Antihistamines are valuable in treating skin disorders mediated by histamine. These are primarily used for the symptomatic relief of allergic reactions, such as urticaria and angioedema. rhinitis, conjunctivitis and pruritus associated with skin disorders. Antihistarnines have also been found useful as tranquilizers, anticonvulsants, decongestants, local anacsthetics, hypnotics and as antiparkinsonian1,2 drugs. Many antihistamines cause some degree of sedation to which tolerance generally develops. Other adverse effects include antimuscarinic, extrapy ramidal symptoms and hypersensitivity reactions. The newer antihistamines are less prone to cause sedation orantirnuscarinic effects.
The role of histamine in dermatopathology
The highest concentration of histamine is found in the lungs, skin and intestinal mucosa. Histamine is distributed widely in the animal kingdom and is found in venonis, noxious secretions, bacteria and plants. Its main formation and storage occurs in mast cells but it is also found inepidermal, gastric mucosal cells, neurons within the central nervous system and in rapidly growing tissues3. Histamine produces a reaction known as the triple response. Erythema appears due to capillary expansion followed by a diffuse flare secondary to arteriolar dilatation. Lastly, a weal appears due to exudation of fluid through the altered vascular wall.
The biological effects of histamine are the result of its interaction with HI and H2 receptors. Hi receptors mediate vasodilation, increased permeability of small blood vessels, smooth muscle contraction and itching. H2 receptors are known for effecting gastric acid production. They also play a role in skin blood vessels and the immune system, somewhat resembling that of leyamizole4. H3 receptors are found in the brain and are responsible for autoregulation of histamine production and release5.
Classification of antihistamines
HI and H2 antagonists are different structurally, accounting for their differences in pharmacokinetic properties.
HI antagonists: These drugs are lipophilic. They show prominent sedation resulting in decreased attention, increased sleep duration and changes in EEG patterns. This occurs in fifty percent of subjects taking conventional antihistamines and in seven percent of subjects taking terfenadine or astemazole1. Antihistaniines are readily absorbed from the stomach and small intestine, with peak blood concentrations in one to two hours. The newer agents such as, terfenadine, loratadine and astemazole are less lipid soluble and enter the central nervous system with difficulty or not at all. Their effective duration of action is four to six hours, but the newer Hi blockers have a duration of 12 to 24 hours.
Carbinoxamine (clistin): Antimuscarinic, central sedative and antiserotinin effects, 2-4 mg three to four times daily.
Diphenhydramine (Benadryl): Antimuscarinic, marked central sedation and antiemetic effects, 25-50 mg three to four times daily.
Dimenhydrinate (Dramamine): Antimuscarinic. marked central sedation, antiemetic effects, 50-100 mg three to four times daily.
Clemastine (Tavegyl): Antimuscarinic with central sedative properties, I mgtwice daily.
Doxylamine (Decapryn): Marked sedation; antimascarinic effects; now available onl\\\\ in OTC “sleep aids’.
Ethylcncdiamincs Antazoline (Antistine): Component of ophthalmic and nasal drops as well, 50mg thnce daily.
Pvrilamine (Neo-Antergan): Moderate sedation, component of OTC “sleep aids”,
Tripelennamine (PBZ): 25-50mg daily. Antimuscarinic central sedation, 25-50 mg four to six hourly.
Clernizole: Moderate central sedation, 20-40 mg two to four times daily.
Piperazines Cyclizine (Marzine): Antimuscarinic, slight sedation, antiemetic,50 mg thrice daily.
Meclozine (Ancolan): Antirnuscarinic, slight sedation, anti-emetic, 25 mg twice daily.
Buclizine (Longifene): Antimuscarinic, marked sedation, antlemetic, 50mg upto thrice daily.
Mebhydrolin (Incidal): Antimuscarinic, sedation, 50-100mg thrice daily.
Aikvlamines Triprolidine (Actidil): Antimuscarinic, central sedation, 2.5-5 mg thrice daily.
Pheniramine (Avil): Antimuscarinic, central sedation. 25-50mg two to three times daily.
Chiorpheniramine (Pinton): Component of OTC “cold” medications, antimuscarinic, moderately sedating, 4mgfourto six hourly.
Phenothiazines Prorncthazine(Phenergan): Antimuscarinic, marked central sedation 25mg at night.
Trimeeprazine (Vallergan): Antiemetic, antimuscarinic, sedating, 10mg iwo to three times daily.
Methdilazine (Tacaryl): Antimuscarinic, sedating, 8mg two to four times daily.
Ataraxis Hydroxazine (Atarax): Antimuscarinic, sedating, anxiolytic, antiemetic, 25mg three to four times.
Piperidines Astemazole (Mayasen): Long acting with antiserotinin effect, no sedation or antimuscarinic activity, 10mg once daily.
Terfenadine (Teldane): No sedation, 60 mg twice daily.
Miscellaneous Cyproheptadine (Periactin): Antiscrotinin, antimuscarinic, sedating, appetite stimulant, 12-16mg daily in three or four divided doses.
Loratadine (Claritine): Long acting, no sedation, 10mg once daily.
Azatadine (Zadine): Long acting, antiserotinin, sedating, antimuscarinic, 1 mgtwice daily.
Ketotifen (Zaditen): Moderately sedating, appetite stimulant, mast cell stabilizing effects6, 1mg twice daily.
Acrivastine (Semprex): No sedation, 8mg thrice daily.
Ebastine (No-sedat): No sedation, 10mg once daily.
Mequitazine (Metapiexan): Minimal sedation, antimuscarinic 5 mg twice daily.
Certrizine (Rigix, Zyrtec): Long acting, minimal sedation, Additional anti-inflammatory actions and inhibitory effects on monocytes and T lymphocytes7, 10mg once daily.
H2 receptors blockers:
These are highly hydrophilic, weak bases with variable lipophilicity. Ranitidine and cimetidine are the main H2 blockers. Other drugs such as famotidine, nizatidine, roxatidine, niperotidine and investigational H2 receptor antagonists have therapeutic effects similar to the above, but differ in potency and duration of action. They are rapidly and completely absorbed after oral ingestion. Their use in dermatology is quite modest. They may be of value in histamine mediated disorders which fail to respond to Hi antagonists.
Side effects of Hi blockers: Sedation is the most common side effect of these drugs. Other CNS effects may include dizziness, tiniiitus, incoordination, blurred vision and diplopia. in certain instances, stimulatory effects such as nervousness, insomnia, tremor and irritability may occur. Gastrointestinal complaints associated with these drugs include nausea, vomiting, diarrhoea, constipation, anorexia and epigastric distress. Anticholinergic properties such as dry mucous membranes, difficulty in micturition, urinary retention and impotence can be disturbing. The cardiovascular effects like hypotension usually follow intravenous therapy, especially when given rapidly. Cutaneous reactions occurring after administration of Hi blockers include fixed drug eruptions, petechial rashes, urticaria, photosensitivity and cczematous contact dermatitis. The latter usually occurs with topically applied antihistamines. Systemic administration of an antihistamine to which there has been topical sensitization will not only reproduce the original allergic contact dermatitis, but at times, a generalized exfoliative dermatitis8. On rare occasions, parentral administration can result in morbjlliform and scar— letiniform cruption and anaphylactic shock.
Theoretically all antihistamines can be teratogenic in animals and therefore should be avoided in pregnancy9. The dyes used in the outer coating of tablets can give rise to allergy like idiosyncratic reactions. Terfenadine and astemazole have been associated with QT interval prolongation and ventricular arrythmias. Therefore, caution is advised in patients with existing repolarization abnormalities and those at risk of elevated levels of antihistamines10.
Drug interactions of H1 antagonists
These can potentiate the effects of alcohol and other CNS depressants including hypnotics. anxiolytic, sedatives, opioid, analgesics and tranquilizers. The actions of CNS stimulants can be enhanced in children and infants. Hi blockers antagonise the effectiveness of steroids, diphenythydantion, oral anticoagulants. phenyibutazone, griseofulvin and other drugs metabolized by liver enzymes. Significant cardiac toxicity has occurred in patients taking a combination of either terfe nadine11,12 or astemazole13 and ketoconazole, itraconazole or erythromycin. The anticholinergic activity of antihistamines is potentiated by anticholinergic drugs.
Side effects of H2 antagonists:
These drugs are usually well tolerated. Cimetidine has been associated with diarrhoea, headache, dizziness, drowsiness malaise, muscular pains, constipation, gynacomastia galactorrhca, loss of lidido, impotence and reduction of sperm counts in young men14. This resolves after the drug is discontinued. Due to its antiandrogenic activity and reduced sebum secretion, generalized xerosis and asteatotic dermatitis has been reported15. Psoriasis has been reported in an 86 year old woman during her treatment with cimetidine for duodenal ulcer16. Urticarial vasculitis, alopecia. hypersensitivity and induction or exacerbation of lupus erythematosus has been reported with cimetidine13. Reversible elevation of serum transaminases and granulocvtopenia can occur. Symptoms of gastric carcinoma may be masked with cimetidine17. Cimetidine and to a lesser extent, ranitidine can inhibit the cytochrome P450 oxidative drug metabolizing system. Both drugs inhibit the renal clearance of basic drugs.
Drug interactions of H2 antagonists
Due to inhibition of metabolism of drugs, cimetidine can prolong the pharmacological activity of diazepam, chlordiazepoxide, warfarin, the opylline, propanolol, phenytoin, caffeine, alprazolam, tricyclic ant.idepressants, carba.mazepine, calcium channel blockers and sulphonylureas. Ranitidine in ordinar therapeutic doses, does not appear to inhibit the oxidative metabolism of other drugs. Clinically significant drug interactions have not been reported with famotidine and nizatidine1. Therapeutic uses of antihistamines in dermatology
They are mainly used for the symptomatic relief of pruritus and treatment of urticana and angioedeina. A trial of HI antagonists especially at bed-time is helpful for the symptomatic relief of itching. Initially chiorpheniramine (4mg) or hydroxazine (10-25 mg) by mouth is prescribed. Atopic dermatitis is more responsive to sedating antihista= mines such as trimeprazine as compared to non-sedating ones such as terfenadine or astemazole18. These have to be combined with the relevant topical therapy such as emollents, steroids or tar. Sedation is desirable in the treatment of atopic dermatitis as its itching involves a central component.
Psychogenic pruritus benefits from antidepressant therapy with doxepin or sedative therapy with hydroxazine which has anxiolytic effects as well19. In the allergic dermatoses, benefit is most striking in acute urticaria although the itching is better controlled than the erythema and oedema3. Hydroxazine is commonly used in the treatment of urticaria. It is also useful in suppressing histamine induced pruritus in dermographism and in cholinergic urticaria20,21. Cyproheptadine and phenergan have been found to be slightly more effective than chiorpheniramine and hydroxazine in aquagenic urticaria22. Cyproheptadine is often the drug of choice in tatients with acquired cold urti-] caria21/angioedema23.
The non-sedating antihistamines acrivastine24, astemazole25, cetrizine25, loratadine26 and terfenadine 27 have been found to be efiective in chronic urticaria. Comparative studies have generally shown no clear difference in their efficacy. The mast cell stabilizing Hi antagonists such as ketotifen and azatadine have shown efficacy in the treatment of urticaria13.
Hi antihistamines have shown improvement in contact dermatitis, infestations and pruritus secondary to underlying idiopathic disorders. Topically, Hi antihistamines notably diphenhydraniine are effective analgesics particularly on mucous membranes1. H1 antagonists can be used as adiuncts in anaphylactic reactions as they act against the urticarial and angioedemal responses; but do not control the associated hypotension and bronchospasm. In acute anaphylactic reactions, adrenaline is given subcutaneously followed by intravenous hydrocortisone, if necessary.
The combination of Hi and H2 antagonists can be considered in patients in whom Hl antthistamines alone are ineffective. Cimetidine or ranitidine, administered alone or in combination with an H1 receptor antagonists have shown benefit incertaintypes of rticaria, especially those associated with cold or angioedema28. routine use however, cannot be justified. Cimetidine alone or in combination with an HI antagonists has been reported to relieve the gastrointestinal symptoms29,30 pruritus and urticaria31,32 in mastocytosis. Cimetidine has been found useful in treating the pruritus of Hodgkins disease33 and polycythemia vera34.
The antiandrogenic effect of cimetidine has produced favourable results in hirsutism35. However, in a recent study, it was concluded that its weak antiandrogenic action was insufficient for it to effectively treat hirsutism36. H2 receptor antagonists have been used as immune stimulants in chronic fungal infections37. Cimetidine has unpredictably causedboth remission38 and lack of response39 in eosinophilic fascitis.There have been reports of cimetidine producing clinical improvement and sometimes complete remission in malignant melanoma when used in combination with couinari40 interferon41 or topical diphencyprone42. However, response to the latter treatment modalities is not predictable. Some patients do not respond and in some there may be progression of disease.
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