What is Antihistamine Toxicity?
In 1943, during World War II and just as the United States declared the end of The Great Depression, the most widely used modern antihistamine, called diphenhydramine, was created. A few years later, in 1947, another drug called orphenadrine was made to help treat Parkinson’s disease. By the time the 1950s rolled around, the use of antihistamines had increased and, unfortunately, so had the number of deaths among children.
Scientists soon noticed that the effects of antihistamines were quite similar to those of a dangerous poison called atropine. However, the main problem with antihistamines isn’t because they bind to certain receptors (H1-receptors) in the body or cause drowsiness. The real issue arises from their anticholinergic effect, which can disrupt the way nerves communicate in the body.
When exposed to toxic levels of antihistamines, people can experience a variety of symptoms, from mild to severe, which can have different outcomes. The treatment for someone who’s been exposed to a harmful amount of antihistamines can range from carefully watching and supporting their recovery to quickly administering treatment using specific drugs.
What Causes Antihistamine Toxicity?
Antihistamine overdose usually happens when it’s taken by mouth. It can also occur through other methods like injecting it into a vein or muscle, or applying it on the skin. But, these methods are uncommon in everyday settings.
Risk Factors and Frequency for Antihistamine Toxicity
Antihistamine toxicity, or poisoning, can occur if a person takes too much of these medications, either by accident or on purpose. This risk is especially prevalent in two groups: young children and older adults. Kids might be given too much because these drugs are widely available and can help them sleep. Older people are more at risk because of the drugs’ sedative and anticholinergic effects, which means they can slow down the body’s functions.
Signs and Symptoms of Antihistamine Toxicity
When children or other individuals who may be too sick or young to communicate clearly are brought for medical attention, the doctors usually have to rely on the information given by parents or caregivers. Doctors look out for signs of use of other medications like antidepressants or specific drugs like atropine and scopolamine that may interact with existing medications to cause adverse reactions.
During a physical examination, symptoms are categorized by organ systems. For example, there could be neurological symptoms, particularly when dealing with a type of antihistamines called first-generation H1 antihistaminics. These symptoms can appear as soon as two hours after the medication is taken and can include feeling drowsy, hallucinations, clumsiness, or mood swings in children. Seizures may occur at any time after taking the medication, but typically within the first one to two hours.
There could also be visual problems such as dilated pupils, blurred vision, and seeing double. Another set of symptoms affect the cardiovascular system and can lead to symptoms like fast heartbeat, either high or low blood pressure. Certain changes on an electrocardiogram, like increased duration of QRS complexes and QT intervals or manifestations of a heart condition called Brugada syndrome, may also be observed. In some instances, there have been reports of a severe condition called rhabdomyolysis, particularly when the patient experiences agitation, mood swings, or seizures.
Testing for Antihistamine Toxicity
When a person has potentially overdosed on antihistamines, the first step for doctors is to check basic vital signs like heart rate, breathing rate, and blood pressure, along with an EKG to look at heart function. After that, doctors may order more tests, even though some of these tests can be misleading. For example, some common antihistamines can sometimes produce incorrect negative results for drugs like amphetamines. Other tests might incorrectly point towards the presence of drugs like methadone or phencyclidine in a urine sample which could mislead doctors.
More sophisticated tests like gas chromatography/mass spectroscopy (GC/MS) or liquid chromatography/mass spectroscopy (LC/MS) can reliably detect antihistamine levels, but these aren’t often ordered because they take a long time. Doctors will also want to check how the kidneys are functioning by testing levels of Creatinine and BUN in the blood, and if the patient is potentially pregnant, a pregnancy test might also be ordered.
For those experiencing seizures or who seem extremely agitated, doctors will want to test for a substance called creatinine kinase. They’ll likely administer a medication called a benzodiazepine intravenously (through a vein) and give more as required. If a patient has a high body temperature, doctors will want to cool them down using methods that cause evaporation.
Low blood pressure is generally treated with fluids that are the same concentration as your body fluids given through a vein, and this is also the treatment for a condition called rhabdomyolysis. If a patient’s heart rhythms look concerning on the EKG, doctors may use medicines to steady the rhythm, and may treat with sodium bicarbonate, epinephrine, and intravenous glucose.
If a patient’s symptoms of antihistamine overdose become severe, doctors may use a medication called physostigmine, although it’s currently not readily available. Other potential replacements include oral rivastigmine or intravenous pyridostigmine, though these are not officially FDA-approved for this use. These medications work by blocking the action of an enzyme that breaks down a chemical called acetylcholine at the connection between nerve cells.
Physostigmine isn’t suitable for everyone, however, and shouldn’t be used in people who have an abnormal EKG, asthma or other lung diseases, or a slow heart rate.
Treatment Options for Antihistamine Toxicity
There are specific guidelines for handling at-home situations involving the ingestion of diphenhydramine and dimenhydrinate. For children under six, if less than 7.5 mg/kg is ingested, watching them at home is sufficient. For adults and older children, the same applies if less than 300 mg or 7.5 mg/kg has been consumed. If these amounts are exceeded, a quick trip to the nearest emergency room is recommended.
When in the hospital, patients should be continuously monitored for heart activity, and a vein should be accessed for potential IV treatment. Giving activated charcoal may be considered, particularly if the patient arrived soon after ingesting the substance. Although it might seem logical to try to speed up the drug’s elimination through the kidneys, this won’t work due to the drug binding tightly to proteins and spreading widely in the body.
What else can Antihistamine Toxicity be?
When trying to figure out what’s causing your symptoms, your doctor should also consider other possibilities, such as:
- Having taken too much of a type of antidepressant known as tricyclic antidepressants
- Consuming too much acetaminophen
- Experiencing low blood sugar, also known as hypoglycemia
- Having a condition called serotonin syndrome, which happens when there’s an excess of serotonin, a chemical your neurons produce, in your brain.
What to expect with Antihistamine Toxicity
The outcome can greatly differ based on the quantity consumed, but usually, if you quickly go to the emergency room and your heart is closely monitored, the outlook for swallowing too many antihistamines is very good.
Possible Complications When Diagnosed with Antihistamine Toxicity
- Irregular heartbeats
- Breathing failure
- Unconsciousness or coma
- Destruction of muscle tissue (Rhabdomyolysis)
- Extremely high body temperature (Hyperthermia)
- Seizures
Preventing Antihistamine Toxicity
It’s important for patients to realize that just because some medications can be bought without a prescription, doesn’t mean they can’t have serious side effects. Patients should be informed about the symptoms of an overdose on allergy medications (antihistamines).
It’s also crucial that these medications are kept in a secure place out of children’s reach at home. Moreover, these drugs should not be taken with sleeping pills or sedatives. When a patient is ready to leave the hospital, the nurse should re-emphasize the importance of storing all medications in childproof containers.
Anyone who was hospitalized because they intentionally consumed too much of these medications should have a session with a mental health professional before they leave the hospital.
