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This is a very long and technical article so I cut and paste the main points but I also placed the link to the whole article here. The article itself is about antihistamines, not just how they cause seizures.
Pathophysiology
H1, H2, H3, and H4 receptors are the 4 human histamine receptors that have been identified.
All H1 antagonists are reversible, competitive inhibitors of histamine receptors. Some of the first-generation H1-receptor blockers (eg, diphenhydramine, clemastine, promethazine) are also potent competitive inhibitors of muscarinic receptors and may cause anticholinergic syndrome (eg, sinus tachycardia, dry skin, dry mucous membranes, dilated pupils, ileus, urinary retention, agitated delirium). H1-receptor blockers may disrupt cortical neurotransmission and block fast sodium channels. These effects can exacerbate sedation, but they also can result in seizure activity. Sodium channel blockade in the cardiac cells can cause conduction delays manifested by widening of the QRS interval and dysrhythmias.
First-generation H1-receptor antagonists, such as diphenhydramine, may be particularly dangerous because they may cause pronounced agitation and seizures, resulting occasionally in rhabdomyolysis and acidosis. Also, a quinidinelike sodium channel blocking effect, and at high doses, a potassium channel blocking effect (HERG1K), may cause delayed conduction (prolonged QRS) and repolarization (prolonged QT) and contribute to ventricular dysrhythmias.
Diphenhydramine is Benadryl
This article explains why I used to wake up with a racing heart rate when I used to use Benadryl to fall asleep. It would also cause me to have seizures. I can't speak for others, but I don't have any problems with Claritin or Zyrtec.
Here's the whole article-
http://emedicine.medscape.com/article/812828-overview
Pathophysiology
H1, H2, H3, and H4 receptors are the 4 human histamine receptors that have been identified.
All H1 antagonists are reversible, competitive inhibitors of histamine receptors. Some of the first-generation H1-receptor blockers (eg, diphenhydramine, clemastine, promethazine) are also potent competitive inhibitors of muscarinic receptors and may cause anticholinergic syndrome (eg, sinus tachycardia, dry skin, dry mucous membranes, dilated pupils, ileus, urinary retention, agitated delirium). H1-receptor blockers may disrupt cortical neurotransmission and block fast sodium channels. These effects can exacerbate sedation, but they also can result in seizure activity. Sodium channel blockade in the cardiac cells can cause conduction delays manifested by widening of the QRS interval and dysrhythmias.
First-generation H1-receptor antagonists, such as diphenhydramine, may be particularly dangerous because they may cause pronounced agitation and seizures, resulting occasionally in rhabdomyolysis and acidosis. Also, a quinidinelike sodium channel blocking effect, and at high doses, a potassium channel blocking effect (HERG1K), may cause delayed conduction (prolonged QRS) and repolarization (prolonged QT) and contribute to ventricular dysrhythmias.
Diphenhydramine is Benadryl
This article explains why I used to wake up with a racing heart rate when I used to use Benadryl to fall asleep. It would also cause me to have seizures. I can't speak for others, but I don't have any problems with Claritin or Zyrtec.
Here's the whole article-
http://emedicine.medscape.com/article/812828-overview
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