What is the most likely diagnosis in a patient who develops a metallic taste and dizziness immediately after an interscalene nerve block for pain control?

Local Anesthetic Systemic Toxicity (LAST)

This patient is experiencing local anesthetic systemic toxicity (LAST), a potentially life-threatening complication characterized by early CNS symptoms including metallic taste, dizziness, circumoral numbness, and facial tingling that can rapidly progress to seizures, cardiovascular collapse, and cardiac arrest. 1

Clinical Presentation and Pathophysiology

The constellation of metallic taste and dizziness immediately following an interscalene nerve block represents the early CNS excitation phase of LAST, which occurs when protein-unbound plasma concentrations of local anesthetic rise to toxic levels. 1, 2

Classic Progression of LAST Symptoms:

Early CNS excitation (current presentation):

• Metallic taste 1
• Circumoral numbness and facial tingling 1
• Dizziness and auditory changes 1
• Pressured or slurred speech 1
• Hypertension and tachycardia may accompany these symptoms 1

Progressive CNS toxicity:

• Seizures (most common serious manifestation, occurring in 77-89% of LAST cases) 1
• Loss of consciousness and respiratory arrest 2

Cardiovascular toxicity (32-55% of LAST cases):

• Bradycardia and conduction disturbances 2
• Ventricular tachycardia or fibrillation (13% of cases) 1
• Asystole (12% of cases) 1
• Circulatory collapse 1

Critical Recognition Points

However, 40% of LAST cases do NOT follow the classic progression and may present with substantially delayed symptoms or isolated cardiovascular compromise without preceding CNS signs. 3 This makes early recognition of prodromal symptoms like metallic taste and dizziness absolutely critical for preventing progression to life-threatening complications.

The temporal relationship—symptoms occurring "shortly after injection"—strongly supports LAST rather than other differential diagnoses such as vasovagal reaction, anxiety, or unrelated neurologic events. 4

Immediate Management Algorithm

Stop the local anesthetic injection immediately and call for help. 1, 2

Step 1: Airway and Oxygenation (First Priority)

• Administer 100% oxygen 2
• Secure the airway and prepare for tracheal intubation if needed 2
• Prevent hypoxia and acidemia, which worsen LAST 1

Step 2: Seizure Management

• Administer benzodiazepines to treat or prevent seizures (Class 1, Level C-LD recommendation) 1
• Benzodiazepines are the first-line anticonvulsant for LAST-induced seizures 2

Step 3: Cardiovascular Support

• Secure intravenous access if not already established 2
• Monitor for bradycardia and arrhythmias 2
• Administer atropine for life-threatening bradycardia (Class 2a, Level C-EO recommendation) 1
• Administer sodium bicarbonate for life-threatening wide-complex tachycardia (Class 2a, Level C-LD recommendation) 1

Step 4: Lipid Emulsion Therapy

Administer 20% intravenous lipid emulsion (ILE) immediately for LAST (Class 1, Level C-LD recommendation). 1 This is the cornerstone of LAST treatment and functions as a "lipid sink" to sequester lipophilic local anesthetics from plasma. 4

ILE dosing protocol:

• Bolus: 1.5 mL/kg of 20% lipid emulsion over 1 minute 1, 2
• Followed by continuous infusion: 0.25 mL/kg/min 2
• Repeat bolus and increase infusion rate if cardiovascular instability persists 2

Step 5: Advanced Support if Refractory

• Consider VA-ECMO for refractory cardiogenic shock (Class 2a, Level C-EO recommendation) 1
• Continue CPR if cardiac arrest occurs, but recognize that resuscitation may be prolonged with LAST 1

Post-Stabilization Monitoring

Extended observation for at least 24 hours is mandatory, even after apparent clinical resolution, because biphasic toxicity can occur with recurrent symptoms after initial stabilization. 4 The patient in the referenced case was observed for 24 hours and discharged without complications, demonstrating the importance of this monitoring period. 4

Prevention Strategies for Future Blocks

• Use ultrasound guidance to visualize needle placement and avoid intravascular injection 2, 5
• Always aspirate before injection to detect intravascular placement 2, 5
• Use dose fractionation with incremental injections 2, 5
• Consider using lower-toxicity agents like ropivacaine or levobupivacaine instead of bupivacaine 2, 5
• Adhere to maximum recommended doses 1
• Maintain continuous verbal contact with the patient during injection to detect early symptoms 6

Common Pitfalls to Avoid

Do not dismiss early prodromal symptoms (metallic taste, dizziness, perioral numbness) as anxiety or vasovagal reactions—these are red flags for impending LAST that require immediate intervention. 1

Do not delay lipid emulsion therapy while waiting for cardiovascular collapse—early administration improves outcomes. 1, 4

Do not use propofol as a substitute for lipid emulsion—the lipid concentration in propofol is insufficient for LAST treatment. 6

Do not discharge the patient prematurely—24-hour observation is essential to monitor for biphasic toxicity. 4