Local anaesthesia in dermatology

Meet the patient

A dermatology trainee is about to excise a 3 cm epidermoid cyst from a 90 kg man's upper back. The lidocaine with epinephrine is drawn up, the field is marked, a 30-gauge needle is on the syringe. Two questions should already be answered on the trolley before the first wheal goes in: what is this patient's maximum safe lidocaine dose? and where is the lipid-emulsion kit if this goes wrong?[1][8]

Ninety seconds after the first infiltration he goes pale, clammy and nauseated, pulse thready. The nurse reaches for the "allergy" label. Hold that thought — because the three things this might actually be, in descending order of likelihood, are vasovagal syncope, an epinephrine effect, and early local-anaesthetic systemic toxicity (LAST). Telling them apart at the bedside, fast, is the single most examinable judgement in cutaneous surgery, and getting it wrong in either direction harms the patient.[1][8]

Amides inject, esters topical — the two-i trick

The drug you draw up in dermatology is almost always an amide, and the one trick that sorts the two families is counting the letter i in the name. Amides carry two i's before the -caine — lidocaine, bupivacaine, mepivacaine, prilocaine, ropivacaine — and they are hepatically metabolised, injectable, and rarely the true cause of allergy. Esters — procaine, tetracaine, benzocaine — are broken down by plasma cholinesterase to para-aminobenzoate (PABA) metabolites that account for their historical allergy reputation, and they live mainly on the topical shelf.[1][8]

[1] [8]

Epinephrine rides along at roughly 1:100,000 to 1:200,000 in skin-surgery mixes, vasoconstricting the field so you bleed less, the drug washes out slower, and the block lasts longer — which is exactly why the safe ceiling per kilogram roughly doubles when epinephrine is in the syringe.[1][9]

Etymology for viva gold: the -caine suffix was coined for procaine in 1905 as a deliberate echo of cocaine, the parent local anaesthetic isolated from coca leaves and the original -caine. Every synthetic cousin has worn the family name since.[1]

How a -caine actually numbs — and why pus blocks poorly

Local anaesthetics have to get inside the nerve to work, which is the whole reason inflamed tissue defeats them. The molecule crosses the axonal membrane in its uncharged, lipid-soluble form, then plugs the intracellular face of the voltage-gated sodium channel, blocking the sodium current that fires the action potential.[1]

The catch is pKa. Commercial lidocaine is formulated acidic, around pH 4, for shelf-stability, so most of it sits in the charged form at the moment of injection — which is precisely why onset is not instantaneous and why buffering toward physiologic pH with sodium bicarbonate both speeds onset and stings less. Inflamed, acidotic tissue (an abscess wall, a cellulitic leg) pushes even more drug into the charged, membrane-impermeant form, so the block comes on slowly and patchily. The practical answer is to ring-block around the inflamed edge, not inject into it.[1][2]

Convert before you draw — percent to milligrams, every single case

The recurring, embarrassing, sometimes fatal error in cutaneous surgery is drawing up a percent and never converting it to milligrams. Burn the conversion into muscle memory: a 1 percent solution is 10 mg/mL, so 2 percent is 20 mg/mL, and a 20 mL vial of 1 percent holds 200 mg total. Until you have done that multiplication you have not calculated a dose — you have guessed one.[3][8]

[3] [8]

The classic trap: under stress, the 7 mg/kg number floats free of its units and a registrar multiplies the patient's weight by seven and calls the answer millilitres of 1 percent — out by a factor of ten. Survey evidence shows clinician fluency with these conversions is patchy, so the rule is mechanical: convert percent to mg per mL, then multiply, every single case, and keep a running tally across staged procedures.[3][8]

In obesity, reason from lean body weight, not actual weight — lipophilic lidocaine does not distribute into fat the way water does, so dosing on total body weight overestimates the safe ceiling. Hepatic disease lowers amide clearance, so the same milligrams climb to a higher peak: halve your ambition and document the cumulative dose.[1][3]

Tumescent is a different country — don't borrow its headline dose

Tumescent anaesthesia is a deliberately dilute, high-volume technique with its own pharmacokinetics, and borrowing its headline dose number for a concentrated infiltration is a direct route to LAST. The principle, introduced for liposuction, is to flood the subcutaneous fat with very dilute lidocaine plus epinephrine, so much of the drug stays bound in fat and the vasoconstriction slows systemic absorption to a crawl. Under that specific regimen, controlled series report mean cumulative doses around 81 mg/kg and estimated maximal safe doses well above the classical infiltrative ceiling.[4][5]

[4] [5] [3]

The classic trap: a registrar mid-way through a large excision, already near the 7 mg/kg infiltrative ceiling, remembers that "tumescent goes to 81 mg/kg" and keeps injecting concentrated 1 percent lidocaine. The tumescent number is safe only because the drug is dilute and fat-bound under a controlled regimen — it is not a free pass for concentrated infiltration, and applying it to a routine excision is a preventable cause of LAST.[3][4][5]

Etymology for viva gold: tumescent comes from the Latin tumescere, "to begin to swell" — the tissue physically swells as the dilute volume opens and hydrodissects the subcutaneous plane, which is precisely the effect the technique exploits.[4]

The kinder injection — buffer, warm, slow, small needle, pinch

A patient who dreads the next injection will refuse the next procedure, so pain-minimising technique is not cosmetic — it is how you keep a practice. The evidence-backed bundle is five moves, applied together:[1][2]

[1] [2]

A consultant confession: mark your cosmetic margins before you infiltrate, because over-infiltration blurs the border of a subtle tumour and you will excise too much or too little. Anaesthetise thoughtfully, not generously, wherever the cosmetic result matters.[9]

The epinephrine-in-digits myth, busted (mostly)

"Never put epinephrine in a finger" is the most quoted and most outdated rule in cutaneous surgery — the modern evidence has retired the absolute ban. The myth dates from the procaine-with-epinephrine era, when case reports of digital necrosis appeared long before dilute, controlled mixtures existed. Krunic and colleagues revisited the question and found no cases of digital necrosis from epinephrine in the modern dermatologic-surgery literature; thousands of digital blocks with dilute epinephrine have since been performed safely.[6]

So the revised rule: dilute epinephrine in a digital block is acceptable in the otherwise vascularly well digit, but it is not a blanket licence. Avoid it in peripheral vascular disease, vasospastic and Raynaud-type disorders, the poorly perfused diabetic digit, or any finger that looks ischaemic — and use the lowest effective concentration. The same caution extends to the penis, an end-artery organ: know your local protocol before adding epinephrine there.[6]

Children — weight, topical prep, and a calm room

Paediatric cutaneous surgery is weight-based dosing, a topical anaesthetic first, and behavioural technique — the milligram error here is the one with the narrowest margin. Calculate the maximum lidocaine from kilograms before you draw up, because a small child's window between effective and toxic dose is small. Apply a topical anaesthetic (a eutectic lidocaine-prilocaine or an amethocaine preparation) under occlusion ahead of the needle to numb the entry site, and pair it with distraction, parental presence, and a warm, buffered injectate. Published paediatric dermatologic-surgery series confirm LA is safe in children when weight-based limits are respected and the whole technique is built around comfort.[7]

Two more groups deserve a named line. Pregnancy: lidocaine is the usual amide of choice when a procedure is unavoidable; keep the dose to the minimum effective. Beta-blockers and drug interactions: take a medication history before adding epinephrine, because selected combinations can produce an exaggerated pressor response that a wary clinician foresees.[1][7]

When the anaesthesia fails — think systematically, blame allergy last

When the block does not take, the cause is almost never allergy — it is volume, plane, time, pH, or anxiety, in roughly that order. Run the checklist before reaching for the allergy label:[1][8]

• Insufficient volume or concentration for the field size.
• Wrong tissue plane — subdermal, not intradermal, for many blocks.
• Not enough time — onset is minutes, not seconds, especially unbuffered.
• Highly vascular face — drug washes out before it binds; add epinephrine.
• Acidotic, infected tissue — ring-block around it, do not inject into it.
• Extreme anxiety — amplifies perceived pain; address it directly.
• Expired or wrongly diluted product — check the vial and the dilution.[1][8]

True IgE-mediated amide allergy is a diagnosis of careful exclusion, made only after the common mimics — vasovagal, epinephrine effect, preservative reaction, plain underdosing — have been ruled out. Labelling a patient "allergic to lidocaine" on thin evidence strips them of the safest injectable for the rest of their life.[1][8]

Mark first, anaesthetise second — the biopsy link

Punch, shave and elliptical biopsies live or die on adequate field anaesthesia and, when used, the haemostasis epinephrine provides — but over-infiltration will blur the very border you are trying to define. The discipline is to mark the lesion and the margins first, then anaesthetise thoughtfully so tissue distortion does not pull your excision off-track.[9][10]

LAST — the thing that kills in the procedure room

Local-anaesthetic systemic toxicity is the kill-or-cure emergency of cutaneous surgery, and it is almost always preventable — it follows intravascular injection or absolute overdose, both of which technique should avoid. The progression is a clinician you must recognise the instant it begins, because minutes decide the outcome:[1][3]

[1] [3] [8]

Why bupivacaine is the feared one: it binds cardiac sodium channels tightly and dissociates slowly, so bupivacaine-induced cardiovascular collapse is refractory to standard ACLS and demands lipid emulsion early plus a prolonged resuscitation. The lesson for office surgery is dosing — the long-acting amides have a narrower therapeutic window than lidocaine, and their ceilings are lower, not higher.[1][3]

Methaemoglobinaemia — the blue baby after a topical

Prilocaine and benzocaine can oxidise haemoglobin's iron to the ferric state, producing methaemoglobin that cannot release oxygen — and infants are the patients it silences. The trigger is high topical exposure: benzocaine sprays and gels, or prilocaine in a eutectic mixture applied over a large area, above all in infants. The child becomes cyanotic, the pulse oximetry reads higher than the cyanosis suggests, the blood looks chocolate-brown, and the cyanosis does not resolve with oxygen.[1]

Treat with methylene blue once the diagnosis is made (after considering G6PD status where time allows), and avoid the offending agent thereafter. The preventive rule is simple and an exam favourite: limit topical prilocaine and benzocaine in infants — smallest area, shortest time — and never use adult benzocaine teething products in babies.[1]

Pale and sweaty after the needle — four mimics, one discriminator each

The allergic-reaction label is slapped on most post-injection events, and it is usually wrong. Four entities account for nearly all of them, and each carries one clean discriminator:[1][8]

[1] [8]

How patients come to harm — the preventable list

• LAST from an intravascular injection — aspirate where relevant, calculate the dose, and never exceed the ceiling. The preventable catastrophe of cutaneous surgery.[1][3]
• LAST from borrowing the tumescent 81 mg/kg number for concentrated infiltration — separate paradigms, separate ceilings.[3][4]
• Methaemoglobinaemia in an infant after liberal topical prilocaine or benzocaine — limit the area, the agent, and the patient age.[1]
• A falsely labelled lidocaine allergy that strips the safest injectable from a patient for life — distinguish the mimics first.[1][8]
• A failed block in an acidotic abscess injected head-on instead of ring-blocked — pain, poor cover, an unhappy patient.[1]
• Over-infiltration that blurs a tumour margin before marking is final — excise too much or too little.[9]
• Epinephrine in a poorly perfused digit — the myth is mostly busted, but ischaemic-appearing and vasospastic digits still exclude it.[6]
• A weight-based error in a child — the milligram mistake with the narrowest margin.[7]

The mantra, and the mnemonic

[1]

The mantra: Amides inject, esters topical; 1% is 10 mg per mL; convert before you draw; LAST is stop, ABC, lipid.[1][3]

Ward-round test — three stems, thirty seconds each

Stem 1 — metallic taste mid-injection (answer)

A registrar is infiltrating 1% lidocaine with epinephrine around a back cyst when the patient says their mouth tastes metallic and their lips are tingling. What is happening, and what is your first move? Model: This is early LAST — the classic CNS prodrome (perioral paraesthesia, metallic taste, tinnitus, agitation) that precedes seizures or cardiovascular collapse. Stop injecting immediately, call for help, secure the airway and give high-flow oxygen, treat seizures with a benzodiazepine, and start intravenous lipid emulsion per the local or ASRA-type LAST checklist, with modified ACLS if it progresses. Do not label this an allergy — the discriminator is the neurological prodrome, and the cause is almost always intravascular injection or absolute overdose.[1][3]

Stem 2 — the blue baby after a topical (answer)

A six-month-old becomes cyanotic after a liberal application of a topical anaesthetic gel for a minor procedure. Pulse oximetry reads 88%, the cyanosis does not improve with oxygen, and the blood looks chocolate-brown. Diagnosis and treatment? Model: This is methaemoglobinaemia from the topical agent (prilocaine or benzocaine) oxidising haemoglobin iron to the ferric state. The discriminator is cyanosis that does not resolve with oxygen, plus chocolate-brown blood. Treat with methylene blue once the diagnosis is made (considering G6PD status where time allows), and avoid the offending agent in future. Prevention is to limit topical prilocaine and benzocaine in infants — smallest area, shortest time.[1]

Stem 3 — pale, sweaty, and labelled allergic (answer)

Immediately after an infiltration a young adult goes pale, clammy and nauseated with a thready, slow pulse. The nurse calls it an allergic reaction. What is it, and what do you do? Model: This is vasovagal syncope, the commonest post-injection event — pallor, clamminess, nausea, and bradycardia with hypotension, typically resolving when laid flat. Lay the patient down, raise the legs, reassure, and monitor. The discriminator from epinephrine effect (tachycardia), early LAST (a neurological prodrome), and true allergy (urticaria or airway involvement) is the bradycardia with pallor. Reserve the allergy label for urticaria, angio-oedema, or bronchospasm — and never document lidocaine allergy on a vasovagal.[1][8]

References

1. [1]Nadir U, Scaburi IR, Cerci FB, et al. How to minimize pain with local anesthesia and improve patient experiences: a review An Bras Dermatol, 2026.PMID 42107926
2. [2]Desai SR, Vidimos AT, Elder EJ, et al. In-office compounding of buffered lidocaine and epinephrine - A stability and safety review J Am Acad Dermatol, 2026.PMID 41665574
3. [3]Wang A, Grushchak S, Kaul S, et al. Toxicity of Infiltrative Lidocaine in Dermatologic Surgery: Are Current Limits Valid? Dermatol Pract Concept, 2021.PMID 34631267
4. [4]Lillis PJ. Tumescent Anesthesia With a Mean Dose of 81 mg/kg of Lidocaine Dermatol Surg, 2023.PMID 37801576
5. [5]Klein JA, Jeske DR. Estimated Maximal Safe Dosages of Tumescent Lidocaine Anesth Analg, 2016.PMID 26895001
6. [6]Krunic AL, Wang LC, Soltani K, et al. Digital anesthesia with epinephrine: an old myth revisited J Am Acad Dermatol, 2004.PMID 15523354
7. [7]Chessa MA, Brunetti T, Robuffo S, et al. Safety of Local Anesthesia in Pediatric Dermatologic Surgery: Our Experience and a Concise Review of the Existing Literature Dermatol Pract Concept, 2026.PMID 42190206
8. [8]Walsh AM, Moran B, Walsh SA. Knowledge of local anesthetic use among dermatologists Dermatol Surg, 2012.PMID 22530750
9. [9]Pickett H. Shave and punch biopsy for skin lesions Am Fam Physician, 2011.PMID 22046939
10. [10]Greenwood JD, Merry SP, Boswell CL. Skin Biopsy Techniques Prim Care, 2022.PMID 35125151