Several medications indicated for antiemetic use produce antimuscarinic effects, most often as part of their adverse effect profiles. This article will focus on scopolamine, a medication that acts as an antiemetic primarily through its antimuscarinic mechanism of action.
Two common indications for the use of scopolamine are for motion sickness and postoperative nausea and vomiting (PONV). Less common indications are also listed below.
Acetylcholine Receptors The nervous system utilizes several neurotransmitters for communication and function. The primary neurotransmitter of the parasympathetic nervous system is acetylcholine (ACh). ACh is synthesized in the cytoplasm of presynaptic neurons by the enzyme choline acetyltransferase (ChAT). It is subsequently relocated to synaptic vesicles by vesicular acetylcholine transporter (VAChT), where it is stored. When the presynaptic neuron becomes stimulated via depolarization, ACh gets released through exocytosis into the synaptic cleft, where it acts on postsynaptic receptors by binding to them. These receptors include muscarinic and nicotinic types, each with their respective subtypes. There are five subtypes of muscarinic receptors, M1 to M5, all of which are G protein-coupled receptors.
ScopolamineScopolamine (also known as hyoscine) is an alkaloid compound. It competitively the action of ACh at muscarinic receptors both centrally and peripherally. It is strongly selective to muscarinic receptors but is non-selective between the muscarinic receptor subtypes (M1 to M5). Because ACh is the principal neurotransmitter of the parasympathetic nervous system, scopolamine acts as a parasympatholytic agent.
Nausea and VomitingMultiple proposed mechanisms of nausea and vomiting involve various systems (e.g., vestibular, gastrointestinal, etc.). Furthermore, the specific pathways that these systems utilize to stimulate nausea and vomiting are ambiguous and still debated. The precise locations and functions of nervous system structures involved in nausea and vomiting are also not fully understood.
For these reasons, the exact mechanisms by which scopolamine prevents nausea and vomiting are unclear. The proposed pathways generally involve scopolamine inhibiting cholinergic communications between higher CNS centers, the reticular formation in the brainstem, the vestibular nuclei, the cerebellum, the glossopharyngeal nerve, and the vagus nerve. Additional structures may be involved, and scopolamine’s exact role is unclear. Furthermore, all five muscarinic receptor subtypes M1 to M5 show expression in the brain, and the roles of each subtype relative to nausea and vomiting remain unclear.
A discussion of the mechanisms involved in scopolamine’s adverse effects is in “Adverse Effects.”
Note: Although the following methods of administration appear in the published literature, every patient should receive a plan specific to them based on their medical team’s assessment and expertise. Prior to the administration of scopolamine, a thorough history, and physical exam are necessary to rule out contraindications and to anticipate adverse effects accurately. See “Contraindications” and “Adverse Effects” for more detail.
Transdermal vs. Oral
The preferred method for the administration of scopolamine is via a transdermal therapeutic system (TTS-patch) due to increased bioavailability and decreased adverse effects. There is a 1.5 mg patch commercially available and is generally the specific patch used. As previously mentioned, oral scopolamine has been shown to be effective when combined with the transdermal patch. However, this mode of administration is uncommon.
Placement Location and Timing
The 1.5 mg patch should be placed behind the patient’s ear on an area of skin without hair. Placement should occur several hours before the patient’s surgery. Reports range from 4 hours before the anticipated end of the procedure to the night before surgery. If used for the prophylaxis of motion sickness, placement should occur 6 to 8 hours before the nausea-inducing event. Due to the timing requirement, patients presenting for surgery should be evaluated for PONV risk as early as possible to properly formulate an anesthetic plan with enough time to permit the ideal usage of scopolamine if indicated.
The patch is designed to release an initial priming dose to achieve steady-state concentrations in an acceptable amount of time. It then continues to release medication slowly, a topic discussed in greater detail in “Mechanism of Action.” Due to its mechanism of release, if the continual antiemetic effect is required, the patch can be replaced every 72 hours. If scopolamine is used preoperatively for the prophylaxis of PONV, the patch can be removed the day following surgery.
Importance of Hand Hygiene
A crucial element of the administration of scopolamine is the counseling of the patient and/or their family. In addition to remaining vigilant about adverse effects, anyone who will be removing the patch needs to understand the importance of hand hygiene. After removal of the scopolamine patch, the skin location where the patch was, and the operator’s hands, require thorough washing with soap and water. Patients should also be instructed to avoid touching their faces, especially their eyes, after touching a scopolamine patch. A significant number of adverse effects related to the scopolamine patch are due to contamination by the patient or by an individual changing or touching the patch.
Scopolamine’s adverse effect profile is due to its antagonism of muscarinic receptors. The more commonly-reported adverse effects are listed below, with xerostomia and CNS symptoms being significantly more common than ophthalmic symptoms. However, scopolamine non-selectively affects all five muscarinic receptor subtypes (M1 to M5), and anticholinergic symptoms, or anticholinergic syndrome, are theoretically possible. Additionally, there are over 600 medicinal products with anticholinergic activity. The antimuscarinic effects of these products have the potential to exacerbate the adverse effects of scopolamine.
Discussion on Ophthalmic Symptoms and GlaucomaMydriasis is a potential complication of scopolamine. Although reports exist of bilateral cases, the literature suggests that the majority of cases of mydriasis are unilateral and ipsilateral to the side of their scopolamine patch, caused by the patient self-contaminating by touching their patch and then touching their eyes.
There are case reports of acute angle-closure glaucoma after the administration of scopolamine. This complication is rare. Patients affected by this complication likely have underlying pathology or abnormal anatomy that predisposes them to acute angle closure in the event of mydriasis through at least two mechanisms. Aqueous humor becomes blocked from flowing from the posterior to the anterior chamber of the eye, building up intraocular pressure. Patients can present with the following signs and symptoms:
Glaucoma is an ophthalmic emergency that can result in blindness if untreated promptly, and an ophthalmology consultation is important.A distinction should be made with patients with open-angle glaucoma as these patients can safely use transdermal scopolamine patches.
Elderly PatientsElderly patients are at an increased risk of and are more vulnerable to adverse anticholinergic effects due to differences in metabolism and excretion of medications. In the elderly, antimuscarinic medications may correlate with increased morbidity, for several reasons, including due to increased risk of falls. Scopolamine and antimuscarinic medications should be used cautiously in elderly patients.
The following are contraindications to scopolamine:
Although one study mentions pregnancy as a contraindication for scopolamine in one study, another study indicates that although the scopolamine crosses the placenta, it’s considered nonteratogenic.
The reversal of scopolamine toxicity is not a widely reported topic. However, based on the mechanism of toxicity, physostigmine can be used to reverse anticholinergic symptoms. There are reports of the use of physostigmine in cases of scopolamine toxicity. The patient then requires monitoring for a cholinergic crisis with atropine available at the bedside. Additionally, as with most poisonings, vital signs and ECG should be obtained and monitored.
Postoperative nausea and vomiting (PONV) is a ubiquitous complaint when caring for postoperative patients. The incidence is estimated to be about 30% in the average patient and can rise as high as 70% in the high-risk patient. PONV is a significant concern because it prolongs recovery room time, can lead to increased hospital admissions, and can lead to unanticipated complications. Additionally, the prevention of PONV is vital to patients.
A patient’s risk of experiencing PONV can be reliably anticipated and predicted using several measures, including the Apfel score. In patients with increased risk, a multimodal approach, including the use of multiple antiemetic medications, in the prevention of PONV, is the proper approach. [Level I] One potential antiemetic medication that has shown effectiveness in reducing the incidence of PONV is the transdermal scopolamine patch. [Level I]
Due to the pharmacokinetics and pharmacodynamics of the scopolamine patch, it requires application several hours before the start of the patient’s surgery. It can also be applied the previous night. The patient’s healthcare team must remain vigilant about identifying patients who are at high risk for PONV so that if scopolamine is indicated, there is sufficient time before the procedure to administer the drug effectively.
Scopolamine commonly causes dry mouth, blurred vision, and sedation. As an antimuscarinic medication, scopolamine has the potential to cause anticholinergic symptoms, including tachycardia, urinary retention, and acute angle-closure glaucoma. There are also reports of withdrawal symptoms from the medication. The patient’s health care team must be aware of these potential complications to recognize them and begin management as soon as possible; this is especially true for elderly patients who are at increased risk of complications, including falls. [Level II]
A collaborative healthcare team approach is the best method for addressing PONV. Clinicians need to accurately identify which patients will be candidates for anti-emetic prophylaxis or therapy and coordinate with nursing and pharmacy for its delivery. Nursing will be administering the medication and needs to fully understand the points discussed in this activity regarding proper administration and handling of the drug, particularly in patch form. The pharmacist needs to complete a full drug-drug interaction check and verify dosing, while also watching for mitigating factors like age, that may require therapy modification. Both the pharmacist and nurse must report any issues or concerns to the treating clinician. These points highlight some of how an interprofessional team approach will be most successful in providing PONV care. [Level V]
Finally, a common mechanism of ophthalmic symptoms from scopolamine is self-contamination by the patient. The health care team, including the pharmacist, must counsel the patient on hand washing and proper hand hygiene after touching the scopolamine patch. In addition to the morbidity associated with the actual adverse effect, the clinical signs associated with unilateral mydriasis may often be misinterpreted, leading to misdiagnosis, increased costs, and delay of proper treatment. [Level V]
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