Approach
In most cases, diagnosis is based on a history of exposure with characteristic signs of cholinergic excess. This can be difficult when the patient is inadvertently exposed, unconscious, or confused. A therapeutic trial of atropine should be ordered in all suspected cases or if diagnosis is in doubt, as this is a quick and safe way to confirm diagnosis. Cholinesterase activity may also be used to confirm diagnosis; however, delays in getting results back make this test less useful.
History
The onset of symptoms and signs may be rapid or delayed by up to 1 day, depending on whether the agent requires metabolic activation for its toxicity. A history of working with pesticides and/or previous self-harm, depression, drug or alcohol addiction, or mental illness may support the diagnosis. Co-ingestion of other substances, particularly alcohol, is common in self-poisonings. If the patient is conscious, they may report ingestion of pesticide; however, not all patients will admit to this.
Minimal exposure (e.g., dermal) may result in an influenza-like syndrome (e.g., fatigue, runny nose, headache, dizziness, anorexia, sweating, diarrhoea, and muscle weakness). Nausea and vomiting are common. The patient may also report visual disturbances such as blurred vision or incontinence.
Physical examination
The most specific features are fasciculations (e.g., of the peri-orbital, chest, or leg muscles) and excessive secretions (e.g., lacrimation, salivation, or bronchorrhoea). A distinctive odour from the solvent may be noticed. The pupils are typically pinpoint and will not respond to naloxone. Chest crackles and rhonchi may be present from excess mucous secretions, indicating bronchospasm or pulmonary oedema. Faecal or urinary incontinence may be noted. Mild to moderate hypothermia is often present on admission if atropine treatment has not been given. Seizures and respiratory failure are more common with severe poisoning (e.g., due to deliberate ingestion or chemical warfare). The mnemonic DUMBELS (diarrhoea, urination, miosis, bronchorrhoea, emesis, lacrimation, salivation) is often used for the cholinergic features.
Deep tendon reflexes are frequently increased early on and decreased or absent later. Delayed-onset central nervous system and peripheral (predominantly motor) neuropathy are uncommon, but may be severe and can lead to permanent disability.
Heart rate and blood pressure are not helpful signs, as they may be increased or decreased. Oxygen saturation is usually low. The patient may be semi-conscious or in a coma.
Response to atropine
All patients with suspected poisoning should receive a therapeutic trial of atropine. After atropine administration, patients who have not been poisoned by organophosphates will tend to have dry skin and mucous membranes, increased heart rate, moderately dilated pupils, and decreased bowel sounds. If few or none of these features are seen, the likelihood of organophosphate poisoning is greatly increased.
Cholinesterase activity
This test is used when diagnosis needs confirmation, but is considered less useful than an atropine trial because treatment needs to be initiated before the test results are available. Plasma cholinesterase is usually depressed below normal in significant organophosphate poisoning; however, while this is a sensitive marker of exposure, it is not a good marker of severity.[10]
Red blood cell acetylcholinesterase (RBC-AChE) is the same enzyme as neuronal AChE, and levels correlate much better with the severity of poisoning; they can also be used to monitor response to oxime agents (the antidote for poisoning - usually pralidoxime). There are a few organophosphates where the correlation of cholinesterase activity with clinical severity is very poor. For example, profenofos may cause undetectable activity in asymptomatic patients.[11] There are numerous technical issues in terms of the method of collection and storage before analysis, and in practice this test is often difficult to interpret.[12]
The RBC-AChE test should only be used when it can be done rapidly on site. Two RBC-AChE point of care devices that provide reliable results within minutes are available.[13]
Other investigations
A chest x-ray may often be useful, as aspiration pneumonia is a very common complication, and should be ordered if chest signs are focal or not responsive to atropine. QT prolongation and arrhythmias may occasionally be seen on ECG; therefore, an ECG should be ordered in symptomatic patients and repeated if an abnormal heart rate or hypotension are persistent. Blood gases should be ordered to monitor for respiratory failure and metabolic acidosis.
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