The blocked limb, the contested question, and the late toxicity

Why it matters

Two things arrive in the PACU under a working regional block, and both are easy to misread as the block. The first is compartment syndrome in a limb that cannot feel it, a question on which the strongest society statement and everyday clinical practice pull in different directions ivani-2015. The second is local anesthetic systemic toxicity, which the current ASRA advisory describes as presenting in changed patterns, including delayed and atypical presentations, so that it looks unlike the block regression it is mistaken for neal-2018.

The consequence of getting this wrong is a limb, or a patient, whose deterioration is filed under the block.

Mechanism

A peripheral nerve block interrupts conduction in the nerves that would carry the pain of a rising compartment pressure, which is the whole basis of the worry that it masks compartment syndrome. Against that, ischemic pain is intense and often breaks through a sensory block as the compartment pressure rises, which is the basis of the clinical practice that treats breakthrough pain as the signal. Both mechanisms are real; the disagreement is about which dominates, and the evidence cannot yet say.

Local anesthetic toxicity is a function of plasma concentration at the heart and the brain. After a block, that concentration depends on the dose, the site’s vascularity and the rate of systemic absorption, which can be slow. Absorption from a large-volume infiltration or from a depot preparation can produce a peak long after the injection, which is why toxicity can present in the PACU rather than in the block room, and why it can present with cardiovascular rather than neurological signs first neal-2018.

Evidence

The joint ESRA/ASRA committee practice advisory states that there are no current evidence-based data that the use of regional anesthesia increases the risk of acute compartment syndrome or delays its diagnosis, and that high-level evidence is not available for the topics it evaluated, with most recommendations based on Evidence B studies ivani-2015. Its scope is pediatric, which is a real limitation for an adult curriculum and is stated here rather than buried.

The 2015 ASRA advisory on neurological complications supplies the framework for distinguishing prolonged block, block failure and neurological injury on examination neal-2015.

The 2018 ASRA interim update on local anesthetic systemic toxicity covers the mechanisms of lipid resuscitation, updated frequency estimates for toxicity, the preventive role of ultrasound guidance, changes in case presentation patterns, and the limited evidence around local infiltration anesthesia and liposomal bupivacaine; it is the source for the atypical and delayed presentations that make toxicity look unlike block regression neal-2018. The 2021 ASRA checklist is the current cognitive aid, updated in response to user feedback and simulation studies; it carries a correction notice, because its Figure 1 was replaced after online-first publication, so any reproduced checklist must come from the corrected version or from the society’s own site neal-2021-checklist.

What this does not show

The compartment syndrome question is not resolved on this page and the page does not pretend otherwise. The advisory finds no evidence of delayed diagnosis; it does not find evidence against delay, and it says so by recording that high-level evidence is not available ivani-2015. The opposing position now has a society source, read in full on 2026-09-08. The Association of Anaesthetists guideline on regional analgesia for lower leg trauma advises that neuraxial or peripheral techniques producing dense blocks of long duration significantly exceeding the duration of surgery should be avoided, and records that single-shot or continuous peripheral blocks using lower concentrations without adjuncts are not associated with delays in diagnosis provided post-injury and postoperative surveillance is appropriate and effective nathanson-2021. It asks that patients at risk be identified on admission or at operation and managed within agreed multidisciplinary protocols, that ward observations be made at set frequencies by staff trained in the recognition of compartment syndrome, that objective scoring charts be used, and that the equipment to measure intracompartmental pressure be available on the wards caring for them nathanson-2021. On the professional disagreement it is blunt: neither the surgeon nor the anaesthetist has the right to veto a treatment recommended by the other, and where consensus cannot be reached the anaesthetist’s expertise in pain relief should be respected nathanson-2021.

The sentence this page was built on needs its qualifier carried with it. Breakthrough pain in a blocked limb is described there as a cardinal feature of compartment syndrome, and the same sentence adds that it may not always be a feature of the syndrome nathanson-2021. Teach both halves. A resident told to wait for breakthrough pain has been given a sign that the guideline itself says can be absent.

Grade the source by its own account of itself. It states that no other guidelines are currently available, and that a systematic review of the literature is absent, offering instead a multi-professional consensus opinion based on an objective review of case reports and case series nathanson-2021. That is why this day’s status chip reads uncertain. The compartment syndrome question is not resolved on this page and the page does not pretend otherwise. The advisory finds no evidence of delayed diagnosis; it does not find evidence against delay, and it says so by recording that high-level evidence is not available ivani-2015. The opposing position now has a society source. The Association of Anaesthetists guideline on regional analgesia for lower leg trauma advises avoiding neuraxial or peripheral techniques that produce dense blocks of long duration significantly exceeding the duration of surgery; records that single-shot or continuous peripheral blocks with lower concentrations and no adjuncts are not associated with diagnostic delay provided surveillance is appropriate; requires ward observations at set frequencies by staff trained to recognize compartment syndrome, objective scoring charts, and documented consent; and states that neither surgeon nor anesthesiologist may veto the other’s recommended treatment nathanson-2021. The specific wording that breakthrough pain in a blocked limb is the trigger to examine sits in the full text of that guideline, which this project has not read; the bedside rule below rests on the parts that were read. The two positions are not in fact opposed on the bedside rule; they differ on whether a block delays the diagnosis, and the status chip says uncertain.

Every source here is an advisory or a checklist. None is a trial, and the frequency estimates in the 2018 update have not been extracted, so no incidence figure appears neal-2018.

The lipid emulsion dose is on the corrected ASRA checklist: for a patient over 70 kg, a bolus of about 100 mL over 2 to 3 min then an infusion of about 250 mL over 15 to 20 min; under 70 kg, a bolus of about 1.5 mL/kg over 2 to 3 min then about 0.25 mL/kg/min; if unstable, repeat the bolus and double the infusion; maximum lipid dose 12 mL/kg; epinephrine in smaller than normal doses, starting below 1 µg/kg; avoid local anesthetics, beta-blockers, calcium channel blockers and vasopressin; benzodiazepines preferred for seizures; observe for 2 h after a seizure or 4 to 6 h after cardiovascular instability neal-2021-checklist. The society states that the checklist is under review with an update expected in early 2027 neal-2021-checklist. The interscalene block’s effect on the diaphragm is not a complication but a consequence: ipsilateral hemidiaphragmatic paresis on ultrasound in 13 of 13 patients within 5 min of a paresthesia-technique block with 34 to 52 mL of 1.5% mepivacaine, resolving by 3 to 5 h urmey-1991. In a prospective series of 520 interscalene blocks for shoulder surgery, acute complications were one pneumothorax (0.2%) and one episode of central nervous system toxicity (0.2%); symptoms such as paresthesia or non-surgical pain were present in 14% at 10 days, 7.9% at one month, 0.9% at six months and 0.2% at nine months borgeat-2001. That paper has now been read in full, from cover to tables, and Horner syndrome is not in it anywhere: not the abstract, not Table 2 on acute complications, not Table 3 on non-acute ones, not the discussion borgeat-2001. The question of a prospective Horner denominator is therefore closed for this source, and the fuller picture it does give is worth carrying: symptoms in 14% at 10 days, 7.9% at 1 month, 3.9% at 3 months, 0.9% at 6 months and 0.2% at 9 months, with catheter and single injection not differing significantly at any timepoint, transient burning pain on injection in 21%, every symptom appearing within 23 days, and an overall rate of short-term and severe long-term complications of 0.4% borgeat-2001. A retrospective consecutive series of 1322 interscalene blocks does report it, at 9 cases (0.7%), alongside hoarseness in 8 (0.6%), and its own authors note that this is lower than previously reported takayama-2021. Read that figure as what it is. It is passive surveillance of a case record, not a rate from patients who were examined for the sign, and Horner syndrome is far commoner when someone looks for it. This page therefore quotes 0.7% as what one record review found, not as the incidence. Pneumothorax after ultrasound-guided supraclavicular or infraclavicular block occurred in 4 of 6366 blocks (0.06%, 95% CI 0.001 to 0.124), three of the four presenting after a two-day latency, against a historical non-ultrasound risk quoted as up to 6.1% gauss-2014. The latency is the PACU point: the pneumothorax that follows a supraclavicular block is usually not in the PACU yet. The interscalene block’s effect on the diaphragm is not a complication but a consequence: ipsilateral hemidiaphragmatic paresis on ultrasound in 13 of 13 patients within 5 min of a paresthesia-technique block with 34 to 52 mL of 1.5% mepivacaine, resolving by 3 to 5 h urmey-1991. In a prospective series of 520 interscalene blocks for shoulder surgery, acute complications were one pneumothorax (0.2%) and one episode of central nervous system toxicity (0.2%); symptoms such as paresthesia or non-surgical pain were present in 14% at 10 days, 7.9% at one month, 0.9% at six months and 0.2% at nine months borgeat-2001. Horner syndrome is not reported in that series, and no source in this ledger gives its incidence, so this page does not quote one. Pneumothorax after ultrasound-guided supraclavicular or infraclavicular block occurred in 4 of 6366 blocks (0.06%, 95% CI 0.001 to 0.124), three of the four presenting after a two-day latency, against a historical non-ultrasound risk quoted as up to 6.1% gauss-2014. The latency is the PACU point: the pneumothorax that follows a supraclavicular block is usually not in the PACU yet.

At the bedside

Pain that escalates through a block that was working is a finding to explain, not a top-up to give. Examine the limb, measure what your institution measures, and involve the surgeon early; the advisory’s absence of evidence for delay is not a reason to wait ivani-2015. Local practice differs. whether a block is used at all in limbs at risk of compartment syndrome, and who is called when breakthrough pain occurs, differ between institutions and surgical services.

Know the current ASRA checklist and where the corrected version lives, because toxicity in the PACU may present hours after the block and with the heart rather than the brain neal-2021-checklist neal-2018. Have lipid emulsion where you can reach it and know the local dose.

When a block seems prolonged or a new deficit appears, use the advisory’s framework to separate prolonged block from failure from injury before you reassure anyone neal-2015.

Check yourself

Three items. Every option carries an explanation. Progress is not saved.