Thermal Injuries — Burn Depth, Fluid Resuscitation, Antemortem vs Postmortem Burns, Electrical and Lightning Injuries

A thermal injury is damage caused by heat transferred to the body. In the clinic the usual sources are flame and flash (most adult admissions), scalds from hot liquids (most paediatric burns), and contact with hot objects; electrical and lightning injuries convert electrical energy into heat inside the tissues. StatPearls reports that about 86% of burns are thermal (43% flame, 34% scald, 9% hot objects), with 4% electrical and 3% chemical.

Heat radiating from the point of contact produces three concentric zones: a central zone of coagulation (irreversible cell death and protein denaturation), a surrounding zone of stasis (damaged circulation that can recover with good resuscitation and wound care), and an outer zone of hyperaemia (increased blood flow). Hypotension or poor wound care can convert the zone of stasis into deeper, wider necrosis — the reason early fluids matter. Burns larger than about 20% of body surface area also trigger a systemic inflammatory response.

Burned area is expressed as a percentage of total body surface area (TBSA). The adult rule of nines gives the head and neck 9%, each upper limb 9%, the front and back of the trunk 18% each, each lower limb 18% and the perineum 1%. In children the head is relatively larger (18%) and each leg smaller (13.5%). The patient's palm is about 1% for scattered burns, and the Lund-Browder chart is the more accurate age-adjusted method. Superficial (first-degree) burns are not included in the TBSA calculation.

• Ringer lactate is the preferred crystalloid because it corrects both hypovolaemia and the extracellular sodium deficit.
• Urine output is the most reliable guide to adequate resuscitation: target 0.5–1 mL/kg/h in adults and 1–1.5 mL/kg/h in children; insert a urinary catheter.
• The 2012 American Burn Association consensus uses 20% TBSA as the threshold for formal resuscitation (children may need lower thresholds).
• The formula is only an estimate — titrate to urine output and vital signs. Older formulas: Evans (2 mL with plasma and saline) and Brooke (2 mL, one-quarter colloid).

StatPearls classifies a major burn as burns over 25% TBSA in adults (20% in children), full-thickness burns over 10% TBSA, or burns of the face, perineum or extremities with significant functional or cosmetic risk. The American Burn Association criteria for burn-centre referral include burns of the face, hands, feet, genitalia and perineum, any full-thickness burn, chemical and electrical burns, inhalation injury, and burns over 10% TBSA.

• Suspect inhalation injury with singed nasal hairs, burns around the nose and mouth, sooty sputum or respiratory distress — usually after flame burns in an enclosed space.
• Measure carboxyhaemoglobin and consider cyanide: both are released in house fires, and raised carboxyhaemoglobin warns of carbon monoxide poisoning and impending respiratory failure. Give oxygen.
• Immersion scalds with sharp lines of demarcation, or a history that does not fit the pattern, should raise suspicion of child or elder abuse.

A body found in a fire raises one question: was the person alive when the fire started? Burning is sometimes used to conceal a homicide or destroy identity. The answer rests on signs of vitality — changes that need breathing or a circulation.

Intense heat produces changes after death that can be mistaken for assault. None of them proves the person was alive in the fire.

• Pugilistic (boxer's) attitude — heat shrinks and retracts the muscles, so the arms take a boxer-like flexed posture and the legs flex. In cremation observations at 670–810 °C, bodies showed the pugilistic attitude after about 10 minutes — it forms in bodies burned after death, so it is no proof of life.
• Heat ruptures (skin splits) — the skin splits as it dries and contracts, producing wounds that resemble stab or slash wounds but without swelling or haemorrhage in the surrounding tissue.
• Heat fractures — the skull and exposed bones crack; heat fractures have irregular edges and can cross suture lines, whereas antemortem fractures generally stop at sutures.
• Heat haematoma — a brick-red, loose-textured collection in the extradural space, from blood extruded from the diploë or torn vessels, typically over the convexities. It is a postmortem artefact; a subdural haematoma in a charred body points more to antemortem head injury (rare exceptions are reported).
• Thermal amputations — burned-off limbs with smooth, rounded bone ends and no soft tissue, unlike traumatic amputations.
• Tongue protrusion and charring of the body are further postmortem thermal artefacts.

Electrical injuries are low-voltage (< 1000 V) or high-voltage (≥ 1000 V). Damage depends on current type, voltage and tissue resistance. By Ohm's law current is proportional to voltage and inversely proportional to resistance; by Joule's law heat produced = I² × R × time. Bone, fat and skin have high resistance; nerves, muscle and blood vessels conduct well. Thick, dry, keratinised skin resists more; moist mucosa and broken skin resist less. Alternating current (AC) is about 3–5 times more damaging than direct current at the same voltage. Low-voltage currents are more likely to cause muscle tetany, which can prolong contact.

• Joule burn (electric mark) — the entry mark of low-voltage contact: a small lesion with a raised border and a central crater, often a pale, hard base with surrounding hyperaemia or blistering.
• Usual entry: palms and fingertips; usual exit: soles of the feet. The exit wound is often atypical, and metallisation (metal deposited from the conductor) can be found at entry and exit.
• Death from electrocution can occur with or without an electric mark on the body. The electric mark remains the key autopsy evidence, with histology (elongated, palisaded epidermal nuclei).
• Management: cardiac monitoring, fluids with a target urine output of 1–2 mL/kg/h if there are significant burns or myoglobinuria, and watch for compartment syndrome. Late sequelae include cataracts, neuropathy and psychological effects.

Lightning delivers an extremely brief, very high-voltage current that largely flashes over the skin surface. StatPearls describes four mechanisms: direct strike (about 5%, rare), contact with a struck object, side splash (about one-third — current jumps from a nearby object such as a tree) and ground current (about one-half — the commonest).

• Lichtenberg figures — transient 'ferning' or 'feathering' red skin pattern, pathognomonic of lightning; transient and self-resolving.
• Cardiac arrest is typically asystolic from simultaneous depolarisation of the myocardium; sinus rhythm often returns before spontaneous breathing, and a second arrest follows if ventilation is not started promptly.
• Reverse triage: with several victims, treat those who appear dead first — survivors of the initial strike rarely die later.
• Keraunoparalysis — temporary, mostly lower-limb paralysis with cold, blue, pulseless limbs from vascular spasm; usually resolves without treatment.
• Tympanic membrane rupture in 50–80%; cataracts (often bilateral) are the commonest eye injury.

• Depth from a description — blisters and pain (superficial partial) vs painless leathery skin (full thickness).
• Calculation — rule of nines plus Parkland volume, and how much in the first 8 hours.
• Best sign of antemortem burning — soot below the vocal cords / in the trachea, and raised carboxyhaemoglobin.
• Postmortem artefact identification — pugilistic attitude, heat ruptures, extradural heat haematoma.
• Electrical marks — Joule burn with crater; exit on soles; metallisation. Lightning — Lichtenberg (arborescent) figures, reverse triage.