Anticholinergic drugs are sometimes called parasympatholytic drugs because they inhibit the action of acetylcholine at specialreceptors in the parasympathetic nervous system.
Anticholinergics used to treat Parkinson’s disease are classified in two chemical categories according to their chemical structure:
· synthetic tertiary amines, such as benztropine, biperiden hydrochloride, biperiden lactate, procyclidine, and trihexyphenidyl
· antihistamines (such as diphenhydramine) that have anticholin-ergic properties, which are effective in treating the symptoms of Parkinson’s disease.
Typically, anticholinergic drugs are well absorbed from the GI tract and cross the blood-brain barrier to their action site in the brain. Most are metabolized in the liver, at least partially, and are excreted by the kidneys as metabolites and unchanged drug. The exact distribution of these drugs is unknown.
Benztropine is a long-acting drug with a duration of up to 24 hours in some patients. For most anticholinergics, half-life is undeter-mined. In addition to the oral route, some anticholinergics can also be given I.M. or I.V.
High acetylcholine levels produce an excitatory effect on the CNS, which can cause a parkinsonian tremor. Patients with Parkinson’s disease take anticholinergic drugs to inhibit the action of acetyl-choline at receptor sites in the CNS and autonomic nervous sys-tem, thus reducing the tremor.
Anticholinergics are used to treat all forms of Parkinson’s disease. They’re used most commonly in the early stages of Parkinson’s disease when symptoms are mild and don’t have a major impact on the patient’s lifestyle. These drugs effectively control sialorrhea (excessive flow of saliva) and are about 20% effective in reducing the incidence and severity of akinesia and rigidity.
Anticholinergics can be used alone or with amantadine in the ear-ly stages of Parkinson’s disease. In addition, anticholinergics can be given with levodopa during the later stages to further relieve symptoms.
Interactions can occur when certain medications are taken with anticholinergics:
· Amantadine can cause increased anticholinergic adverse ef-fects.
· Absorption of levodopa can be decreased, which could lead to worsening of parkinsonian signs and symptoms.
· Antipsychotics taken with anticholinergics (such as pheno-thiazines, thiothixene, haloperidol, and loxapine) decrease the ef-fectiveness of both anticholinergics and antipsychotics. The inci-dence of anticholinergic adverse effects can also be increased.
· Over-the-counter cough or cold preparations, diet aids, and analeptics (drugs used to stay awake) increase anticholinergic ef-fects.
· Alcohol increases CNS depression. (See Adverse reactions toanticholinergics)