Muscarinic antagonists

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Muscarinic antagonists

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USMLE® Step 2 style questions USMLE

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A 65-year-old man comes to the emergency department during the winter due to shortness of breath and a productive cough. The symptoms have progressively worsened over the past two years, and the patient has had to limit his daily activities to avoid becoming short of breath. Each morning, he coughs up large quantities of viscous sputum. Past medical history is significant for hypertension and type II diabetes mellitus. Current medications include lisinopril and metformin. Temperature is 36.4°C (97.6°F), pulse is 120/min, respirations are 24/min, and blood pressure is 147/98 mmHg. Oxygen saturation is 93% on room air. Physical examination reveals a markedly increased anteroposterior chest diameter. Cardiopulmonary examination reveals tachycardia and poor air movement bilaterally with scattered expiratory wheezes. A chest radiograph is obtained and shown below. Which of the following medications is most appropriate for managing this patient’s symptoms?  


Image reproduced from Radiopedia

External References

First Aid

2024

2023

2022

2021

Adverse effects/events

atropine p. 240

Atropine p. 240

antimuscarinic reaction p. 250

for β -blocker overdose p. 327

cholinesterase inhibitor poisoning p. 239

toxicity treatment p. 247

Bradycardia

atropine for p. 240

Cycloplegia

atropine p. 240

Geriatric patients

atropine in p. 240

Glaucoma p. 551

atropine p. 240

Hyperthermia

atropine as cause p. 240

Urinary retention

atropine p. 240

Transcript

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Muscarinic antagonists, or antimuscarinic medications, are a class of medications that prevent muscarinic receptors of the parasympathetic nervous system from getting stimulated by acetylcholine.

Okay, first things first, the nervous system is divided into the central nervous system, so the brain and spinal cord; and the peripheral nervous system.

The peripheral nervous system can be divided into the somatic nervous system, which controls voluntary movement of our skeletal muscles; and the autonomic nervous system, which is further divided into the sympathetic and parasympathetic nervous systems, and controls the involuntary movement of the smooth muscles, and the glands of our organs.

Now, the autonomic nervous system is made up of a relay that includes two neurons.

We’ll focus on just the parasympathetic nervous system.

Signals for the parasympathetic nervous system start in the hypothalamus.

These hypothalamic neurons synapse with nuclei in the brainstem or spinal cord, which send out signals to preganglionic neurons that travel to the rest of the body.

Their targets are the parasympathetic ganglion, which consist of many postganglionic neuron cell bodies and are located nearby or directly in the target organs.

The postganglionic neurons extend the rest of the way to the target cell, where they release the neurotransmitter acetylcholine, which is why they are also called cholinergic neurons.

Acetylcholine binds to a type of receptor, known as muscarinic receptors, on the cells of target organs, which allow the parasympathetic nervous system to trigger a ‘rest and digest’ response, meaning that it keeps body-energy use as low as possible to stimulate activities like digestion.

It acts in the heart, slowing the heart rate and reducing the cardiac output.

In the gastrointestinal tract, it increases its motility to stimulate digestion and defecation.

In the bladder, it causes constriction of the bladder muscle, called the detrusor muscle, which stimulates urination.

In salivary, sweat, and lacrimal glands, it increases their secretions.

In the liver, it triggers glucose storage to reduce blood glucose levels.

In the lungs, it causes bronchoconstriction, since in a relaxed state our cells do not consume as much oxygen.

In the eyes, it triggers miosis, or constriction of the pupils, to improve close vision and stimulates the contraction of the ciliary muscles, increasing the outflow of aqueous humor, which is the fluid in the anterior chamber of the eye, and this decreases the intraocular pressure.

Finally, its effects on the brain are extremely complicated, but generally, they cause overall stimulation, and they participate in many wanted and unwanted functions, such as movement control and vomiting, respectively.

Alright, so medications that block the effects of acetylcholine on muscarinic receptors are called muscarinic antagonists or antimuscarinic medications.

Now, the most famous muscarinic antagonist is atropine.

Atropine blocks the “rest and digest” effect from the parasympathetic system, so clinically, it can be used to treat bradycardia, or slowed heart rate.

It decreases bladder smooth muscle contraction so it’s useful for preventing nocturnal enuresis, or bedwetting, in children.

Ophthalmologists can also use atropine to dilate the pupils.

Finally, it’s an antidote for poisoning by anticholinesterases like organophosphates found in pesticides. These agents prolong the effect of acetylcholine by inhibiting their break down, so atropine can help by simply blocking the muscarinic receptors.

Now, for side effects, atropine can produce: tachycardia, or increased heart rate; constipation and urinary retention; dry mouth, skin and eyes; and blurry vision.

The more severe side effects include hyperthermia, dizziness, confusion, and delirium.

It’s also contraindicated in individuals suffering from narrow angle glaucoma since it can worsen the obstruction of aqueous humor drainage.

Summary

Muscarinic antagonists, also known as antimuscarinic medications, are a class of drugs that block the activation of muscarinic receptors of the parasympathetic nervous system. Examples of muscarinic antagonists include atropine, scopolamine, and ipratropium. These drugs are used to treat a variety of conditions, such as asthma, glaucoma, and urinary incontinence. They can also be used to treat the symptoms of poisoning from organophosphate insecticides.

Sources

  1. "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)
  4. "Muscarinic receptor subtypes in airways" Life Sci (1993)
  5. "Pharmacologic therapy of obstructive airway disease" Clin Chest Med (1990)
  6. "[Atropine. Principles and rules of utilization]" Rev Prat (2001)
  7. "Antimuscarinic drugs" Prof Nurse (2004)