EMLA Cream

EMLA Cream: A Guide to Topical Anesthesia

EMLA stands for Eutectic Mixture of Local Anesthetics. It is a topical anesthetic used to provide temporary, superficial analgesia of intact skin. Its utility lies in its ability to reduce the pain of needle procedures, making it an invaluable tool, especially in pediatrics and for patients with needle phobia.

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Composition

EMLA is a 1:1 mixture by weight of two amide-type local anesthetics:

• Lidocaine 2.5%
• Prilocaine 2.5%

The "Eutectic" Property: This is the key to its formulation. A eutectic mixture is one where the melting point of the combination is lower than the melting point of either individual component. By mixing lidocaine (MP ~68°C) and prilocaine (MP ~37°C), they form an oil at room temperature, allowing for much more efficient dermal penetration than if they were simply mixed in a water-based cream.

Mechanism of Action

Both lidocaine and prilocaine work by blocking voltage-gated sodium channels in the neuronal cell membrane. This prevents the influx of sodium ions, which inhibits the depolarization phase of the action potential. The result is a reversible block of nerve impulse conduction, leading to a loss of sensation (analgesia) in the area.

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Pharmacokinetics & Dynamics

Understanding the PK/PD is crucial for safe and effective use.

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Clinical Applications in Anesthesia

EMLA is used to reduce procedural pain. Its primary role is to facilitate painless vascular access and other minor procedures.

• IV Cannulation & Venipuncture: The most common use, especially in children and adults with difficult veins or high anxiety.
• Arterial Line Placement: Can reduce the pain of local anesthetic infiltration prior to arterial puncture.
• Lumbar Puncture: Significantly reduces pain in pediatric patients undergoing LP.
• Facilitating Peripheral Nerve Blocks: Applying EMLA under the site of a planned block (e.g., femoral, sciatic) can reduce the initial sting of the local anesthetic infiltration, improving patient comfort and cooperation.
• Minor Dermatological Procedures: Shave biopsies, curettage of molluscum contagiosum, or suturing of small, superficial lacerations.

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Application Technique

Improper application is a common reason for failure.

1. Calculate Dose: The dose is 1-2g per 10 cm² of skin. Do not exceed the maximum recommended dose.
   • Adults: Do not apply more than 10g at any one time.
   • Children >1 year: Do not exceed the adult dose of 10g. The dose should be calculated based on weight and surface area.
2. Apply Thickly: Squeeze a thick layer of cream directly onto the center of the intended area. Do not rub it in.
3. Occlusive Dressing is MANDATORY: Cover the cream with the provided Tegaderm (or similar) dressing. This is not optional. The occlusion prevents evaporation and is essential for forcing the anesthetic into the dermis.
4. Time It: Mark the time of application. A minimum of 60 minutes is required for reliable analgesia.
5. Removal: Just before the procedure, remove the dressing and wipe off all the cream with a gauze pad. Clean the skin with an appropriate antiseptic (e.g., chlorhexidine or alcohol) as you normally would.

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Contraindications

This is a critical safety section.

Absolute Contraindications:

• Known hypersensitivity to lidocaine, prilocaine, or other amide-type local anesthetics.
• Congenital or idiopathic methemoglobinemia.

Relative Contraindications & Cautions:

• Infants < 3 months or < 5 kg: They have immature enzyme systems (specifically NADH-dependent methemoglobin reductase), making them highly susceptible to prilocaine-induced methemoglobinemia. Use with extreme caution and limit the application area and duration.
• Patients with Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency: They are more prone to oxidative stress and methemoglobinemia.
• Application on broken skin or mucous membranes: This leads to rapid and unpredictable systemic absorption, increasing the risk of toxicity.
• Patients on Class I anti-arrhythmic drugs (e.g., tocainide, mexiletine): Due to additive effects on the cardiac conduction system.

Adverse Effects

• Local Reactions (Common & Benign): Transient local skin reactions are seen in up to 50% of patients. These include:
  • Pallor (due to vasoconstriction)
  • Erythema (reactive hyperemia after removal)
  • Edema
  • Itching or burning sensation
• Methemoglobinemia (Rare but Serious): Caused by the prilocaine metabolite o-toluidine. It oxidizes the iron in hemoglobin from the ferrous (Fe²⁺) to the ferric (Fe³⁺) state, creating methemoglobin, which cannot bind oxygen.
  • Signs: Cyanosis, "chocolate brown" blood, dyspnea, tachycardia, SpO₂ reading ~85% that does not improve with supplemental O₂.
  • Treatment: Mild cases may resolve. Severe cases are a medical emergency requiring methylene blue.
• Systemic Toxicity: Rare, but can occur with excessive application (large surface area, long duration, or on broken skin), leading to CNS excitation/seizures or cardiovascular depression.

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Practical Pearls for the Residents

• The "Cold Trick": The vasoconstriction caused by EMLA can make veins harder to cannulate. If you're struggling to find a vein after removing the cream, applying a warm pack for a minute can cause vasodilation and make the vein more prominent.
• Blistering: Prolonged application (>2 hours) can lead to localized blistering or superficial skin ulceration, especially in patients with atopic dermatitis.
• Don't Trust It Completely: Always have a backup plan. If a patient still reports pain after EMLA application, believe them and proceed with infiltrative local anesthesia as needed. It is an adjunct, not a guarantee of a painless procedure.
• Communicate: Explain to the patient that the skin will feel numb, but they may still feel pressure or pushing. This manages expectations and improves cooperation.

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Summary Table

EMLA is a powerful tool for improving patient experience. When used correctly and with a full understanding of its pharmacology and risks, it is an excellent addition to your anesthetic technique.