Lidocaine is one of the most widely used local anesthetics in medicine and dentistry. It is highly effective for pain relief, minor surgical procedures, nerve blocks, and even as an antiarrhythmic drug. However, when administered in excessive doses, injected intravascularly, or metabolized abnormally, lidocaine toxicity can occur.
Toxicity from local anesthetics, including lidocaine, is clinically termed Local Anesthetic Systemic Toxicity (LAST). It is a rare but potentially life-threatening condition, affecting the central nervous system (CNS) and the cardiovascular system. Understanding its symptoms, early recognition using mnemonics like SAMS, and prompt treatment can save lives.
What is Lidocaine Toxicity?
Lidocaine toxicity occurs when blood concentrations of lidocaine rise above safe therapeutic levels. Normally, when used in controlled amounts, lidocaine is safe. But accidental intravascular injection, impaired metabolism (e.g., liver disease), or overdose can overwhelm the body’s ability to detoxify the drug, leading to neurological and cardiovascular symptoms.
Causes and Risk Factors
Several factors predispose patients to lidocaine toxicity:
- Overdose: Exceeding the recommended dose (generally not more than 4.5 mg/kg without epinephrine or 7 mg/kg with epinephrine).
- Accidental intravascular injection: During nerve blocks or infiltration anesthesia.
- Rapid absorption: Use in highly vascular areas (scalp, mucous membranes).
- Liver dysfunction: Impaired metabolism as lidocaine is primarily metabolized hepatically.
- Reduced cardiac output: Prolonged clearance of the drug.
- Drug interactions: Concomitant use of medications affecting liver enzymes.
- Infants and elderly patients: Altered pharmacokinetics increase risk.
Pathophysiology of Lidocaine Toxicity
Lidocaine works by blocking sodium channels, preventing nerve impulse transmission. At toxic levels, however, it interferes with sodium channels in the brain and heart, leading to:
- CNS Toxicity: Hyperexcitability due to initial sodium channel blockade in inhibitory neurons → seizures → CNS depression.
- Cardiac Toxicity: Depressed myocardial conduction, contractility, and automaticity, causing bradycardia, hypotension, dysrhythmias, and cardiac arrest.
Clinical Features of Lidocaine Toxicity – The SAMS Mnemonic
The mnemonic SAMS helps recall the progressive symptoms of lidocaine toxicity:
S – Slurred or Difficult Speech
- Paresthesias (tingling) of lips and tongue
- Numbness of lips/tongue
- Metallic taste
- Early neurological signs due to CNS excitation
A – Altered Cardiovascular and Neurological System
- Neurological: Drowsiness, dizziness, restlessness, confusion
- Cardiovascular: Dysrhythmias, hypotension, bradycardia, heart block
- Other: Visual and auditory disturbances
M – Muscle Twitching
- Fine tremors progressing to generalized twitching
- Warning sign of impending seizures
S – Seizures
- Generalized tonic-clonic seizures
- Respiratory depression
- Severe cases progress to respiratory and cardiac arrest
Complications of Lidocaine Toxicity
If untreated, lidocaine toxicity can lead to:
- Respiratory depression and arrest
- Cardiac arrhythmias including ventricular fibrillation
- Severe hypotension and cardiogenic shock
- Cardiac arrest
- Neurological damage due to prolonged seizures and hypoxia
- Death if management is delayed
Diagnosis of Lidocaine Toxicity
Diagnosis is mainly clinical, based on recognition of early symptoms after administration of lidocaine.
Key diagnostic approaches include:
- Patient history and recent exposure to local anesthetics.
- Observation of early neurological signs (tingling, numbness, dizziness).
- ECG changes showing bradyarrhythmias or conduction blocks.
- Serum lidocaine levels (rarely required in emergency but confirmatory).
Management of Lidocaine Toxicity
Immediate Steps
- Stop administration of lidocaine immediately.
- Call for resuscitation support.
- Maintain airway, breathing, and circulation (ABCs).
Supportive Management
- Oxygen supplementation and airway support.
- IV benzodiazepines (e.g., diazepam, midazolam) for seizures.
- Treat bradycardia with atropine.
- Manage hypotension with IV fluids and vasopressors.
Specific Therapy – Lipid Emulsion Rescue (Intralipid Therapy)
- 20% intravenous lipid emulsion (ILE) is the antidote for severe local anesthetic systemic toxicity.
- Acts as a “lipid sink,” binding lipophilic lidocaine molecules and reducing their plasma concentration.
- Standard regimen: 1.5 mL/kg IV bolus over 1 min → continuous infusion at 0.25 mL/kg/min → repeat bolus if necessary.
Advanced Management
- CPR in case of cardiac arrest.
- Extracorporeal membrane oxygenation (ECMO) for refractory cases.
- Hospitalization for monitoring.
Prevention of Lidocaine Toxicity
Prevention is better than cure. Key preventive strategies include:
- Careful calculation of maximum safe dose.
- Aspiration before injection to avoid intravascular delivery.
- Using epinephrine-containing solutions to reduce systemic absorption.
- Slow injection with fractionated dosing.
- Close monitoring of high-risk patients.
- Proper training of healthcare professionals in LAST management.
Quick Reference Table
| Stage (SAMS) | Symptoms | Clinical Relevance |
|---|---|---|
| S – Slurred Speech | Tingling, numbness of lips/tongue, metallic taste | Early CNS warning signs |
| A – Altered System | Dizziness, drowsiness, arrhythmias, hypotension | CNS and cardiac involvement |
| M – Muscle Twitching | Tremors, localized twitching | Pre-seizure stage |
| S – Seizures | Convulsions, respiratory/cardiac arrest | Life-threatening stage |
FAQs on Lidocaine Toxicity
1. What is the maximum safe dose of lidocaine?
Without epinephrine: up to 4.5 mg/kg (max ~300 mg).
With epinephrine: up to 7 mg/kg (max ~500 mg).
2. How quickly does lidocaine toxicity occur?
Symptoms can appear within minutes if injected intravascularly, or more gradually if overdosed through infiltration.
3. What is the first sign of lidocaine toxicity?
Early neurological symptoms such as tingling around the mouth, metallic taste, dizziness, or slurred speech.
4. How is lidocaine toxicity treated?
Stop the drug, support airway and circulation, treat seizures, and administer IV lipid emulsion therapy in severe cases.
5. Can lidocaine toxicity be fatal?
Yes. Without prompt recognition and treatment, it may cause cardiac arrest and death.
