Question

cholinergic antagonist are contraindicated I MG, tachy-dysrhythmias, MI, glaucoma, prostatic hypertrophy, hyperthyroidism, pregnancy and lactation. why is cholinergic antagonist contraindicated and what is the mechanism and disease pathophysiology.

Answer 1

• Cholinergic antagonist are drugs that block the action of acetylcholine
  • Examples of the cholinergic antagonist are - Atropine, homatropine, tropicamide
• Contraindications are conditions in which a particular drug should not be used.
• The contraindications for cholinergic antagonist are:
  1. Myasthenia gravis
     • Myasthenia gravis is a condition characterized by antibodies to post-synaptic acetylcholine receptors.
     • This leads to the blockage of transmission of nerve impulses at the neuromuscular junction.
     • This leads to weakness
     • Anticholinergic drugs or cholinergic antagonists will block the action of acetylcholine and aggravated the defect.
  2. Tacy-arrhythmias:
     • Tachyarrhythmias are pathological conditions characterized by irregular rhythms with a rapid heart rate.
     • Acetylcholine increases the permeability of the cardiac muscle to potassium.
     • This leads to hyperpolarization of the cardiac muscle.
     • Therefore action potential doesn't get transmitted. This leads to a reduction in the heart rate.
     • By administering a cholinergic antagonist, there will be no hyperpolarization.
     • This will leads to a further increase in the heart rate.
  3. Glaucoma:
     • Glaucoma is a condition caused due to an increase in intraocular pressure.
     • Cholinergic antagonist lead to pupillary dilatation.
     • This results from the contraction of the dilator pupillary muscles.
     • Dilation of the pupil leads to shortening of the iris.
     • In this situation, the iris can attach to the anterior surface of the lens.
     • This can obstruct the normal aqueous humor flow.
     • This increases intraocular pressure.
     • Therefore, cholinergic antagonist drugs are contraindicated in glaucoma.
  4. Prostatic hypertrophy:
     • The urinary bladder receives its parasympathetic supply from the pelvic nerves.
     • This nerve leads to contraction of the detrusor muscle and relaxation of the urethra.
     • Prostatic hypertrophy causes mechanical blockage of the prostatic urethra.
     • The cholinergic antagonist will contract the urethra and relax the detrusor muscle.
     • This will cause urinary retention.
  5. Hyperthyroidism
     • Hyperthyroidism is characterized by an increase in the level of thyroxine.
     • This can leads to tachycardia, tremor, and increase sweating.
     • Cholinergic antagonists will cause tachycardia.
     • Therefore, the cholinergic antagonist will exacerbate the symptoms of hyperthyroidism.
  6. Pregnancy and lactation:
     • The cholinergic antagonist can pass through the milk and cross the placenta.
     • They can enter the fetus or baby's blood and lead to tachycardia.
     • Therefore, the cholinergic antagonist is contraindicated in pregnancy and lactation.