Antihistamine Toxicity - Causes, Symptoms, and Management

Introduction:

Antihistamines, sometimes known as allergy drugs, decrease or block the effects of histamine, a substance the body creates in response to allergens. These drugs are of two categories. The H1 antihistamines are known to more commonly cause antihistamine toxicity. The side effects of antihistamine toxicity include sedation, anticholinergic effects (blocking the action of acetylcholine, a neurotransmitter), impaired or loss of memory, headaches, lowered cognitive functions, altered behavior, nervousness, anxiety, loss of sleep, etc.

What Are Histamines?

Histamine is a natural compound associated with the immune response of the body. In addition, it is also responsible for the regulation of various functions of the body, like working as a neurotransmitter for the brain, spinal cord, uterus, etc., and regulation of the gut (digestive tract). Therefore, histamines are considered local hormones or biological factors and central neurotransmitters. In addition, histamine has a vital role as the itching mediator and is associated with the inflammatory response.

What Are Antihistamines?

Antihistamines are a class of drugs used to treat conditions mediated by the action of histamines. Histamine receptors fall into two categories: H-1 receptors and H-2 receptors. Antihistamine medications that bind to H-1 receptors treat allergic rhinitis and other allergic reactions. At the same time, antihistamine drugs that bind to H-2 receptors can cure upper gastrointestinal tract diseases due to too much stomach acid. Moreover, H-1 antihistamines are classified as first-generation agents and second-generation agents. This is because the blood-brain barrier (a network of tissues and blood vessels that prevents harmful chemicals from entering the brain) is easily crossed by first-generation H-1 antihistamines but not second-generation H-1 antihistamines in the central nervous system (CNS). In addition, second-generation drugs only preferentially bind to peripheral histamine-1 receptors. Conversely, first-generation drugs will bind to both central and peripheral histamine-1 receptors. This results in distinct therapeutic indications, results, and side effects of these drugs.

Why Are Antihistamines Used?

Antihistamine drugs are used in the following conditions:

• Allergies.

• Allergic rhinitis (the nose gets irritated by certain allergic agents and causes symptoms like sneezing, runny nose, etc.).

• Allergic conjunctivitis (inflammatory response of the conjunctiva of the eyes due to allergens leading to eye inflammation).

• Allergic dermatological reactions (skin reactions to allergic agents leading to itching, rashes, redness, etc.).

• Sinusitis (inflammations of the sinuses).

• Urticaria (skin allergy leading to itchy welts).

• Angioedema (swelling under the skin due to exposure to allergens).

• Atopic dermatitis (eczema, a condition where the skin becomes itchy and red).

• Bronchitis (the lining of the bronchial tubules becomes inflamed).

• Motion sickness.

• Nausea and vomiting.

• Peptic ulcers (ulcers on the stomach lining, intestines, etc.).

• Acid reflux (backward flow of the stomach acids).

• Gastritis (swelling in the stomach lining).

What Is Antihistamine Toxicity?

Toxicity occurring due to the antihistamine class of drugs, especially H-1 receptor antihistamines, is known as antihistamine toxicity. The toxicity occurs due to an overdose of the drug when used to treat a certain medical condition. Antihistamine’s competitive H1-receptor binding and sedative effects are not the main causes of antihistamine toxicity; rather, it is their anticholinergic action (blocking the action of the neurotransmitter acetylcholine). Therefore, different symptoms and consequences develop on exposure to antihistamine toxicity.

What Causes Antihistamine Toxicity?

Antihistamine toxicity occurs due to antihistamine drugs, especially H-1 receptor antihistamine drugs. These drugs are safe when taken in the prescribed dosage. However, when an excess amount of these allergy drugs are consumed, it can lead to various side effects, including toxicity. Antihistamine toxicity is due to H-1 receptor binding, the sedative effect of the drug, and the anticholinergic action of the drugs (blocking the action of acetylcholine, a neurotransmitter). Antihistamine toxicity nearly always results from oral consumption. Other kinds can be topical, intramuscular, and intravenous (IV), which are uncommon in everyday use.

What Are the Symptoms of Antihistamine Toxicity?

The symptoms of antihistamine toxicity are:

• Neurological symptoms usually manifest after two hours of drug consumption. These symptoms include; drowsiness, irritability, poor muscle control, hallucinations, etc.

• Seizures (fits).

• Visual impairments include; enlargement of the black center of the eye, blurred vision, double vision, etc.

• Increased heart rate (tachycardia).

• Increased or decreased blood pressure.

• Bread down of muscle tissues leads to muscle fiber content being released into the bloodstream.

Is Antihistamine Toxicity Common?

• Pediatric patients and the elderly are at the most risk for antihistamine toxicity due to accidental toxic dose consumption or suicide attempts.

• Antihistamines can frequently be administered excessively to children due to their therapeutic sedative effects and widespread accessibility. Their calming and anticholinergic effects are particularly potent in the elderly.

How Is Antihistamine Toxicity Diagnosed?

The diagnostic procedures involved in evaluating a case of antihistamine toxicity are:

• Signs and Symptoms: It is crucial to closely study the signs and symptoms of the patient. Moreover, it is essential to record the patient’s medical history to determine the intake of any antihistamine drugs.

• Monitoring the Vitals: The patient’s body temperature, heart rate, respiratory rate, and blood pressure should be done to check for any variations in their values. Any alterations in the values of these parameters should be immediately addressed.

• Electrocardiogram (ECG): ECG should be done to check for the presence of any heart conditions that may be occurring due to antihistamine toxicity.

• Other Tests: Beta-hCG or other serum pregnancy tests should also be performed, as well as creatinine and blood urea nitrogen (BUN) measurements to examine kidney function.

• Extensive Tests: It is possible to reliably determine antihistamine concentrations using extensive testing such as gas chromatography-mass spectroscopy or liquid chromatography-mass spectroscopy. However, these tests are only sometimes requested because of the lengthy diagnostic process.

How Is Antihistamine Toxicity Treated?

Treatment of various antihistamine toxicity include:

• If a child under the age of six consumes Diphenhydramine or Dimenhydrinate, at-home supervision is advised. At-home observation is advised if an adult or older kid consumes less than 300 mg (milligrams) or 7.5 mg/kg (milligrams per kilogram).

• If these turn out to be insufficient, an immediate evaluation at the nearest emergency department is required. Initial management should involve initiating intravenous (IV) access and ongoing cardiac monitoring. In addition, administration of activated charcoal may be necessary, particularly if the patient presents soon after ingesting the drug.

What Are the Complications of Antihistamine Toxicity?

Some of the complications of antihistamine toxicity are:

• Arrhythmia: Irregular heartbeat.

• Respiratory Failure: A severe condition characterized by an inability to breathe and failure to supply oxygen to the blood.

• Coma: Prolonged period of unconsciousness.

• Rhabdomyolysis: Breakdown of muscles leading to the release of muscle fibers into the blood.

• Hyperthermia: Higher than normal body temperature.

Conclusion:

Antihistamine drugs are commonly used to treat allergies. However, toxicity due to an overdose of antihistamine drugs is very common. The treatment of antihistamine toxicity depends greatly on the class of drug used and the amount of drug ingested. The prognosis for antihistamine toxicity might vary significantly depending on how much of the drug was consumed, but the prognosis is typically fairly good with prompt emergency department admission and thorough heart monitoring.