Antimuscarinics · What Are They, How Do They Work, What Are They Used For, Side Effects, and More

Antimuscarinic medications are a group of anticholinergic agents, specifically known for blocking the activity of muscarinic receptors. These receptors play an important role in mediating the functions of the parasympathetic nervous system, which controls many involuntary functions to conserve energy, including the contraction of smooth muscle, dilation of blood vessels, increased bodily secretions, gastrointestinal activity, and heart rate. 

A type of molecule that can cross the blood brain barrier is lipophilic molecules. Some antimuscarinic medications are lipophilic and can therefore cross the blood brain barrier, such as atropine and benztropine. When these medications pass through to the brain, they are able to target the central nervous system and influence certain neurological functions. 

Antimuscarinic medications can be prescribed for a variety of different reasons. One of the more common cases is in individuals with overactive bladder syndrome, a lower urinary tract condition that causes strong urges to urinate and sometimes urine leakage. Antimuscarinics help by decreasing the motility of smooth muscle cells in the urinary tract and increasing the tone of the sphincters controlling urination. 

Antimuscarinics also use a similar mechanism to treat irritable bowel syndrome, an intestinal condition that causes abdominal discomfort, gas, diarrhea, and constipation. By decreasing the motility of smooth muscle cells and the activity of the gastrointestinal tract, antimuscarinics can relieve symptoms associated with irritable bowel syndrome. 

Additionally, individuals with bradycardia, or a slower than usual heart rate, may be prescribed an antimuscarinic in order to increase their heart rate. 

Antimuscarinics are also prescribed to treat motion sickness, as they can minimize vestibular disturbances. 

Individuals with Parkinson disease may be prescribed antimuscarinics to help improve motor function, since it is believed that blocking acetylcholine receptors may increase the activity of neurons responsible for movement. 

Individuals with peptic ulcers may also benefit from antimuscarinic medications, which can decrease the amount of gastric acid secretion in the stomach known for causing or worsening peptic ulcers.  

Finally, antimuscarinics can be helpful for individuals with certain respiratory conditions, such as asthma and chronic obstructive pulmonary disease. These medications serve as muscle relaxants in the lungs, which help individuals breathe more easily. 

Antimuscarinic medications have several potential side effects that often correspond to the various receptors targeted for treatment. Adverse effects resulting from the impaired secretion by exocrine glands are often a dry mouth and sore throat. Antimuscarinics can also cause tachycardia, or a higher than usual heart rate, when interfering with receptors that typically slow the heart rate. They may also result in urinary retention (the inability to empty the bladder) and obstipation (the inability to pass stools or gas). This is a result of decreased smooth muscle motility and tone. Additional side effects include blurred vision and light sensitivity. Some antimuscarinics may also cause mood changes, hallucinations, confusion, or disorientation.

Because of these side effects, antimuscarinic medications are contraindicated in individuals with certain medical conditions, such as narrow angle glaucoma, myasthenia gravis, urinary obstruction, or gastric ulcers. In addition, antimuscarinic medications may interfere with other medications that an individual might already be taking, such as antihistamines, antiparkinsonism agents, and tricyclic antidepressants (TCAs). For that reason, individuals should consult with their healthcare provider before taking antimuscarinics medications.

Overdosing on antimuscarinics may lead to anticholinergic syndrome, which can present with dry mouth, flushed skin, tachycardia, confusion, and agitation. It may also lead to anticholinergic delirium, hallucinations, and psychomotor symptoms. With lower doses, acute anticholinergic syndrome can resolve on its own after the anticholinergic medication has been excreted. On the other hand, higher doses can be life-threatening cases, so acetylcholinesterase inhibitors can be prescribed as an antidote.