Uses of H1 Antagonists / Conventional Antihistaminics

USES

The uses of H1 antihistaminics are based on their ability to block certain effects of histamine released endogeneously, as well as on sedative and anticholinergic properties.

1. Allergic Disorders

Antihistaminics do not suppress AG: AB reaction, but block the effects of released histamine—are only palliative. They effectively control certain immediate type of allergies, e.g. itching, urticaria, seasonal hay fever, allergic conjunctivitis and angioedema of lips, eyelids, etc. However, their action is slow—Adr alone is lifesaving in laryngeal angioedema. Similarly, they cannot be relied upon in anaphylactic shock and have a secondary place to Adr. Benefits are less marked in perennial vasomotor rhinitis, atopic dermatitis and chronic urticarias; combination with an H2 antagonist succeeds in some cases of chronic urticaria not responding to H1 antagonist alone.

The antihistaminics are ineffective in bronchial asthma: reasons may be—

a.   Leukotrienes (C4, D4) and PAF are more important mediators than histamine.

b.     Concentration of antihistamines attained at the site may not be sufficient to block high concentration of histamine released locally in the bronchi.

Certain newer compounds like cetirizine have adjuvant role in seasonal asthma.

Antihistaminics are also ineffective in other types of humoral and cell mediated allergies because histamine is not involved. They do suppress urticaria and swellings in serum sickness, but not other components of the syndrome.

Type I hypersensitivity reactions to drugs (except asthma and anaphylaxis) are suppressed. Some skin rashes also respond.

2. Other Conditions Involving Histamine

Antihistaminics block symptoms produced by histamine liberators; afford symptomatic relief in insect bite and ivy poisoning. Abnormal dermographism is suppressed. They have prophylactic value in blood/saline infusion induced rigor.

3. Pruritides

Many conventional antihistamines have antipruritic action independent of H1 antagonism. Though relief is often incomplete, older antihistaminics remain the first choice drugs for idiopathic pruritus.

4. Common Cold

Antihistaminics do not affect the course of the illness but may afford symptomatic relief by anticholinergic (reduce rhinorrhoea) and sedative actions. The newer nonsedating antihistamines are less effective in this respect.

5. Motion Sickness

Promethazine, diphenhydramine, dimenhydrinate and cyclizine have prophylactic value in milder types of motion sickness; should be taken one hour before starting journey. Promethazine can also be used in morning sickness, drug induced and postoperative vomiting, radiation sickness. Cyproheptadine has appetite stimulating effect; has been used in underweight children.

6. Vertigo

Cinnarizine is the H1 antihistamine having additional anticholinergic, anti5HT, sedative and vasodilator properties which has been widely used in vertigo. It modulates Ca2+ fluxes and attenuates vasoconstrictor action of many endogenous substances.

Cinnarizine inhibits vestibular sensory nuclei in the inner ear, suppresses postrotatory labyrinthine reflexes, possibly by reducing stimulated influx of Ca2+ from endolymph into the vestibular sensory cells. Beneficial effects have been reported in Méniére’s disease and other types of vertigo. Side effects are sedation and mild g.i. upset.

Drugs For Vertigo

The therapy of vertigo occurring in Méniére’s disease and other conditions is imperfect. A variety of approaches have been tried and have met with partial success.

1. Labyrinthine Suppressants

They suppress endorgan receptors or inhibit central cholinergic pathway (in vestibular nuclei).

a.     Antihistaminics (with anticholinergic action)— cinnarizine, cyclizine, dimenhydrinate, diphenhydramine, promethazine.

b.     Anticholinergics—atropine,  hyoscine.

c.      Antiemetic phenothiazines—prochlorperazine, thiethylperazine.

2. Vasodilators

They improve blood flow to labyrinth and brainstem—betahistine, codergocrine, nicotinic acid, naftidrofuryl.

3. Diuretics

They decrease labyrinthine fluid pressure —acetazolamide, thiazides, furosemide.

4.   Anxiolytics, Antidepressants

These drugs appear to modify the sensation of vertigo—diazepam, amitriptyline.

5. Corticosteroids

They suppress intra-labyrinthine edema due to viral infection or other causes.

Parenteral prochlorperazine is the most effective drug for controlling violent vertigo and vomiting.

7. Preanaesthetic Medication

Promethazine has been used for its anticholinergic and sedative properties.

8. Cough

Antihistaminics like chlorpheniramine, diphenhydramine and promethazine are constituents of many popular cough remedies. They have no selective cough suppressant action, but may afford symptomatic relief by sedative and anticholinergic property.

9. Parkinsonism

Promethazine and some others afford mild symptomatic relief in early cases— based on anticholinergic and sedative property.

10. Acute Muscle Dystonia

Caused by antidopaminergicantipsychotic drugs is promptly relieved by parenteral promethazine or hydroxyzine. This is again based on central anticholinergic action of the drugs.

11. As Sedative, Hypnotic, Anxiolytic

Antihistamines with CNS depressant action have been used as sedative and to induce sleep, especially in children. However, promethazine has produced serious respiratory depression in young children; few deaths are on record; it is not indicated in children aged 2 years or less. For promoting sleep, antihistaminics are not as dependable as benzodiazepines. Hydroxyzine has been used in anxiety associated with autonomic manifestations.

(Combinations of antihistaminics with antidiarrhoeals or bronchodilators, or those containing more than one antihistaminic are banned in India.)