Corrosive Acid & Alkali Poisoning

Corrosives are chemical agents that destroy tissue on contact by coagulating or liquefying proteins. In forensic toxicology they are classic agents of homicidal vitriolage (acid throwing), suicidal ingestion, and accidental poisoning. The fountainhead of high-yield MCQs lies in the type of necrosis, the postmortem mucosal colour, and the unique systemic toxicity of carbolic acid (phenol).

Definition & Classification

A corrosive (caustic) poison is one that chemically burns or destroys living tissue at the site of application. Corrosion = local destruction; if absorbed, some also produce systemic effects. They are broadly divided into acids and alkalis, with phenol (carbolic acid) occupying a special intermediate place.

Category	Examples	Type of necrosis	Mechanism
Mineral (inorganic) acids	Sulphuric (oil of vitriol), Nitric (aqua fortis), Hydrochloric (muriatic/spirit of salt)	Coagulative	Dehydration + protein coagulation, forms a tough eschar limiting deep penetration
Organic acids	Oxalic, Acetic, Carbolic (phenol), Formic	Mostly coagulative; oxalic is also a systemic poison (hypocalcaemia)	Variable
Alkalis (caustics)	Caustic soda (NaOH), Caustic potash (KOH), Ammonia, Lime (CaO), Sodium/potassium carbonate	Liquefactive (colliquative)	Saponification of fat + dissolution of protein → deep, penetrating burns

High-yield: Acids cause coagulative necrosis (eschar limits depth); alkalis cause liquefactive necrosis (penetrate deep → higher perforation risk). This single contrast is the most repeated corrosive MCQ.

Strength matters (corrosive vs irritant)

• Concentrated mineral acids/alkalis → corrosive (frank tissue destruction).
• Dilute forms act as irritants (inflammation without destruction).

Etiology & Pathophysiology

Acids liberate H⁺ ions that abstract water from tissues and precipitate proteins as acid proteinates, forming a firm, dry, leathery eschar (slough). This coagulum acts as a barrier, so acid burns tend to be more superficial but produce intense immediate pain and well-demarcated lesions.

Alkalis combine with tissue proteins to form gelatinous, soluble alkaline proteinates and saponify fats. There is no protective barrier, so the burn keeps advancing (penetrating necrosis), making the wound soft, slimy, translucent, and prone to late perforation and stricture.

Phenol (carbolic acid) is a protoplasmic poison — it denatures and precipitates protein producing a hard, white, corrugated eschar locally, but is also readily absorbed producing severe CNS depression and renal toxicity.

High-yield: The most dangerous segment of GIT for alkali ingestion is the oesophagus (stratified squamous, prolonged contact) → liquefactive necrosis → strictures. For acids, the stomach (pylorus/antrum) bears the brunt → coagulative gastritis → later pyloric stenosis.

Clinical Features

Local + systemic depending on route (ingestion vs splash vs throwing).

Ingestion:

1. Immediate intense burning pain in mouth, throat, retrosternal and epigastric region.
2. Dysphagia, drooling, odynophagia, blood-stained vomiting (haematemesis with coffee-ground or altered blood).
3. Burns/stains around lips, angle of mouth, chin (dribble marks track downward and outward).
4. Glottic oedema → stridor, dyspnoea, aphonia (laryngeal involvement → may need emergency airway).
5. Shock, circulatory collapse; later perforation (mediastinitis/peritonitis).

External (vitriolage / splash): corrosive burns with discolouration of skin and clothes; eyes may be permanently damaged (corneal opacity, blindness).

Colour clues of the eschar / mucosa (very high-yield)

Corrosive	Colour of burn / mucosa	Notes / mechanism
Sulphuric acid	Black (charring)	Most powerful dehydrating mineral acid; greyish-white → brown → black
Nitric acid	Yellow (xanthoproteic reaction)	Forms yellow xanthoproteic acid with proteins; brownish-yellow stains
Hydrochloric acid	Greyish-brown / brownish	Least corrosive of the three mineral acids
Carbolic acid (phenol)	Greyish-white, hard, corrugated, opaque ("cooked"/leathery)	Mucosa looks "boiled"; urine turns olive-green to smoky black on standing
Oxalic acid	Whitish; mucosa bleached	Systemic hypocalcaemia (tetany, cardiac)
Caustic alkalis	Soft, swollen, soapy, translucent / greyish	Liquefactive; greasy to touch

High-yield: Nitric acid → yellow stains (xanthoproteic reaction); Sulphuric acid → black (charring); carbolic acid → greyish-white opaque corrugated mucosa with olive-green/smoky urine. These colour pairings are repeatedly examined.

Carbolic Acid (Phenol) — Special Focus

Carbolic acid is a frequent NEET PG favourite because of its dual action: local corrosive + systemic protoplasmic poison.

• Synonyms: phenol, hydroxybenzene. Lysol = cresol (methylphenol) + soap; Dettol = chloroxylenol.
• Carboluria: urine darkens to olive-green → brown → smoky black on standing/exposure to air (oxidation products — hydroquinone/pyrocatechol).
• Local: white painless eschar (phenol has local anaesthetic action → relatively painless corrosion, unlike mineral acids). Mucosa appears hard, corrugated, opaque-white.
• Systemic: giddiness, CNS depression, coma, shock, pinpoint or normal pupils, shallow respiration, renal failure, smell of phenol/carbolic in breath and vomitus.
• Carbolic acid marasmus / dermatitis: chronic occupational exposure → anorexia, weight loss, dark urine, vertigo (ochronosis-like darkening).
• Fatal dose ~ 10–15 g; fatal period ~ 3–4 hours.

High-yield: Phenol corrosion is comparatively painless because of its local anaesthetic effect — contrast with the excruciating pain of mineral acids. Urine on standing turns olive-green/smoky black (carboluria).

Mnemonic — "Phenol is a SILENT burner": Smoky urine, I anaesthetic (painless), Lysol/cresol related, Eschar white & corrugated, Nervous system depression, Ten–fifteen grams fatal.

Diagnosis & Investigation of Choice

Clinical diagnosis is usually evident from history, dribble pattern, breath odour, and characteristic burns. Confirmatory and assessment steps:

Flow of acute assessment: Airway (look for stridor/oedema) → Breathing/Circulation, treat shock → Identify agent (history, stain colour, pH of vomitus) → Endoscopy 12–48 h to grade burns → Imaging if perforation suspected → Definitive care.

• Upper GI endoscopy is the investigation of choice to grade mucosal injury (Zargar grading) and prognosticate stricture risk. Ideally done within 12–48 hours (early enough to grade, late enough to demarcate; avoid > 48 h–5 days when perforation risk is highest).
• Chest/abdominal X-ray (erect) / CT: free air/mediastinal air suggests perforation.
• Contrast (barium) swallow: later, to assess stricture (avoid if perforation suspected — use water-soluble contrast).
• ABG, electrolytes, calcium (oxalic), renal function (phenol), CBC.

Zargar endoscopic grade	Findings	Risk
Grade 0	Normal	Nil
Grade I	Mucosal oedema/erythema	Low; no stricture
Grade IIa	Superficial ulcer, exudate	Moderate
Grade IIb	Deep/circumferential ulcers	Higher stricture
Grade III	Necrosis (deep brown/black eschar), perforation	High mortality, stricture

Management / Drug of Choice

General principles (acute):

1. Do NOT induce emesis — re-exposes oesophagus to corrosive.
2. Do NOT do gastric lavage / pass stomach tube blindly — risk of perforation (a soft NG tube may be passed under direct endoscopic vision if needed).
3. Do NOT use chemical neutralisation / "neutralising" antidotes — the exothermic reaction generates heat and gas → worsens injury and may perforate.
4. Avoid activated charcoal — poor binding, obscures endoscopy.
5. Dilution: small sips of water or milk only if the patient can swallow, is conscious, and presents very early (controversial; never force large volumes — may induce vomiting).
6. Secure airway (early intubation/tracheostomy if laryngeal oedema), IV fluids for shock, analgesia (opioids), PPIs, NPO, nutritional support (TPN/feeding jejunostomy).

Surgery: emergency laparotomy/oesophagectomy for perforation or full-thickness necrosis; later dilatation/colonic interposition for strictures.

Agent-specific:

• Phenol: wash skin with water then polyethylene glycol (PEG) or olive/castor oil; supportive for CNS/renal failure. (Sodium sulphate cathartic historically.)
• Oxalic acid: calcium gluconate IV for hypocalcaemia/tetany; calcium lactate orally to precipitate insoluble calcium oxalate.
• Hydrofluoric acid (HF) burns: calcium gluconate topical gel / local infiltration / intra-arterial — DOC for the painful, deep HF burns and hypocalcaemia.
• Alkali splash to eye: copious continuous irrigation with water/saline for ≥ 30 min (alkali eye injury is an ophthalmic emergency — penetrates deeper than acid).

High-yield (board favourites):

• No gastric lavage, no emetics, no chemical neutralisation in corrosive ingestion.
• Endoscopy within 24–48 h is investigation of choice; steroids are controversial (may reduce stricture in grade IIb but risk masking perforation — not routinely recommended).
• HF acid burn → calcium gluconate. Oxalic acid → calcium gluconate (hypocalcaemia).

Complications

• Acute: glottic oedema/asphyxia, shock, GI haemorrhage, perforation → mediastinitis/peritonitis, aspiration pneumonitis.
• Systemic: renal failure (phenol, oxalic), hypocalcaemia/tetany & cardiac arrhythmia (oxalic), CNS depression/coma (phenol), metabolic acidosis.
• Late: oesophageal stricture (especially alkali), pyloric/antral stenosis & hour-glass contracture of stomach (especially acids), oral/laryngeal scarring, oesophageal carcinoma (long-term, decades after lye stricture — squamous cell).

High-yield: Chronic alkali (lye) stricture is a recognised risk factor for oesophageal squamous cell carcinoma decades later.

Postmortem Appearances (Forensic Angle)

• External: corrosive stains on lips, chin, angles of mouth running downward (gravity); stained clothing; in vitriolage, skin burns.
• Internal: mucosa of mouth, pharynx, oesophagus, stomach softened, eroded, discoloured in the agent-specific colour.
• Stomach wall may be perforated; with sulphuric acid the stomach can be converted to a black, charred, brittle mass; contents are dark, acidic.
• "Acid contact" perforation may occur postmortem if acid leaks — must distinguish ante-mortem (vital reaction, congestion) from postmortem effect.
• Alkali: mucosa soft, swollen, soapy, greyish; deeper penetration.

High-yield: Sulphuric acid can perforate the stomach and even the diaphragm/abdominal wall postmortem; mucosa is black, sodden, charred. Nitric acid stains are yellow.

Key Differentials

Feature	Acid poisoning	Alkali poisoning	Phenol
Necrosis	Coagulative (eschar)	Liquefactive (penetrating)	Coagulative + systemic
Pain	Severe, immediate	Severe	Mild/painless (anaesthetic)
Main GI site	Stomach (pylorus)	Oesophagus	Oesophagus + systemic
Stricture risk	Pyloric stenosis	Oesophageal stricture (high)	Both
Perforation	Less common	More common	Possible
Urine	—	—	Olive-green/smoky black
Systemic poison	Oxalic (Ca²⁺), HF (Ca²⁺)	Usually local	CNS/renal

Distinguish corrosive from:

• Irritant poisoning (dilute → inflammation, no destruction).
• Diphtheria / candidiasis (membranous lesions, no chemical stain history).
• Hot liquid (thermal) burns of mouth — no chemical odour/stain pattern.

Recently asked / exam angle

• Colour matching: "Yellow staining of mucosa in poisoning is due to ___" → Nitric acid (xanthoproteic reaction). "Black charring of stomach" → Sulphuric acid.
• Type of necrosis: "Liquefactive necrosis is produced by ___" → Alkali; "Coagulative" → acid. Frequently flipped.
• Phenol: "Urine turning green/smoky on standing" → carbolic acid (carboluria); phenol is painless corrosive.
• Management negative-stem: "Which is contraindicated in corrosive ingestion?" → gastric lavage / emesis / neutralisation.
• Investigation of choice for corrosive ingestion → upper GI endoscopy.
• HF acid burn antidote → calcium gluconate.
• Vitriolage = throwing of sulphuric acid (most common acid used in acid attacks in India); legal aspect under amended IPC sections on grievous hurt by acid.
• Site most affected: acid → stomach, alkali → oesophagus.
• Oesophageal Ca risk after lye stricture (long latency).

Rapid revision

1. Acids → coagulative necrosis (eschar, superficial); alkalis → liquefactive necrosis (deep, perforate).
2. Sulphuric acid = black charring; strongest dehydrating mineral acid; can perforate stomach postmortem.
3. Nitric acid = yellow stains (xanthoproteic reaction); brownish-yellow vomit.
4. Hydrochloric acid = greyish-brown; least corrosive mineral acid.
5. Carbolic acid (phenol) = greyish-white, hard, corrugated, painless eschar; urine turns olive-green/smoky black (carboluria); fatal dose 10–15 g.
6. Alkali ingestion damages the oesophagus most → strictures; acid damages the stomach/pylorus most → pyloric stenosis.
7. Investigation of choice = upper GI endoscopy (within 12–48 h); Zargar grading.
8. Contraindicated: emesis, gastric lavage, activated charcoal, chemical neutralisation.
9. Oxalic acid & HF acid → hypocalcaemia → calcium gluconate is the antidote.
10. Alkali eye injury penetrates deeper than acid → emergency copious irrigation.
11. Vitriolage = acid throwing, classically sulphuric acid; dribble stains run downward/outward.
12. Long-term lye stricture → oesophageal squamous cell carcinoma.