POISON IDENTIFICATION

Wherever possible the constituents of the substance ingested and its dosage per kilo body weight should be identified as accurately as possible.

In younger children the substance taken is often easily identifiable but the dosage can be difficult to ascertain. Some idea of the maximum amount of substance that could have been ingested can be gathered from comparing the number of tablets, or volume of liquid remaining, with details on packaging.

Care must be taken not to overlook the involvement of other children in a poisoning incident.² When children share a poisonous substance, it must be assumed that each child has taken the maximum amount.

In older children, a clear history of ingestion may not be forthcoming and illicit drugs are more likely to be involved. Specific enquiry should be made into the medicines prescribed to each member of the household, both currently and in the past.

RECOGNISABLE POISON SYNDROMES

In cases where poisoning is suspected, but cannot be confirmed by clinical history, a detailed physical examination, including a full neurological assessment, is an essential part of substance identification. Table 2 shows collections of signs and their possible causes.

A number of toxins acting on the autonomic nervous system can produce a mixed clinical picture because of effects on both muscarinic and nicotinic receptors.

In most cases of poisoning, clinical suspicion is raised because symptoms do not fit a common pattern. Certain poisons can produce symptoms that mimic common diseases. A high index of suspicion is necessary in such cases (see table 3).

PREVENTING ABSORPTION

There is no place for the use of emetics.⁶ The routine use of gastric lavage⁷ or activated charcoal⁸ is inappropriate. While the latter techniques may have a role in the early management of poisoning with a small number of specific substances, their effectiveness in these circumstances remains unproven.^7,8 Gastric lavage is contraindicated if a corrosive substance or volatile hydrocarbon has been ingested. Adequate airway protection is essential if any of these procedures is to be performed in the presence of an altered level of consciousness. Table 4 lists poisons for which activated charcoal has been proven to be ineffective.

ENHANCING EXCRETION

Active elimination techniques have a limited role in the management of poisoning. Their use should be restricted to situations where prolonged exposure to high concentrations of toxin is predictably deleterious. Examples of such situations would include haemodynamic instability despite supportive measures, intractable seizures, or organ failure.

The use of repeated doses of activated charcoal to remove toxins undergoing enterohepatic circulation is one of the simplest active elimination techniques.⁹ Table 5 lists substances for which this technique may prove useful. The technique is not without its complications; these include bowel obstruction and perforation. Careful monitoring of bowel sounds is essential.

Forced diuresis has previously been recommended as a method of enhancing elimination of salicylates and barbiturates. The risk of fluid overload is high and this technique should be avoided.

Urinary alkalinisation can be used to enhance the excretion of weakly acidic drugs. The unionised form of the drug is filtered and reabsorbed. Urinary alkalinisation increases the proportion of ionised drug in the tubule, preventing its reabsorption. Examples of substances which may undergo significantly enhanced excretion include salicylate, isoniazid,¹⁰ phenobarbitone, and dichlorophenoxyacetic acid.¹¹

Conversely, decreasing urinary pH can be used to enhance the excretion of weakly alkaline drugs. Urinary acidification, using ammonium chloride, has previously been used to enhance excretion of amphetamine, strychnine, quinine, quinidine, and phencyclidine.¹⁰ The dangers of acidosis and hyperammonaemia outweigh the benefits of this technique.

Whole bowel irrigation¹² can be used to physically eliminate highly toxic substances that are not absorbed by activated charcoal and have a long gastrointestinal transit time. Treatment is based on the enteral administration of large quantities (30 ml/kg/h) of osmotically balanced polyethylene glycol electrolyte solution to induce a liquid stool. Treatment is continued until rectal effluent clears. Substances for which this technique may prove useful include iron and sustained release or enteric coated preparations.

Dialysis, haemoperfusion, and haemofiltration have all been used to actively enhance toxin excretion. While many case reports exist in the literature, the efficiency of such methods is very difficult to assess clinically. Estimates of efficacy based on blood levels before and after treatment are likely to be misleading as they cannot take into account enterohepatic circulation, hepatic metabolism, or urinary excretion.¹³ It is generally accepted that extracorporeal elimination is worthwhile if it increases total body clearance by 30% or more.¹⁴

For dialysis to be effective, a toxin must be of low molecular weight (<500 relative molecular mass (RMM)) and highly water soluble. It must have a small volume of distribution (<2 l/kg) and bind poorly to protein. Examples include salicylate, methanol, ethylene glycol,¹⁵ vancomycin,¹⁶ lithium,¹⁷ and isopropanol¹⁸ poisoning. Dialysis is of particular value where concomitant electrolyte or acid-base disturbance exists.

Haemoperfusion is better suited to toxins with low water solubility. Such substances must have a high affinity for the adsorbent, a fast rate of equilibrium from peripheral tissues to the blood, and a low affinity for plasma proteins.¹⁴ Examples include carbamazepine, barbiturates, and theophylline.¹¹

Haemofiltration can remove compounds with a high molecular weight (>500–40 000 RMM). It is of particular use in aminoglycoside¹⁴ and theophylline¹⁹ overdose. Haemofiltration may also be of benefit in iron²⁰ and lithium overdose.²¹

Substances not amenable to significant extracorporeal removal include benzodiazepines, tricyclic compounds,²² phenothiazines, chlordiazepoxide, and dextropropoxyphene.

LABORATORY INVESTIGATIONS

A careful history may obviate the need for blood tests. Particular attention should be paid to safe ingestion levels.

Routine measurement of plasma paracetamol and salicylate may be considered in older children presenting with deliberate ingestion. The value of such practice has been questioned in adults.^23,24 However, Ashbourne et al found that 1 in 500 adult overdose patients, not suspected of having taken paracetamol, had levels above the treatment threshold.²⁵ In adolescents, the cost of missing serious paracetamol ingestion probably outweighs the price of the test.

Techniques exist to identify a wide variety of toxins, particularly drugs of abuse, from samples of blood and urine. These tests are expensive and rarely provide immediate results. They may have important medicolegal and social consequences, but rarely alter clinical management.²⁶ Samples are best obtained acutely and stored for future use as deemed necessary.

ADMISSION CRITERIA

Decisions about the need for hospitalisation of children presenting with possible poisoning are sometimes difficult. Most children will be asymptomatic and a short period of observation, in an emergency department or admission ward, is often all that is required. The nature, and quantity, of the substance consumed must clearly be taken into account. Reports are available on the speed with which symptoms develop for a variety of poisons.²⁷ Suggested observation times, for asymptomatic children, are based on these figures.

The circumstances surrounding a poisoning episode may have an important impact. Deliberate ingestion may signal significant psychosocial problems. An inconsistent history of poisoning in a younger child may raise the possibility of abuse or neglect.

Other factors that need consideration include family circumstances, parental confidence, and the availability of emergency care should the child deteriorate unexpectedly.

SPECIFIC ANTIDOTES

A wide range of specific antidotes exists (see table 6).

SUPPORTIVE MANAGEMENT

Initial management should focus on assessment of airway, breathing, and circulation. Depression of the central nervous system is a common symptom. This may lead to airway compromise, respiratory failure, or aspiration.

Diuresis, vomiting, and diarrhoea may all contribute to profound dehydration and shock. Aggressive fluid resuscitation, guided by invasive monitoring, may be necessary. Hypotension, unresponsive to adequate fluid replacement, requires treatment with inotropes. Dopamine and dobutamine are the agents most commonly used. The inotropic effect of glucagon has been used in the management of β blocker and tricyclic antidepressant induced hypotension.

Metabolic acidosis is frequently encountered. In many cases, this is mild and does not require specific therapy. In some cases, the correction of a mild metabolic acidosis may decrease toxin clearance (see Urinary Alkalinisation).

Hepatic and renal function should be monitored closely. Urine samples should be checked regularly for blood, haemoglobin, protein, glucose, and myoglobin.

Hypoglycaemia should be identified rapidly and corrected using intravenous boluses (5 ml/kg) of 10% dextrose.

Convulsions can usually be treated with benzodiazepines.

Children developing symptoms after ingestion, other than perhaps mild nausea, vomiting, or diarrhoea, require hospital admission. For most poisons, treatment is supportive.

Children with nausea and vomiting may require intravenous fluids. Treatment with antiemetic drugs is best avoided.

MANAGEMENT OF ARRHYTHMIAS

A variety of common household poisons may produce arrhythmias in overdose. Arrhythmias are rarely encountered in general paediatric practise and can prove daunting.

The presence of an arrhythmia in a poisoned child does not necessarily indicate direct cardiac drug toxicity.³² The first step in treatment is to ensure adequate resuscitation and supportive therapy. Underlining hypoxia or hypercarbia must be corrected. Abnormalities of electrolytes or acid-base balance should be addressed.

While many arrhythmias have serious consequences for children and adults with significant cardiac pathology, they may be relatively benign in healthy children.³³ Only if supportive measures prove inadequate should specific therapy, aimed at correcting an arrhythmia, be considered. Table 7 lists specific antiarrhythmic drugs, and also drugs to avoid.

MANAGEMENT OF CORROSIVE INJURY

The investigation and treatment of caustic ingestion in children is controversial. Alkalis tend to cause more damage than acids, while liquids cause more scars than powders. Products that can become trapped in the oesophagus cause the most damage, for example, batteries, Clinitest, or dishwasher tablets.

The outcome for most children following corrosive ingestion is good. Superficial oesophageal burns occur in 20% of cases, deeper burns are seen in 5%, and stricture formation in 1–3%.⁴⁴ Attempts at neutralisation of corrosives, or gastric decontamination, are best avoided. The utility of early upper gastrointestinal endoscopy in symptomatic children, followed by steroid treatment if oesophageal burns are identified, has been called into question. Early signs and symptoms do not predict the presence of oesophageal burns,⁴⁵ and Anderson et al found no evidence that steroid treatment improved outcome.⁴⁶

Children should be managed symptomatically. Particular care is needed over fluid balance and respiratory function. Drooling and dysphagia persisting beyond 12–24 hours are good predictors of oesophageal scar formation⁴⁴ and should prompt upper gastrointestinal endoscopy.

FOLLOW UP OF DELIBERATE SELF HARM

Deliberate poison ingestion is a common presentation in older children. While the precipitating factors are often minor, arguments with friends or parents being the most common, these episodes should not be regarded as trivial. Children who display self harming behaviour have a significantly increased risk of underlying psychiatric illness, particularly depression.⁴⁷ In many children self harm is a manifestation of significant difficulties in other parts of their lives. Social disadvantage and disturbed family relationships are common.⁴⁸

Adolescents presenting with self harm should undergo psychiatric assessment. This is ideally undertaken during an in-patient stay, as outpatient attendance rates are poor.⁴⁹ This assessment can also offer adolescents, and their parents, a useful forum in which to discuss ongoing difficulties.

SEEKING FURTHER ADVICE

Specific, expert advice on all aspects of poisoning is available to medical professionals in the United Kingdom via the National Poisons Information Service (NPIS). The regional centres that make up this service have recently introduced a single national enquiry number: 0870 600 6266.

A wide range of easily accessible and highly practical advice is available through the NPIS website.⁵⁰ This free service is restricted to medical professionals. On line registration is available at http://www.spib.axl.co.uk/toxbase/.

While contact with the NPIS is an important way of keeping up to date with developments in this field, information gleaned should be disseminated to reduce the need for multiple enquires from individual hospitals.