Antiemetic Histamine H1 Receptor Blockers

Mechanism of Action

H1 receptors in central nervous system areas include the area postrema and vomiting center in the vestibular nucleus. Also, many of the antihistamines have anticholinergic properties that block muscarinic receptors at the same sites. Antihistamines must cross the blood-brain barrier to affect the central nervous system.[5][6]

Administration

Dosing for Nausea and Vomiting

• Diphenhydramine: 25 to 50 mg orally (PO) every 6 hours, as needed
• Dimenhydrinate: 50 to 100 mg PO every 6 hours, as needed
• Meclizine (mainly used for motion sickness and vertigo): 25 every 6 hours, as needed
• Promethazine: PO, intramuscular (IM), rectal, or intravenous (IV): 12.5 to 25 mg every 6 hours, as needed

Adverse Effects

These agents can exhibit many anticholinergic effects, including central nervous system (CNS) depression, fatigue, paradoxical vomiting, blurred vision, xerostomia, and worsening of narrow-angle glaucoma.

Warnings/Precautions

According to Beers criteria, all first-generation antihistamine histamines carry a higher risk for severe adverse events to the elderly with diphenhydramine and hydroxyzine, posing an even higher risk for adverse events, including CNS depression. They block CYP2D6, which diminishes the effects of codeine, tramadol, and tamoxifen in the geriatric population.[7]

Neuroleptic malignant syndrome has an increased risk of occurring when combined with phenothiazines and antipsychotic drugs. Hyperpyrexia, muscle rigidity, altered mental status, tachycardia, irregular pulse, irregular blood pressure, and cardiac dysrhythmias are typical manifestations of the disease.[8]

Promethazine (FDA Boxed Warning): Injection can cause tissue injury, including but not limited to tissue necrosis, gangrene, abscesses, venous thrombosis, and amputation. The preferred route of injection is intramuscular and never subcutaneous. Discontinue injections if there is any burning or pain out of fear of perivascular extravasation or arterial injection. Using injectable promethazine is not advised.[9]

Children younger than two years of age are at increased risk of respiratory depression while taking promethazine.[10]

Contraindications

Hypersensitivity to the medications or other phenothiazines is a contraindication. Patients with lower respiratory tract symptoms, including asthma, should cautiously use antihistamines, especially for children under the age of 2.

Pregnancy/Breastfeeding Considerations

Ten percent to 15% of females have taken antihistamines during pregnancy to treat allergic reactions and nausea. Diphenhydramine, loratadine have been safely used during pregnancy.[11] However, antihistamine usage during breastfeeding is contraindicated because it may inhibit lactation due to decreased maternal serum prolactin concentrations. When antihistamines are needed, second-generation antihistamines are preferable in the breastfeeding population.[12]

Monitoring

The therapeutic index of first-generation antihistamines depends on several factors, including the disease (allergic rhinitis, vomiting, nausea, urticaria, motion sickness, vertigo, parkinsonian, insomnia), formulation (by mouth, intranasal, intravenous, intramuscular), and the population (pediatric, adult, elderly). All antihistamines can cause anticholinergic effects; however, diphenhydramine causes the most anticholinergic and cardiac toxicity effects. Anticholinergic effects include cutaneous vasodilation, hydroceles, hidrotic hyperthermia, noninteractive eye injuries, delirium, hallucinations, urinary retention. Cardiac toxicity effects include tachycardia and prolonged QTc.[13] Special monitoring of patients with epilepsy, chronic alcohol abuse, or pre-existing heart disease because these conditions may decrease the safety profile of antihistamines, especially with a narrow therapeutic index medication such as diphenhydramine.[14]

Toxicity

Initial treatment should be with stabilization of the airway and monitoring breathing and circulation. The patient should receive supplemental oxygen, continuous pulse oximetry, intravenous access, and an ECG. In patients with prolonged QRS intervals or arrhythmias, sodium bicarbonate is an option.[15] Benzodiazepines can be useful to address agitation and seizures. Hypothermia treatment includes evaporative cooling. If no altered mental status is present and ingestion of anticholinergic agents is likely, activated charcoal is an option.[16] If the airway access is threatened, consider intubation instead. Supportive care is usually adequate; however, in patients with peripheral and central anticholinergic toxicity, administration of the anticholinesterase inhibitor, physostigmine, may be warranted. Consultation with a medical toxicologist or regional poison center is a recommended process before administration. Caution is necessary for patients with cardiac abnormalities, reactive airway disease, or gastrointestinal obstruction. Atropine should be available at the bedside in case of an overdose of physostigmine.

Enhancing Healthcare Team Outcomes

Antihistamine toxicity usually manifests itself as anticholinergic poisoning and requires an interprofessional team of nurses, physicians, laboratory technologists, pharmacists to narrow down the differential diagnosis. Without proper management, a patient can receive incorrect treatment and could cause cardiac arrest. A patient with delirium, seizures, or tachycardia has a wide range of diagnoses when initially admitted. When a diagnosis of anticholinergic toxicity is suspected, a healthcare provider should perform a screening test. Finger-stick glucose, acetaminophen concentrations, salicylate concentrations, ECG, and a pregnancy test for women of childbearing age are necessary. Monitoring the patient for any cardiac arrhythmias or altered mental status will further dictate treatment. Clinicians should use benzodiazepines for agitation, but physostigmine may be more effective.[17][18] [Level 4, Level 5]

A toxicologist consult is in order if physostigmine is needed. The patient should be placed on a cardiac monitor, have resuscitation equipment and atropine available at the bedside when administering physostigmine. Pharmacists should review the patient's medication record before and after the toxicity and consult with the clinicians regarding a plan to move forward. Nursing will monitor treatment and report any concerns to the treating clinicians. If there is any question of the etiology of the poisoning, please call the United States Poison Control network at 1-800-222-1222.

After patient stabilization, the patient will need a thorough history to assess for underlying psychiatric problems, medication nonadherence, exposure, or medical issues, causing an increased concentration of the offending medication. All these interprofessional strategies can result in safer use of antihistamines and help patients who encounter toxicity from these medications. [Level 5]