This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today’s case focuses on a 35-year-old-woman who has difficulty breathing. Physical examination reveals a lethargic and diaphoretic adult female with bilateral pinpoint pupils and watering from the eyes. Can you figure it out?
A 35-year-old woman is brought to the emergency department due to difficulty breathing. The patient is a farmer and developed symptoms while working in the fields. Past medical history is significant for major depressive disorder and two prior suicide attempts. Temperature is 36.7°C (98.1°F), pulse is 53/min, respirations are 32/min, and blood pressure is 90/65 mmHg. Oxygen saturation is 87% on ambient air. Physical examination reveals a lethargic and diaphoretic adult female with bilateral pinpoint pupils and watering from the eyes. The patient’s body and clothes are soiled in vomit and feces. Lung auscultation reveals diffuse wheezing with rhonchi. Which of the following agents most likely precipitated this patient’s symptoms?
A. Atropine
B. Organophosphate
C. Amitriptyline
D. Caustic substances
E. Amphetamine
The correct answer to today’s USMLE® Step 1 Question is…
B. Organophosphate
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 1 Question are…
A. Atropine
Incorrect: Atropine is a muscarinic antagonist. Atropine toxicity can present with mydriasis (dilated pupils), increased body temperature, constipation, and dry flushed skin. This patient’s presentation is inconsistent with the symptoms of atropine toxicity and instead more suggestive of organophosphate toxicity.
C. Amitriptyline
Incorrect: Amitriptyline is a tricyclic antidepressant with anticholinergic properties. Tricyclic antidepressant toxicity can present with mydriasis, increased body temperature, dry mouth, tachycardia, and flushed skin. This patient’s presentation is inconsistent with the symptoms of amitriptyline toxicity.
D. Caustic substances
Incorrect: Ingestion of caustic substances (e.g., bleach, drain cleaners) can cause stridor, hoarseness, dysphagia, and oropharyngeal ulceration. In contrast to this patient’s presentation, it would not typically cause pinpoint pupils, bradycardia, or diaphoresis.
E. Amphetamine
Incorrect: Amphetamine is a sympathomimetic agent. Although amphetamine overdose can present with tachypnea and increased sweating, the pupils are typically dilated, and blood pressure is elevated.
Main Explanation
This patient presents with dyspnea, bradycardia, miosis, and excessive secretions (e.g., diaphoresis, lacrimation). These symptoms are highly suggestive of organophosphate poisoning.
Organophosphates can be found in insecticides, herbicides, and nerve agents. Agricultural workers are at increased risk for organophosphate poisoning. The substance binds to acetylcholinesterase (AChE), which is normally responsible for the hydrolysis of acetylcholine and renders the enzyme non-functional. With AChE blocked, acetylcholine subsequently accumulates and leads to hyperstimulation of nicotinic and muscarinic receptors. Nicotinic stimulation leads to neuromuscular blockade, and muscarinic activation results in increased visceral and glandular secretions. Organophosphates may also contribute to central respiratory depression, lethargy, and ultimately coma. A brief overview of the clinical features of organophosphate toxicity is found in the table below.
P.S. If you or someone you know needs this, here’s the number for the National Suicide Prevention Lifeline 1-800-273-8255 or text HOME to the Crisis Text Line at 741741.
Major Takeaway
Organophosphate poisoning can lead to symptoms of nicotinic hyperstimulation (neuromuscular blockade), muscarinic hyperstimulation (increased visceral and glandular secretions), as well as central nervous system symptoms. Organophosphates are found in insecticides and herbicides. As a result, agricultural workers are at increased risk of organophosphate poisoning.
References
Eddleston, M., Buckley, N.A., Eyer, P., Dawson, A.H. (2008) Management of acute organophosphorus pesticide poisoning. The Lancet. 371(9612), 597-607. Doi: 10.1016/S0140-6736(07)61202-1.
Eddleston, M., Phillips, M.R. (2004) Self-poisoning with pesticides. BMJ. 328(7430), 42-44. Doi: 10.1136/bmj.328.7430.42.
Khurana, D., Prabhakar, S. (2000) Organophosphorus intoxication. Archives of Neurology. 57(4), 600-602. Doi: 10.1001/archneur.57.4.600.
King, A.M., Aaron, C.K. (2015) Organophosphate and carbamate poisoning. Emergency Medicine Clinics of North America. 33(1), 133-151. Doi: 10.1016/j.emc.2014.09.010.
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