INDICATIONS AND USAGE Trihexyphenidyl HCl is indicated as an adjunct in the treatment of all forms of parkinsonism (postencephalitic, arteriosclerotic, and idiopathic).

It is often useful as adjuvant therapy when treating these forms of parkinsonism with levodopa.

Additionally, it is indicated for the control of extrapyramidal disorders caused by central nervous system drugs such as the dibenzoxazepines, phenothiazines, thioxanthenes, and butyrophenones.

WARNINGS Patients to be treated with trihexyphenidyl HCl should have a gonioscope evaluation prior to initiation of therapy and close monitoring of intraocular pressures.

The use of anticholinergic drugs may precipitate angle closure with an increase in intraocular pressure.

If blurring of vision occurs during therapy, the possibility of narrow angle glaucoma should be considered.

Blindness has been reported due to aggravation of narrow angle glaucoma (see CONTRAINDICATIONS and ADVERSE REACTIONS ).

Trihexyphenidyl HCl should be administered with caution in hot weather, especially when given concomitantly with other atropine-like drugs to the chronically ill, alcoholics, those who have central nervous system disease, or those who do manual labor in a hot environment.

Anhidrosis may occur more readily when some disturbance of sweating already exists.

If there is evidence of anhidrosis, the possibility of hyperthermia should be considered.

Dosage should be decreased so that the ability to maintain body heat equilibrium via perspiration is not impaired.

Severe anhidrosis and fatal hyperthermia have occurred with the use of anticholinergics under the conditions described above.

Neuroleptic Malignant Syndrome A potentially fatal symptom complex sometimes referred to as Neuroleptic Malignant Syndrome (NMS) has been reported in association with dose reduction or discontinuation of trihexyphenidyl.

Clinical manifestations of NMS are hyperpyrexia, muscle rigidity, altered mental status and evidence of autonomic instability (irregular pulse or blood pressure, tachycardia, diaphoresis and cardiac dysrhythmias).

The diagnostic evaluation of patients with this syndrome is complicated.

In arriving at a diagnosis, it is important to identify cases where the clinical presentation includes both serious medical illness (eg, pneumonia, systemic infection, etc.) and untreated or inadequately treated extrapyramidal signs and symptoms (EPS).

Other important considerations in the differential diagnosis include central anticholinergic toxicity, heat stroke, drug fever, and primary central nervous system (CNS) pathology.

ADVERSE REACTIONS Minor side effects, such as dryness of the mouth, blurred vision, dizziness, mild nausea or nervousness, will be experienced by 30 to 50 percent of all patients.

These sensations, however, are much less troublesome with trihexyphenidyl HCl than with belladonna alkaloids and are usually less disturbing than unalleviated parkinsonism.

Such reactions tend to become less pronounced, and even to disappear, as treatment continues.

Even before these reactions have remitted spontaneously, they may often be controlled by careful adjustment of dosage form, amount of drug, or interval between doses.

Isolated instances of suppurative parotitis secondary to excessive dryness of the mouth, skin rashes, dilatation of the colon, paralytic ileus, and certain psychiatric manifestations such as delusions, hallucinations, and paranoia, all of which may occur with any of the atropine-like drugs, have been reported rarely with trihexyphenidyl HCl.

Potential side effects associated with the use of any atropine-like drugs, including trihexyphenidyl HCl, include cognitive dysfunctions, including confusion and memory impairment; constipation, drowsiness, urinary hesitancy or retention, tachycardia, dilation of the pupil, increased intraocular pressure, choreiform movements, weakness, vomiting, and headache.

Exacerbation of parkinsonism with abrupt treatment withdrawal has been reported.

Neuroleptic malignant syndrome with abrupt treatment withdrawal has been reported ( see WARNINGS , Neuroleptic Malignant Syndrome ).

The occurrence of angle-closure glaucoma in patients receiving trihexyphenidyl HCl has been reported (blindness has been reported in some cases).

Paradoxical sinus bradycardia, dry skin, and cycloplegia have been reported.

In addition to adverse events seen in adults, the following adverse events have been reported in the literature in pediatric patients: hyperkinesia, psychosis, forgetfulness, weight loss, restlessness, chorea, and sleep alterations.