GASTRO Anti-emetics/ constipation/ diarrhoea/ colic

Question 1

Brief physiological summary of nausea

Answer 1

Vomiting centre is in the medulla.

Pathways converge here from input in the chemoreceptor trigger zone - this is innervated by the vagus nerve.

Dopamine acts on D₂ receptors which is the main receptor in the chemoreceptor trigger zone.

Dopamine is also an important neurotransmitter in the gut - it relaxes the stomach.

Drugs that BLOCK D₂ receptors have a PROKINETIC effect - promote gastric emptying.

Question 2

Dopamine and nausea

Answer 2

D₂ receptors are in the chemoreceptor trigger zone (CTZ).

Drugs that block D₂ receptors relieve nausea by blocking CTZ stimulation, and reduce gut motility.

Question 3

Since D₂ blocking drugs treat nausea by their prokinetic action (and < CTZ stimulation), what is their main side effect?

Answer 3

Diarrhoea

Question 4

Name two dopamine D_2-receptor antagonists used to treat nausea

Answer 4

Metoclopramide

Domperidone

Question 5

What are the side effects of Metoclopramide?

Answer 5

1. Diarrhoea
2. Dystonia, such as oculogyric crisis - extrapyramidal syndrome

(note - domperidone doesn’t cross the blood-brain barrier, thus doesn’t cause extrapydramidal symptoms)

Note: warn patient of possibility of muscle spasms/ abnormal movements.

Question 6

What anti-emetic can be tried if normal PPIs are ineffective for indigestion?

Answer 6

Metoclopramide - as an add-on treatment.

Question 7

Name the two phenothiazines for treating nausea

Answer 7

prochlorperazine

chlorpromazine

Question 8

How to phenthiazines work? (prochlorperazine)

Answer 8

They block dopamine D₂receptors in the CTZ and gut

Question 9

What are the problems with phenothiazine (e.g. prochlorperazine) - 2

Answer 9

Postural hypotension (and drowsiness)

Extrapyramidal symptoms (due to dopamine blockage) - short-term use can cause oculogyric reaction

QT-interval prolongation

Reduce dose in elderly - RISK OF INCREASED FALLS

Question 10

Name four anti-spasmodics

Answer 10

Alverine citrate

Mebeverine (Colofac)

Hyoscine (Buscopan)

Peppermint oil

Question 11

What can alverine citrate be used for? (3)

Answer 11

Abdo cramping,

e.g. IBS, diverticular disease, painful cramping periods.

Question 12

Name two bowel stimulants

Answer 12

Senna

Bisacodyl (Dulcolax)