Constipation causes

 

 

Treatment induced:

Drugs include opioids commonly used in palliative care for pain and

anticholinergics such as phenothiazines, tricyclic antidepressants, some classes of

antipsychotic drugs. 

 

 Opioids

Opioids can cause constipation by increasing the muscle tone in the anal sphincter, and inhibition of peristalsis. This occurs by stimulation of opioid receptors (mu and delta) which is present on the smooth muscles and the enteric nervous system of the intestine. The former receptor (mu) exerts a direct effect on the myenteric plexus which is a system of nerve fibers and neurones that enclose the oesophagus, stomach and intestine. The myenteric plexus plays a vital role in the forward peristalsis of food and since opioids possess an inhibitory effect, this reduces the excitability of neurones, the motility in GI tract is lowered. 

 

Anticholinergics 

 

The process of peristalsis can be classed into contraction (ascending) and relaxation

(descending) phases with neurotransmitters such as acetylcholine (Ach) and vasoactive

intestinal peptide (VIP) involved in the control. Ach mediates contraction and VIP mediates

relaxation. Anticholinergics tend to affect only part of the process i.e. the contraction phase and this causes constipation.

 

 Other drugs  

Diuretics disturb the level of potassium, (hypo and hyperkalaemia) which alter the activity of the nerve cells that innervate the GI system and subsequently  affect the excitability of the muscle cells involved in peristalsis.