Trihexyphenidyl hydrochloride is used for the adjunctive treatment of all forms of parkinsonian syndrome including the postencephalitic, arteriosclerotic, and idiopathic types. Trihexyphenidyl is also used for the relief of parkinsonian signs and symptoms of antipsychotic agent-induced (e.g., butyrophenones, phenothiazines, thioxanthenes) extrapyramidal effects.
Trihexyphenidyl may also be effective in diminishing the frequency and duration of oculogyric crises, in decreasing salivation, in reducing spastic contractions and involuntary movements characteristic of dyskinesia, and in relieving mental inertia and depression characteristic of all forms of parkinsonian syndrome. As with other antiparkinsonian drugs, tolerance to trihexyphenidyl may develop during prolonged use. The maximum therapeutic response attainable with trihexyphenidyl is in the range of 20-30% symptomatic improvement in 50-75% of patients. Frequently, the maximum response requires empiric combination of trihexyphenidyl with other antimuscarinic drugs or with antihistaminic or dopaminergic agents. Some clinicians believe trihexyphenidyl to be of little value, but the majority have found it a useful adjunct in the multidimensional therapeutic approach to parkinsonian syndrome. Trihexyphenidyl is effective as adjunctive therapy for parkinsonian syndrome in patients receiving levodopa.
Clinical results from preliminary trials with trihexyphenidyl in the treatment of other dyskinesias, Huntington's chorea, spasmodic torticollis, and associated disorders have been equivocal. The drug generally is not helpful in spastic states such as cerebral palsy and hemiplegia.