Local anaesthesia

What are local anaesthetics?

'Local anaesthesia' refers to the reversible loss of sensation in a defined area of the body. This loss of sensation is achieved by the topical application or injection of agents that block the sodium channels that facilitate nerve impulses in tissue.

• The use of 'regional anaesthesia' or 'nerve blocks' refers to when local anaesthetic agents are injected adjacent to larger nerves, resulting in anaesthesia of large areas supplied by that nerve.
• 'Tumescent anaesthesia' is carried out by injecting large amounts of fluids containing diluted local anaesthetic and adrenaline (epinephrine). It is used for liposuction and other plastic, cosmetic, and dermatological surgical procedures involving large areas of skin.

Local anaesthesia is used in many dermatological procedures, surgical operations, and dental procedures. The aim is to minimise pain so that procedures can be conducted as efficiently and comfortably as possible.

Topical lignocaine gel 

Lignocaine solution for injection 

Eutectic mixture of lignocaine with prilocaine 

What types of local anaesthetics are there?

Local anaesthetics are categorised into two different classes based on their structure: para-aminobenzoic acid (PABA)–based anaesthetics known as esters and non-PABA-based anaesthetics are termed amide local anaesthetics [1].

Ester local anaesthetics include:

• Benzocaine
• Chloroprocaine
• Cocaine
• Procaine
• Proparacaine
• Tetracaine
• Amylocaine
• Oxybuprocaine.

Amide local anaesthetics include:

• Articaine
• Bupivacaine
• Dibucaine
• Etidocaine
• Levobupivacaine
• Lidocaine (also known as lignocaine)
• Mepivacaine
• Prilocaine
• Ropivacaine
• Sameridine
• Tonicaine
• Cinchocaine.

Lignocaine (lidocaine)

Both ester and amide local anaesthetics are available in a variety of formulations, including ointments, patches, and injections.

Lignocaine (lidocaine) is the most commonly used anaesthetic in the surgical setting. It is effective, acts rapidly and is relatively free from toxicity and sensitivity. It is available in many different forms, including topical applications (eg, EMLA® Cream and patches) and solutions for injection. It is frequently combined with adrenaline (epinephrine) to prolong the duration of anaesthesia, reduce associated bleeding, and increase the intensity of the nerve blockade through the reduction of systemic absorption.

What is the maximum dose of lignocaine?

The maximum dose of lignocaine varies based on the area and vascularity of the skin being anaesthetised and the condition of the patient. A higher dose may be used if lignocaine and adrenaline are being used.

For cutaneous infiltration, the maximum dose without adrenaline is 3 mg/kg [2].

What are the side effects or complications of local anaesthetic use?

Local anaesthetics are well tolerated, and when used appropriately, have minimal side effects. Local side effects include temporary stinging, burning, and bruising secondary to the injection.

More serious side effects are associated with the infusion or injection of high doses of local anaesthetics. The risk of local anaesthetic systemic toxicity is variable and is dependent on patient factors such as age, end-organ dysfunction, and the area being anaesthetised [3]. Most cases of local anaesthetic systemic toxicity occur after inadvertent intravenous injection.

The symptoms and signs of local anaesthetic systemic toxicity are grouped into the central nervous system (CNS) and cardiac toxicity [3]. Initial CNS symptoms are a result of excitation, and can include:

• Tinnitus
• Perioral numbness
• Metallic taste
• Altered mental status
• Muscle twitching
• Seizures.

Eventually, as toxicity progresses, CNS depression occurs.

Cardiac signs can occur independently of or in addition to CNS signs. Cardiac signs can include:

• Tachycardia
• Hypertension
• Bradycardia
• Hypotension.

As toxicity becomes more severe, cardiac signs can progress to arrhythmias (palpitations) and asystole (cardiac arrest).

Is it possible to be allergic to local anaesthetics?

Ester local anaesthetics are known sensitisers and therefore are associated with allergic reactions [1,4]. Cross-reactivity between ester local anaesthetics is also common (see our page Allergy to benzocaine). A hypersensitivity to amide local anaesthetics is much less common and cross-reactivity is unpredictable.

Local anaesthetic hypersensitivity is nearly always a delayed hypersensitivity reaction (see our page Allergies explained). This requires previous sensitisation. Upon re-exposure to the allergen, patients will develop localised erythema and swelling in the area exposed to the local anaesthetic.

Hypersensitivity to a local anaesthetic injected into the mucosal surfaces of the mouth can present with diffuse swelling of the face, which can appear like urticaria. Immunoglobulin E-mediated hypersensitivity reactions (such as anaphylaxis) are very rare.

How is a local anaesthetic allergy diagnosed?

The diagnosis of a local anaesthetic allergy is difficult because it is rare. Reaction to the preservatives in the solution can also occur [4,5]. Patch testing can be used to identify the cause of a delayed-type hypersensitivity, as well as the presence of cross-reactivity [4].

How is a local anaesthetic allergy treated?

The best management of an allergy to local anaesthetics is the avoidance of the identified allergen by the patient and their doctors (and any allergens that demonstrate a cross-reactivity).