In #KnowingYourJargon we explore common terms to improve nurses confidence,
Today I look at Antihistamine drugs.
We have had antihistamine drugs now for 4 generations. So common are these drugs that most in this category are sold over the counter in pharmacies, and even in some supermarkets.
You know them as Zantac, phenergan, Zyrtec, Loratadine, painstop nighttime, polaramine and Allergeze to name just a few.
To understand these drugs we need to understand a few terms.
Histamine is a protein secreted by specialised white blood cells called Mast cells and Basophils when stimulated. They are mostly associated with an allergic trigger. Histamine attaches to 4 receptors in different tissues in the body, stimulating and chemically instructing target tissues to behave or respond in certain ways. Histamine 1 (H1) receptors are found in the CNS causing pain and drowsiness. They are also found in smooth muscles of the bronchioles of lungs, and vascular smooth muscle. Here they exert classic allergy symptoms like vasodilation and bronchoconstriction in lungs (Asthma).
As vessels in local tissues dilate, capillaries become engorged and leak into interstitial spaces. This swelling is called oedema, and can be life threatening as seen in the swollen airway and lips of someone with angioneurotic oedema. It can be minor and simply manifest in skin as hives (urticaria).
H2 receptors in the gut lining are stimulated to secrete digestive juices and hydrochloric acid in the stomach. Ranitidine (Zantac) is an example of an antihistamine that selectively blocks these H2 receptors.
H3 receptors in the nervous system respond by producing neurotransmitters like acetylcholine and dopamine
H4 receptors on while blood cells and other immune organs are stimulated into action by histamine which causes chemotaxis ( cell attraction ).
So now that stuff is out of the way and we know what histamine does, it stands to reason that antihistamine drugs selectively block one or more of the four Histamine receptors. The vast majority of antihistamines we use are for allergic symptoms of itchiness, hives, rash, and swelling. These are certainly H1 blockers, but the use of H2 blockers has been well established for gastritis and ulcer symptom relief.
In the 70s and 80s, allergy targeting antihistamines were indiscriminate, and having strong CNS effects, drugs like Promethazine (Phenergan, and dexchlorphenaramine (Polaramine) were common antihistamine choices. These lost out in popularity to the newer drugs like Loratidine (Clarytine), and Zyrtec, because the new generation Antihistamines can't cross the blood brain barrier (BBB) so don't cause drowsiness.
That said, few nurses or parents would contest the inherent value of Promethazine as an added "angel" in the PainStop preparation, or as a nightcap during the allergy season's restless night routine.
Years ago the antihistamine was a very common treatment option for Asthma flare ups, but modern evidence has given us faster acting smooth muscle relaxants like the SABA Salbutamol. Notwithstanding, there may still be an adjunctive role for antihistamines in asthma management plans, particularly for well known seasonal allergic asthma.
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