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allergies or NOT?
and peripheral histamine-1 receptors that prevent the hista- mine receptor interaction and subsequent mediator release. In addition, second-generation antihistamines inhibit the release of mast cell mediators and may decrease cellular recruitment. Differences among antihistamines relate to the rapidity and degree to which they penetrate the blood–brain barrier as well as to their receptor specificity. Sedating antihistamines are highly lipophilic molecules that readily cross the blood–brain barrier. Nonsedating antihistamines, large protein-bound lipo- phobic molecules with charged side chains, do not readily cross the blood–brain barrier. Both types of antihistamines are highly selective for H1 receptors but have little effect on H2, H3, or H4 receptors. The sedating antihistamines have anti- cholinergic, antiserotonin, and anti–alpha-adrenergic effects. These agents are indicated for relief of symptoms of allergic rhinitis including itching, sneezing, and rhinorrhea. The adverse effects of antihistamines ultimately depend on receptor activity, chemical structure, and lipophilicity of the drug. The primary adverse effects, CNS effects (depression and stimulation) and anticholinergic effects, are common with first-generation antihistamines but are rarely seen with second-generation agents. CNS-depressive effects include sedation and impaired performance (e.g., impaired driving performance, poor work performance, incoordination, reduced motor skills, and impaired information processing). CNS-stimulatory effects include anx- iety, hallucinations, appetite stimulation, muscle dyskinesias, and activation of epileptogenic foci. Adverse effects related to cholinergic blockage include dryness of the eyes and mucous membranes, blurred vision, urinary hesitancy and retention, constipation, and reflex tachycardia. Sedating antihistamines are contraindicated in newborns or premature infants, lactating women, and patients with narrow-angle glaucoma. Additional contraindications include acute asthma exacerbation, stenosing peptic ulcer, symptomatic prostatic hypertrophy, bladder neck and pyloroduodenal obstruction, and concomitant use of MAOIs. Patients with lower respiratory tract diseases (e.g., emphy- sema, chronic bronchitis) should use sedating antihistamines with caution. People whose activities require mental alertness should not use sedating antihistamines and should use levo- cetirizine and cetirizine with caution. Patients should also be advised to use sunscreen and wear protective clothing when administering sedating antihistamines, due to these drugs being photosensitizing. Below are the common first- and second-generation antihistamines used in allergic rhinitis:
PHARMACOLOGICAL APPROACH TO ALLERGIC RHINITIS
Allergic rhinitis, unfortunately, cannot be cured. The goals of therapy are to reduce symptoms and improve a patient’s functioning status and sense of well-being. The condition is treated in three steps: allergen avoidance, pharmacotherapy, and immunotherapy. Treatment is individualized to provide optimal symptomatic relief and/or control. Since allergen avoid- ance alone is generally not sufficient to provide complete relief of allergic rhinitis, targeted therapy with single-entity drugs is typically initiated. As more medications become avail- able without a prescription, patients can extensively self-treat. Medications commonly used in treating allergic rhinitis include intranasal corticosteroids, antihistamines, decongestants, and cromolyn sodium. The management of allergic rhinitis is influenced by the frequency and severity of symptoms, the age of the patient, and the presence of concurrent conditions. Each type of medication will be discussed further below. Intranasal Corticosteroids Intranasal corticosteroids (INCS), also known as glucocorticoids, are extremely effective agents for the treatment of nasal symp- toms associated with allergic rhinitis such as itching, rhinitis, sneezing, and congestion. These drugs inhibit multiple cell types and mediators, including histamine, and effectively stop the “allergic cascade.” They downregulate inflammatory responses by binding to intracellular glucocorticoid receptors in the cyto- plasm of inflammatory cells and enhance anti-inflammatory genes, while also suppressing the transcription of most cyto- kine and chemokine genes. These agents are the most effective single maintenance therapy for allergic rhinitis and cause few side effects at recommended doses. Concerns with the long- term use of many glucocorticoid preparations include adrenal suppression, slowed growth in children, decrease in bone mineral density, glaucoma, and cataract formation. Due to the drug formulation being topical, the risk of these long-term com- plications appears to be small because of the limited systemic absorption and relatively low doses involved. More common side effects of these agents include local irritation and epistaxis. Septal perforation is another possible adverse effect but is rare on occasion. INCS can be divided into first- and second-gen- eration preparations, with second-generation formulations carrying a lower risk of systemic effects because of markedly lower total bioavailability. First-generation products include beclomethasone, flunisolide, triamcinolone, and budesonide. Second-generation products include fluticasone propionate, mometasone furoate, ciclesonide, and fluticasone furoate. Antihistamines Antihistamines have been used to treat allergies since the 1940s. These agents can be classified as first-generation and second-generation products, where first-generation formulations are significantly more sedating than second-gen- eration formulations. Sedating antihistamines are effective, readily available without a prescription, and relatively inex- pensive. However, these antihistamines expose individuals to risks of anticholinergic effects and should be used with caution. Antihistamines work by competing with histamine at central
First-generation antihistamines • Diphenhydramine • Chlorpheniramine • Hydroxyzine • Brompheniramine • Doxylamine Second-generation antihistamines • Cetirizine • Levocetirizine • Loratadine
• Desloratadine • Fexofenadine
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