, or injecting into inflamed tissue (pulpitis), which makes the anesthetic less effective [1.5.2, 1.3.6].

A 'wear-off' block occurs when the surgical procedure lasts longer than the duration of action of the local anesthetic used. The pain returns as the medication's effect diminishes before the surgery is finished [1.2.3].

Clinicians use several strategies, including detailed patient history, using ultrasound guidance for precise placement, choosing the right anesthetic drug for the procedure's duration, and adhering to safety checklists like a 'stop before you block' to prevent errors [1.2.5, 1.6.1].