1 Introduction
Cyanide poisoning is a form of poisoning resulting from exposure to compounds that contain the cyanide ion (CN⁻). It disrupts cellular respiration by binding to cytochrome c oxidase in the mitochondrial electron transport chain, leading to histotoxic hypoxia and rapid organ failure. Common sources include industrial chemicals, smoke inhalation from fires, certain plant seeds (e.g., apricot pits), and intentional poisoning. Symptoms range from headache and dizziness to seizures, respiratory arrest, and death within minutes. Treatment involves supportive care, antidotes such as hydroxocobalamin or sodium nitrite/sodium thiosulfate, and decontamination.
1.1 Historical perspective
Cyanide has been known since antiquity. Ancient Egyptians used cyanide-containing compounds from peach pits as poisons. During the 19th century, cyanide was isolated and characterized, and its use in gold extraction (cyanidation) became widespread. Cyanide was employed as a chemical warfare agent in World War I and later used in gas chambers during World War II. Accidental and intentional cyanide poisonings have been documented throughout industrial history, prompting the development of antidotes such as nitrites and thiosulfates in the 1930s and hydroxocobalamin in the modern era.
1.2 Chemical forms and sources
Cyanide exists in several chemical forms, each with distinct routes of exposure and toxicity.
1.2.1 Hydrogen cyanide gas
Hydrogen cyanide (HCN) is a colorless, highly volatile liquid or gas with a bitter almond odor (though a significant portion of the population cannot detect it). It is used in industrial processes (e.g., plastics manufacturing, fumigation) and is a major component of smoke from burning synthetic materials (e.g., polyurethane, acrylics). Inhalation is the most rapid and lethal route of exposure.
1.2.2 Cyanide salts
Common cyanide salts include sodium cyanide (NaCN) and potassium cyanide (KCN). These are white crystalline solids used in electroplating, mining, and chemical synthesis. They are highly water-soluble and rapidly release HCN in acidic conditions (e.g., stomach acid). Ingestion or dermal absorption of these salts can cause severe poisoning.
1.2.3 Cyanogenic glycosides in plants
Many plants contain cyanogenic glycosides—sugar-bound cyanide compounds that are released upon enzymatic breakdown (e.g., when plant tissue is crushed or ingested). Notable sources include bitter almonds, apricot pits, peach pits, cassava (manioc), lima beans, and sorghum. Chronic dietary exposure to inadequately processed cassava is a well-known cause of cyanide-related neurological disorders in tropical regions.
1.3 Epidemiology
Cyanide poisoning is relatively uncommon in developed countries but remains a significant occupational and environmental hazard. Most acute poisonings result from house fires (smoke inhalation), industrial accidents, and intentional ingestion (suicide or homicide). In the United States, approximately 200–300 cyanide-related deaths occur annually, with smoke inhalation being the most common cause. Globally, chronic cyanide exposure from cassava consumption affects millions in sub-Saharan Africa and parts of Asia, leading to conditions such as konzo (a spastic paraparesis).
2 Pathophysiology
2.1 Mechanism of toxicity
Cyanide exerts its toxic effects primarily through the inhibition of mitochondrial respiration, leading to cellular hypoxia and metabolic acidosis.
2.1.1 Inhibition of cytochrome c oxidase
The cyanide ion binds with high affinity to the ferric (Fe³⁺) heme a₃ of cytochrome c oxidase (complex IV of the electron transport chain). This binding prevents the transfer of electrons to molecular oxygen, halting aerobic ATP production. Cells reliant on oxidative phosphorylation—particularly in the central nervous system and heart—are rapidly deprived of energy, leading to loss of function and cell death.
2.1.2 Cellular hypoxia and metabolic acidosis
Despite adequate oxygen delivery to tissues, the inability to use oxygen results in histotoxic hypoxia. Anaerobic metabolism is upregulated, producing lactic acid and causing severe metabolic acidosis. The combination of energy failure and acidosis disrupts ion pumps, leading to cellular swelling, membrane damage, and eventual necrosis. High cyanide concentrations can also inhibit other enzymes, including superoxide dismutase and glutamate decarboxylase, contributing to neurotoxicity.
2.2 Toxicokinetics
2.2.1 Absorption
Cyanide is rapidly absorbed via inhalation (HCN gas), ingestion (salts, cyanogenic foods), and dermal contact (especially with high concentrations or damaged skin). Inhalation offers the most rapid absorption, with symptoms appearing within seconds to minutes. Gastrointestinal absorption is slightly slower but still occurs within minutes.
2.2.2 Distribution
Once absorbed, cyanide distributes widely in the body. It readily crosses cell membranes and the blood–brain barrier. The highest concentrations are found in the liver, kidneys, brain, and heart. Plasma protein binding is minimal.
2.2.3 Metabolism (detoxification pathways)
The primary detoxification pathway involves the sulfurtransferase enzyme rhodanese (thiosulfate sulfurtransferase), which catalyzes the transfer of sulfur from thiosulfate to cyanide, forming the relatively non‑toxic thiocyanate (SCN⁻). This reaction occurs mainly in the liver and kidneys. A minor pathway involves reaction with hydroxocobalamin (vitamin B₁₂a) to form cyanocobalamin (vitamin B₁₂), which is excreted in urine or bile. Small amounts of cyanide are also metabolized via oxidation to cyanate or incorporation into the cyanide–methemoglobin complex.
2.2.4 Excretion
Thiocyanate is the main excretory product, eliminated primarily via the kidneys (half‑life 1–3 days in normal renal function). Small amounts of unchanged cyanide are excreted in urine and exhaled air. In acute poisoning, the half‑life of cyanide in blood is short (20–60 minutes) due to rapid metabolism.
3 Clinical features
3.1 Acute poisoning
The clinical presentation of acute cyanide poisoning depends on the dose, route, and speed of exposure. It is classically described by a rapid progression of symptoms.
3.1.1 Early symptoms
After mild to moderate exposure (e.g., low‑dose inhalation or ingestion), symptoms may include headache, dizziness, confusion, weakness, nausea, vomiting, anxiety, and a feeling of “air hunger.” Tachypnea and tachycardia are common. A bitter almond odor on the breath may be noted.
3.1.2 Severe manifestations
With higher doses, the condition deteriorates rapidly. Profound toxicity often leads to loss of consciousness, seizures, and cardiorespiratory collapse within minutes.
3.1.2.1 Neurological: seizures, coma
Central nervous system effects dominate: generalized tonic‑clonic seizures, then coma, decerebrate posturing, and loss of brainstem reflexes. Pupils are often fixed and dilated. Neurological damage can be irreversible if treatment is delayed.
3.1.2.2 Cardiovascular: hypotension, dysrhythmias
Cardiovascular instability includes initial hypertension and reflex tachycardia followed by profound hypotension, bradycardia, and dysrhythmias (e.g., atrial fibrillation, ventricular tachycardia). Cardiac arrest may occur.
3.1.2.3 Respiratory: tachypnea, apnea
Early tachypnea and hyperpnea are followed by respiratory depression, apnea, and death. Pulmonary edema is occasionally seen in smoke‑inhalation victims.
3.2 Chronic low-level exposure
Chronic exposure to sub‑lethal concentrations of cyanide (e.g., from dietary cassava, tobacco smoke, or occupational settings) can cause various non‑specific symptoms: headache, fatigue, vertigo, tremors, and peripheral neuropathy. In cassava‑dependent populations, prolonged exposure may lead to konzo (upper motor neuron damage) or tropical ataxic neuropathy (sensory and motor deficits). Thyroid dysfunction (goiter, hypothyroidism) can occur due to thiocyanate’s inhibition of iodine uptake.
3.3 Specific presentations
3.3.1 Smoke inhalation
Smoke from fires—especially those involving synthetic materials (polyurethane, nylon, acrylics)—contains HCN and carbon monoxide. Victims may present with altered mental status, cherry‑red skin (from CO), or classic cyanide features. Co‑poisoning with CO complicates diagnosis; both agents must be considered.
3.3.2 Ingestion of cyanogenic foods
Ingestion of large amounts of raw or improperly processed cyanogenic plants (e.g., bitter apricot kernels, cassava) can cause acute poisoning. Children are particularly at risk from eating fruit pits. Symptoms are similar to other acute ingestions, with onset typically 15–30 minutes after consumption.
4 Diagnosis
4.1 Clinical suspicion and history
Cyanide poisoning is a medical emergency; treatment should not be delayed for laboratory confirmation. Diagnosis is based on history (fire exposure, industrial accident, known ingestion), rapid onset of neurological and cardiovascular symptoms, and classic signs (bitter almond odor, bright‑red venous blood due to high oxygen saturation in venous blood). A high anion gap metabolic acidosis with elevated lactate is strongly suggestive.
4.2 Laboratory findings
4.2.1 Blood cyanide levels
Whole‑blood cyanide levels are the definitive test. Lethal concentrations are generally >1 mg/L (approximately 40 μmol/L). However, levels may be falsely decreased after antidote administration, and turnaround time often limits clinical utility.
4.2.2 Lactate and acid‑base status
Severe lactic acidosis (lactate >8 mmol/L) is a hallmark of cyanide poisoning. Arterial blood gas may show low arterial‑venous oxygen difference (increased venous oxygen content due to impaired extraction). Metabolic acidosis with increased anion gap is typical.
4.2.3 Methemoglobin and other markers
Methemoglobin levels are usually normal, but may be elevated if sodium nitrite has been given. Carboxyhemoglobin should also be measured in smoke‑inhalation cases to rule out CO poisoning. Serum thiocyanate can be elevated in chronic exposure.
4.3 Differential diagnosis
Differential diagnoses include carbon monoxide poisoning, hydrogen sulfide poisoning, methemoglobinemia, organophosphate poisoning, opioid overdose, stroke, and hypoglycemia. History and oxygen therapy responsiveness help differentiate.
5 Treatment
5.1 Prehospital and supportive care
5.1.1 Scene safety and decontamination
First responders must use appropriate personal protective equipment to avoid secondary exposure. Victims should be removed from the contaminated area immediately (e.g., from fire smoke). For skin or eye exposure, copious irrigation with water or saline is indicated. Contaminated clothing should be removed.
5.1.2 Airway management and oxygen therapy
Aggressive airway support is essential: high‑flow oxygen (100% by non‑rebreather mask or positive‑pressure ventilation). Endotracheal intubation may be necessary for comatose patients. Oxygen saturation should be monitored, though pulse oximetry may be falsely normal due to histotoxic hypoxia.
5.2 Antidote therapy
5.2.1 Hydroxocobalamin (Cyanokit)
Hydroxocobalamin is the preferred first‑line antidote for known or suspected cyanide poisoning. It binds cyanide directly to form cyanocobalamin, which is excreted renally. Adult dose: 5 g IV infused over 15 minutes; may repeat once if needed. Side effects include transient hypertension, red discoloration of skin and urine, and interference with colorimetric laboratory tests.
5.2.2 Sodium nitrite and sodium thiosulfate
The traditional “Lilly kit” contains amyl nitrite (inhalation) followed by sodium nitrite (IV) and sodium thiosulfate (IV). Sodium nitrite induces methemoglobinemia, which competes with cytochrome c oxidase for cyanide binding. Sodium thiosulfate serves as a sulfur donor for rhodanese, enhancing thiocyanate formation. Dosing: sodium nitrite 300 mg IV (adjusted for pediatric/pediatric methemoglobin levels), sodium thiosulfate 12.5 g IV. Risks include excessive methemoglobinemia and hypotension.
5.2.3 Comparative effectiveness and guidelines
Hydroxocobalamin has largely replaced nitrite/thiosulfate due to superior safety profile (no methemoglobin induction) and efficacy in smoke‑inhalation victims (who often also have CO poisoning and cannot tolerate methemoglobin). Major toxicology guidelines (e.g., from the American Heart Association and American College of Emergency Physicians) recommend hydroxocobalamin as first line. Nitrite/thiosulfate may still be used when hydroxocobalamin is unavailable.
5.3 Advanced interventions
5.3.1 Hyperbaric oxygen
Hyperbaric oxygen therapy has theoretical benefits (enhanced oxygen delivery, possible displacement of cyanide from cytochrome oxidase, increased metabolism of cyanide to thiocyanate). However, evidence from human studies is mixed, and it is not routinely recommended except for concurrent CO poisoning or refractory shock.
5.3.2 Extracorporeal elimination
Hemodialysis or hemoperfusion have limited efficacy in removing cyanide due to its rapid metabolism and small volume of distribution. They may be considered for severe toxicity with renal failure or for removal of thiocyanate in chronic cases.
6 Prognosis and complications
6.1 Factors influencing outcome
Outcome depends on dose, speed of treatment, age, and pre‑existing health. Prompt administration of antidotes and supportive care significantly improves survival. Patients who reach hospital alive (especially those with non‑agonal presentation) have good chances of survival. Death usually occurs within 4–8 minutes after massive inhalation or ingestion. Survivors of severe poisoning may suffer permanent neurological deficits.
6.2 Long‑term neurological sequelae
Survivors of acute cyanide poisoning can develop Parkinsonism, choreoathetosis, dystonia, and cognitive impairment. These are thought to result from damage to the basal ganglia, particularly the globus pallidus and putamen. Chronic low‑level exposure may lead to ataxia, optic atrophy, and hearing loss (as seen in tropical ataxic neuropathy). Recovery of motor function is variable.
7 Prevention and public health measures
7.1 Occupational safety
Industries handling cyanide (e.g., gold mining, electroplating, chemical manufacturing) must enforce strict engineering controls: closed‑system processes, ventilation, personal protective equipment (respirators, gloves), and routine air monitoring. Training on emergency procedures and availability of antidote kits (including hydroxocobalamin) at worksites are essential.
7.2 Fire safety education
Public education on fire prevention and the use of smoke detectors is critical. Building codes increasingly require the use of less‑toxic materials in furnishings and insulation. Firefighters should be trained to recognize cyanide poisoning and carry antidotes.
7.3 Food safety and natural toxins
In regions where cassava is a dietary staple, proper processing—soaking, grating, fermentation, and cooking—reduces cyanogenic glycosides to safe levels. Imported apricot kernels and similar products should be labeled with warnings against raw consumption. Regulatory limits for cyanide in certain foods (e.g., lima beans, almonds) exist in many countries.