Toxicology Brain Teaser – What's your poison?

Here are some visual aid type questions, on topics you may not have covered yet, but you absolutely need to know about these toxicology subjects for the exam. The exam questions WILL NOT be structured like this, these are designed as teaching questions, to prompt you to delve deeper into the toxiology textbooks.

 

Click on the thumbnails to enlarge the photos. Click on “Answer” to see the answer.

QUESTION 1

Outline the use of the product depicted in the following photographs:

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Fullers Earth is powdered clay that is used as an adsorbant in cases of paraquat poisoning. Paraquat is a highly lethal herbicide, which can be fatal after as little as a mouthful. Paraquat poisonong is the ONLY poisoning in which decontamination takes precedence over resuscitation or even transport to hospital.  (potential MCQ!) This means that patients should be treated at the scene or immediately upon arrival with decontamination measures – assuming their airway is not threatened. Fullers earth acts as an adsorbant to minimse the amount or paraquat reaching the circulation. Due to the same mechanism of action, activated charcoal (which is commonly available in all ED’s) can also be used (dose: 50g  in adults, 1g/kg in children).  If you are really stuck and in a remote area, getting the patient to eat some food, or even dirt/mud, (perhaps best administered via NGT) may have some benefit. Haemodialysis/haemoperfusion is also used to enhance elimination if it can be started within 2 hours of ingestion.

Extra points for noticing the dust on the lid, combined with how old the bottle looks, is indicative of how infrequently this decontamination product is used.

Why is it called “Fullers Earth”?

In medieval times, “fulling” was the process by which wool was kneaded with this material and water to adsorb the oils (eg lanolin) and other greasy impurities in the wool as part of the finishing process.  Fullers were the people who perfromed this task (usually using their feet), hence the name “Fullers Earth”!

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QUESTION 2

Outline the use of the following product: (Clue – look at the name on the box!)

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Hydroxocobalamin is used in the treatment of cyanide poisoning. It is also known as Vitamin B12.

Indications:

  • Known cyanide poisoning with serious clinical effects: altered mental state, seizures, hypotension, severe metabolic acidosis.
  • Suspected cyanide poisoning with serious clinical effects (as it has minimal adverse effects if given in a case that turns out not to be cyanide poisoning)

Mechanism:

Hydroxocobalamin has a high affinity for cyanide, binding to it & forming cyanocobalamin, which is relatively non-toxic, and is excreted in the urine

It is administered with Sodium Thiosulfate in cases of severe cyanide poisoning. Sodium Thiosulfate (which enhances the endogenous elimination of cyanide) is often sufficient as a sole treatment in conjunction with good supportive care in mild-moderate cases.

Dose: This is a key Fellwoship exam point, and signifies “Consultant level knowledge”.

Hydroxocobalamin comes in 2 preparations (click on links to see more details on each):

1mg (in 1ml) ampoules – used for treating pernicious anaemia

2.5g bottles – used for treating cyanide poisoning. The recommeded dose is 5 grams. Note the writing on the packet, it is especially imported from France, and costs close to $2000. It is NOT available on the PBS.

5g = 5000mg = 5000 ampoules (5 LITRES) of the 1mg/1ml preparation! Therefore failure to stock or administer the correct dose can lead to indaequate treatment.

If you really have a serious case of cyanide poisoning and you don’t have the 2.5g/5g preparation, you will need to contact your pharmacist and get the whole hospital supply of 1mg ampoules, as well as source more (or hopefully the 2.5/5g pack) from nearby hospitals/pharmacies.

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QUESTION 3

Outline the toxicological indications for this product:

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Pralidoxime is a cholinesterase reactivator, that is used in the treatment of Organophosphate eg Malathion, Parathion, (once sufficient atropine has been given), Carbamate (in severe cases) and Nerve Agent poisoning. It is only effective if given before “ageing” (irreversible binding) occurs.

Pitfalls with treatment include starting too late, not treating for long enough, or inadequate dosing.

Note: The recommended dose is 1-2g Pralidoxime in 100ml N.Saline over 15 minutes, followed by an infusion of 250-500mg/hour for up to 24 hours. The box above contains 5 vials, (2.5g) so it will be enough for the initial dose, but you’ll need to source more for the ongoing infusion. Prolonged paralysis (several days) can be caused by inadequate pralidoxime dosing.

Note: Nerve gas ages within seconds, so victims usually die at the scene.  If they make it to hospital you still treat with atropine/pralidoxime.  Fertilisers are designed to age slower, so are therefore “safer”.

Organophosphate/anticholinesterase toxicity is a complex topic which you need detailed knowledge about for this exam. This summary about the antidote is brief, but hopefully has piqued your interest enough to go look it up.

Extra points if you noticed the above pack is out of date!

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