Constipation Flashcards
Pearls - Constipation
- When writing an order for an opioid, order a laxative
2. Check the rectum as part of assessment of patients who are constipated
Risk factors for constipation
- Lack of fluid or fibre in diet
- Malnutrition
- Decreased mobility
- Environmental factors
- Medications (diuretics, antacids, opioids, sedatives, antipsychotics, anticholinergics, TCAs, NSAIDs, antihistamines, iron, ondansetron, vincristine)
- Metabolic disorders (dehydration, hypercalcemia, hyponatremia, uremeia, hypothyroidism, DM)
- Advanced age or altered attention to rectal fullness
- Depression
- Chronic illness
- Malignancy affecting bowel, spinal cord
- Neurologic disorders (spinal cord compression, autonomic dysfunction)
- Anatomic abnormalities (masses in the pelvis/abdo, adhesions)
- Bowel factors (diverticular disease, rectocele, anal fissuer or stenosis, colitis, hemorrhoids)
In advanced cancer, most common culprits are opioids and debility.
Physiology of Gut Motility
- Coordination of gut motility (coordinated through the myenteric plexus autonomic nervous system
- Acetylcholine (ach) and vasoactive intestinal peptide mediate peristalsis. ACh mediates ascending contraction, vasoactive intestinal peptide mediates descending relaxation. Further modulated by opioid receptors and anticholinergics, which can disrupt peristalsis.
- In the intestines, more mixing than propulsion to facilitate bacterial and enzymatic breakdown.
- Colonic transit: Episodic forward peristalsis about 6 times per day, with a largest one on wakening/breakfast and a smaller one at the midday meal. Frequency reduced by inactivity
Gut contents spend 2-4 hours in small bowel, then 24-48 hrs in colon (but may be up to 12 days!)
Pathophysiology of Constipation with Opioids
- Opioids increase fluid reabsorption by prolonging transit time (more reabsorption) and inhibition of secretomotor neurons, resulting in less fluid in the gut.
- Opioids reduce peristaltic propulsion and enhance non-propulsive contractions
- Opioids reduce sensation of gut distention, including rectal distention, which may increase risk of fecal impaction.
- Opioids increase rectal sphincter tone
Typical causes of constipation result in increased fluid reabsorption and prolonged transit time
How to approach laxatives with opioids
- Always prescribe a prophylactic bowel regimen
- PRN laxatives have little place in the prophylaxis of opioid-induced constipation - may cause alternation between constipation and diarrhea
- Titrate laxative dosing according to response (not opioid dose - constipating effects of opioids are not dose dependent)
- TD Fentanyl patch has decreased constipation when compared to morphine (based on inconsistent effect)
- Methadone may also reduce constipation and laxative requirements due to action on NMDA receptors
- Tramadol may be less constipating as actions are partly mediating by other receptor types
Approach to treating constipation
- Individual symptoms, performance status, and preferences need to be considered
- Attention to laxative dosing can halve use of enemas and suppositories, which patients typically dislike.
- Exclude intestinal obstruction and treat correctable causes (e.g. TSH, hypercalcemia, etc.)
- PEG and/or stimulant
- If no improvement, rectal supp/enema, consider methylnaltrexone if on an opioid
- If no improvement, manual evacuation, consider methylnaltrexone if on an opioid
Continue with regimen and encourage lifestyle changes.
Stimulant laxatives (onset, side effects, mechanism of action)
- Bisacodyl, Senna
- Onset within 6-12 hours (dose at bedtime)
Side Effects:
- Cramping (may be reduced by BID dosing)
- Lyte disturbances
Mechanism of action: Stimulates myenteric plexus, increasing motility, decreasing fluid/lyte absorption
Indications: Reasonable choice for combating opiod-induced bowel dysfunction.
Components of a constipation history
- Present pattern of BMs
- quantity, consistency of stool
- Difficulty with defacation (pain, hardness, etc.)
- Normal pattern of BMs prior to illness or med change (‘normal’ is q3 days to 3x a day)
- Colour (GI bleeding)
- Associated GI symptoms
- Medication history
Physical examination for constipation
- Inspection
- Auscultation (all 4 quadrants), list for low-pitched, infrequent sounds.
- Palpation (including of bladder, which may be distended in severe constipation)
- Inspection of the anal area for fissures, tears, fistula, skin changes (resulting in avoidance of defecation and constipation)
- Volume status assessment (cause or consequence of constipation)
- Rectal examination (unless severe neutropenia or septicemia) - assess for loaded rectum or empty but ballooned rectum (fecal mass obstructing beyond the finger)
Assessment of constipation
Visual Analog Scale (1-10)
Victoria Bowel Performance Scale (assess both constipation and diarrhea).
Consider TSH, Corrected calcium if clinical picture is suggestive.
- Ask patient number of episodes of formed stool leading to a sense of reasonably complete rectal evaluation, as compared to previous normal pattern.
May also use Bristol Stool Scale as a way to estimate stool form as an adjunct to measuring frequency.
Imaging of the abdomen for constipation
Flatplat and upright AXR to evaluate fecal loading (note that this is contested)
- Scoring system to determine need for intervention. Colon divided into four sections (ascending, transverse, descending, recto-sigmoid) and stool versus air is noted. Points given:
0 = no stool
1 = stool < 50% of lumen
2 = stool > 50% of the lumen
3 = 100% of the lumen.
If all four quadrants >7, intervention required. - Note that AXR may differentiate between constipation and obstruction, but should otherwise not be a standard procedure.
Docusate for Constipation?
- No more effective than placebo for increasing stool frequency, and no more effective than placebo in alleviating symptoms associated with constipation (e.g. difficulty passing stool, straining, etc.)
Lifestyle Interventions for Constipation
- Adequate fluid intake (at least 1.5 L per day if taking fibre supplements)
- Physical activity
- Defecating when the urge hits, comfort and privacy to do so
- Abdominal massage
- Treating other context (e.g. pain and breathlessness to maximize mobility, nausea to maximize intake)
Bulk-forming laxatives (onset, side effects, mechanism of action)
Bran, Psyllium, Methylcellulose
Onset 24-72 hrs
Side effects:
Bloating and flatulence
Mechanism of action:
Increased stool bulk, decreased transit time, increased GI motility. Requires adequate fluid intake, often inappropriate in advanced disease as it may precipitate obstruction.
Surfactant Laxatives (onset, side effects, mechanism of action)
Docusate (24-72 hours)
Side effects: can cause cramps, diarrhea, nausea
MOA: Increased water penetration, detergent activity. Avoid mineral oil concurrently. Ineffective if overall hydration is poor, not recommended in recent guidelines. Consider in patients with painful defecation.
Mineral oil (6-8 hours)
Side effects: Malabsorption of fat soluble vitamins, other meds, nutritions. Risk of lipoid pna if aspirated, not recommended PO in palliative patients
MOA: Lubricates, softens stool
