Local Anesthetic Systemic Toxicity (LAST)

What is local anesthetic systemic toxicity?

Local anesthetic systemic toxicity (LAST) is a rare, life-threatening complication of receiving local anesthetic medications. LAST typically causes issues with your central nervous system and/or cardiovascular system, like seizures and arrhythmias.

Local anesthetics work by temporarily blocking your peripheral nerves from sending pain and sensory signals to your brain. Healthcare providers use these medications to numb specific areas of your body for various procedures. Local anesthetics come in many forms, like injections, gels and sprays. In rare cases, local anesthetics can cause toxicity within minutes of receiving them.

LAST is treatable. Your provider will act quickly if symptoms develop.

Symptoms of local anesthetic systemic toxicity

Local anesthetic systemic toxicity most commonly causes central nervous system (your brain and spinal cord) issues. Initial signs and symptoms include:

• Seizures (the most common sign)
• Agitation or confusion
• Auditory changes, like tinnitus
• Numbness around your mouth
• Metallic taste in your mouth
• Slurred speech (dysarthria)
• Hallucinations
• Eye movement abnormalities

LAST can also affect your cardiovascular system, causing cardiac toxicity. Signs and symptoms may include:

• Low blood pressure
• Bradycardia (slow heart rate), tachycardia (fast heart rate) or other arrhythmias
• Shortness of breath (dyspnea)
• Cardiac arrest, including failure of your heart’s electrical system (asystole) or ventricular fibrillation

Without quick treatment, LAST can progress to cause respiratory arrest and/or coma.

LAST symptoms typically develop within five minutes of receiving a local anesthetic. The symptoms can range from mild to severe. But it’s impossible to predict how LAST will progress. Your healthcare provider will observe you closely after you receive a local anesthetic to look for signs of LAST, especially if you have risk factors for it.

Local anesthetic systemic toxicity causes

The most common cause of LAST is accidental injection of the local anesthetic medication into a blood vessel.

Nerve blocks that numb an extremity (like an arm or leg) account for about 20% of LAST cases. But there are reports of LAST developing from:

• Topical application of a local anesthetic in gel form
• Application of local anesthetic to the mucosa in the mouth, esophagus and trachea

Based on this evidence, LAST can develop with any use or form of a local anesthetic. But several factors increase the likelihood of it happening.

Risk factors

Factors related to your health that may increase your risk of experiencing local anesthetic systemic toxicity include:

• A buildup of acids in your body (metabolic acidosis)
• Low levels of oxygen in your body (hypoxia)
• Preexisting heart disease, especially ischemia, arrhythmias, conduction abnormalities and low ejection fraction
• Being over 65 (infants under 4 months are also at increased risk of LAST)
• Liver or kidney disease (these may increase your risk of delayed LAST)
• Pregnancy
• Mitochondrial diseases

Certain local anesthetic medications are more likely to lead to LAST than others, like bupivacaine, for example. Higher doses of anesthetics also increase your risk of LAST.

Certain techniques for giving local anesthetic may also increase your risk of LAST. According to a study of 125 cases of LAST:

• Infiltration techniques (injecting the anesthetic subcutaneously) accounted for 20% of cases
• Central neuraxial blocks (epidural and caudal blocks) accounted for 15% of cases
• Continuous infusion of a local anesthetic accounted for 13% of cases

Lastly, local anesthetic injections in areas that have a lot of blood vessels (high vascularity) are more likely to lead to LAST. There’s an increased number of LAST cases for penile nerve blocks, for example.

How doctors diagnose local anesthetic systemic toxicity

Healthcare providers typically diagnose local anesthetic systemic toxicity based on your symptoms. This is because providers must act quickly to treat LAST, so they can’t delay the diagnosis. In rare cases, your provider might do an EKG (electrocardiogram) to help diagnose it.

What is the treatment for LAST?

Local anesthetic systemic toxicity is a medical emergency, so healthcare providers act quickly to treat it. In general, treatment involves:

• Airway management. Your provider will provide immediate ventilation and oxygen therapy to manage your breathing.
• Seizure prevention or treatment. Your provider will give you medication (like benzodiazepines) for this.
• IV infusion of lipid emulsion. Your provider will inject this medication into a vein. It helps reverse LAST by absorbing local anesthetic from blood and tissues. This decreases the concentration of anesthetic in your brain and heart.
• CPR. If LAST affects the functioning of your heart, your provider will do CPR and use other methods to stabilize it.

After treatment, a provider will monitor you for at least six hours to make sure the treatment is effective and you don’t have any other complications.

Is local anesthetic systemic toxicity preventable?

LAST isn’t always preventable. But healthcare providers use certain guidelines and methods to help decrease the risk of it.

For example, your provider will use the lowest effective dose of local anesthetic. They may also use ultrasound imaging to guide a local anesthetic injection to help avoid accidental injection into a blood vessel.

If you have risk factors for LAST, your provider will carefully consider the safest anesthetic medication, dose and application for you.

What is the prognosis for someone with LAST?

People with LAST who receive quick diagnoses and treatment typically have a good prognosis (outlook). The treatments often effectively reverse LAST.

Delayed treatment of severe LAST can result in coma and/or death. This is rare. For example, data from the poison control centers in the U.S. reveals that there were only four deaths from local anesthetics in 2016.