Adjuvants/co-analgesics: anticholinergic drugs
Antimuscarinic drugs include hyoscine salts, atropine, propantheline, oxybutynin and glycopyrrolate. Their smooth muscle relaxant and antisecretory properties are utilised for therapeutic effect, but anticholinergic adverse effects are prominent.
Hyoscine butylbromide is used to treat painful colic resulting from malignant bowel obstruction, and to reduce gastrointestinal secretions. It does not cross the blood–brain barrier so does not cause drowsiness or delirium. Its duration of action is less than 2 hours. It can be used subcutaneously.
The smooth muscle relaxant action of propantheline and oxybutynin are used in the relief of urinary urge incontinence and bladder spasm. Oxybutynin has an oral bioavailability of approximately 6% and a plasma half-life of 2 to 3 hours (longer in the elderly). Its duration of action is 6 to 8 hours. For more information on use of these drugs to treat bladder symptoms,
Anticholinergics used for gastrointestinal indications include propantheline, hyoscine salts and derivatives, and hyoscyamine. These drugs reduce gastric acid secretion and gastrointestinal motility. The quaternary ammonium derivatives (eg hyoscine butylbromide, propantheline) do not cross the blood-brain barrier and therefore have no significant central anticholinergic effects. However, they do inhibit other cholinergic receptors in salivary glands, the pancreas and the bladder. (The other hyoscine salt available in Australia, hyoscine hydrobromide, is a tertiary amine and readily crosses the blood-brain barrier. This is important in its therapeutic use in nausea and vomiting.)
The belladonna alkaloids atropine and hyoscine hydrobromide have been used in palliating accumulated secretions and noisy breathing at the end of life. Atropine is also used as an antiarrhythmic (see Other antiarrhythmic drugs: atropine).
Glycopyrrolate is used to reduce secretions in the terminal phase and to reduce drooling in neurodegenerative conditions. It does not cross the blood–brain barrier. Its onset of action is 20 to 40 minutes and its plasma half-life is approximately 2 hours. Its duration of action is 7 hours.
Adverse effects and precautions: Anticholinergic adverse effects include dry mouth, decreased sweating, blurred vision, urinary hesitancy and retention, constipation and tachycardia. Because of the generalised anticholinergic effects, they should not be given to patients with glaucoma or significant prostatism. Hyoscine hydrobromide and atropine both cross the blood–brain barrier and can cause sedation, cognitive dysfunction and delirium (particularly in the elderly). In the terminal phase their sedative actions may be an advantage.
Comments
Post a Comment