Constipation 1 - Anatomy and pathophysiology Flashcards
What diseases are associated with the Gastrointestinal Tract
- Gastroesophageal Reflux Disease (GERD) : esophagus - where the stomach pushes stomach acid up into the esophagus tract (causes pain and in long term can lead to significant pathology)
- Peptic Ulcer Disease (PUD) : stomach - stomach becomes infected with helicobacter pilori - stomach lining breaks down and stomach acid damages the lining of the stomach
- Nausea & Emesis - stomach - largely driven by the brain
- Duodenal Ulcer - in the duodenum
- Diarrhea & Constipation
- IBD (Crohn’s & Ulcerative Collitis); breakdown of LI lining - happens throughout intestine - can be very painful
- IBS : can happen throughout the gut, associated with constipation and diarrhea
Disorders of the Lower GI Tract
- Constipation
* Diarrhea
Functions of the large intestine
- Reabsorb water and compact material into faeces
- Absorb vitamins produced by bacteria
- Store fecal matter prior to defecation
The areas of the colon are:
- Ascending
- Transverse
- Descending
- Sigmoid
- Rectum
- Anal canal
Which areas of the colon are involved in reabsorption of water and vitamins
- Ascending
- Transverse
- Descending
Which areas of the colon are involved in packaging of the remaining material into faeces
- Sigmoid
- Rectum
- Anal canal
Motility in the Large Intestine
- The proximal half of the colon is concerned with absorption and the distal half with storage
- The transit of small labeled markers through the large intestine occurs in 36-48 hrs
Motility in the Large Intestine
• Movements of the colon
- Mixing movements (Haustrations) : mis contents of colon to aid with reabsorption and packaging remaining material into faeces
- Propulsive movements (Mass Movements) : keeps material moving through colon - peristaltic
The Defecation Reflex - overall summary
- extremely complex process
- once the rectum is built up with faeces it gets stretched and stimulates stretch receptors.
Activates nerves in the colon and rectum which increases peristalsis which pushes more material down into rectum and get more distension. (feedback loop) - second feedback loop where stretch receptors feed back up through vagal and parasympathetic neurons to stimulate motor neurons in spinal cord. this increases peristalsis throughout LI and pushes more material down into rectum
- That eventually leads to relaxation of the anal sphincter and the faeces move from the rectum into anorectal canal but also leads to contraction of external anal sphincter so that the material can be passed straight out of your body
- only when you voluntary relax external sphincter defecation occurs
The Defecation Reflex - each step
Step 1: distension of rectum
Step 2: stimulation of stretch receptors (short or long reflex)
Step 3a: stimulation of myenteric plexus in sigmoid colon and rectum (short reflex)
Step 3b: stimulation of parasympathetic motor neurons in sacral spinal cord long reflex)
Step 3c: stimulation of somatic motor neurons
Step 4a: increased local peristalsis - back to step 1
Step 4b:increased peristalsis throughout LI - back to step 1
draw diagram
Are there any feedback loops in The Defecation Reflex
2 feedback loops
Diagnosis of constipation: The “Rome criteria”
- Two of the following present for >3 of past 6months:
- Straining at stool at least 25% of the time
- Hard stools at least 25% of the time
- A feeling of incomplete evacuation at least 25% of the time
- A feeling of anal blockage at least 25% of the time
- Manual maneuvers for rectal emptying at least 25% of the time
- Two stools or less per week
Pathophysiological components of chronic constipation
- Abnormal intrinsic motility
• Lack of luminal factors (stretching, chemical
and tactile stimuli)
• Medications
• Lack of extrinsic innervation (in paraplegia)
• Hormones (very rarely, e.g., in pheochromocytoma - Impaired defecation)
What type of motility is more prone to constipation
low motility more prone to constipation
What is paraplegia
where neuronal pathways or peristalsis throughout colon may be broken
Iatrogenic constipation
• Many medicines cause constipation
- pain medications, iron, calcium, blood pressure medications, etc
• Opioid-induced constipation in palliative care common
• Major cause of distress for the patient and the caring family.
Iatrogenic meaning
when a disease/ certain symptom is caused by medical intervention
Opioids cause constipation by…
- Increasing smooth muscle tone
• Suppresses forward peristalsis
• Increases tone in anal sphincters (less able to relax)
• Increases transit time and water absorption (more material in gut for longer but less water to lubricate it and make it soft) - Reduces sensitivity to anal distension
• Reduces urge to defecate (leads to build up of faecal matter)
Laxative Misuse
•Most common cause of constipation!
- Longer interval needed to refill colon is misinterpreted as constipation which leads to further laxative use
• Enteral loss of water and salts causes release of aldosterone
- stimulates reabsorption in intestine, but increases renal excretion of K+
• double loss of K+ causes hypokalemia, which in turn reduces peristalsis.
- Misinterpreted as constipation
- Further laxative use
Diagram of colon before and after laxative use
draw
aldosterone and K involving loop
constipation turns to laxative use which leads to loss of K+ with the faeces and that also leads to production of aldosterone which stimulates kidneys to retain sodium and water but loss of K+
All that loss of K+ can lead to hypokalemia which can lead to loss of activity in the bowel (bowel inertia) which can be sensed as constipation so more laxative taken
Impaired defecation
A - holding: puborectalis, external and internal anal sphincters contracted (muscles close anus and prevent faeces from being passed whilst walking)
B - initiation: puborectalis and external anal sphincters relax. Levator ani, abdominals and diaphragm contract (when you get faecal reflex you initial relaxation of puborectalis and external anal sphincter
C - completion: internal and external sphincters relax. Rectum contacts
• Loss of feedback between components of the faecal reflex can lead to impaired defecation so you don’t pass faeces when they’re ready to be passed
Faecal impaction
Impaired defecation leads to faecal matter becoming hard and difficult to pass so leads to further build up due to blockage in descending colon. Leads to constipation
Summary
• Constipation is a symptom not a disease
– Underlying causes of constipation should be
considered
• Treatment can usually be non-pharmaceutical
– Diet (increasing fibre), hydration (increases volume) and exercise (increases motility of colon)
• Only if these fail do we turn to pharmacotherapy (bulking laxative)
What is the GI tract involved in
It is involved in the absorption of nutrients and excretion of waste products
Process of food moving through GI tract
- Start with esophagus where food enters the GI tract from your mouth and it then enters the stomach, SI then LI/colon where it is processed for excretion in faeces by rectum and anus
