Term
| What are the 6 anatomical regions of the colon and their blood supply? |
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Definition
1-3 from midgut- Superior mesenteric A/V
4-6 from hindgut (upper rectum)- IMA/V
Lower rectum from cloaca and supplied by hemorhoidal vessels
1) Cecum
2) Ascending colon
3) Transverse
4) Descending
5) Sigmoid
6) Rectum |
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Term
| What is the functional importance of "taenia coli"? |
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Definition
3 bunches of longitudinal muscle that coalesce at the rectum.
Important for retarding propulsion and enhancing residence time in ascending colon (reabsorption and solidification of fecal material) |
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Term
| What are the 4 primary functions of the large intestine? |
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Definition
1) Segmentation
- Mixing and dehydration of fecal material
2) Compliance
- Storage until evacuation
3) Salvaging electrolytes and water
4) Fermentation of monosaccharides to yield FFA that are taken up by colonic epithelium |
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Term
| How does neural activity regulate colonic motility? |
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Definition
Serves a primarily modulatory role (intrinsic is more critical)
1) Parasympathetic is from vagus (right and proximal transverse) and pelvic (as far as mid-transverse)
- Vagus activates nAChR in VIP and NO-releasing myenteric plexus neurons
- Pelvic nerves evoke generalized colonic contractile activity to speed transit.
2) Sympathetic is from low C to L3, entering paravertebral ganglia and emerging as splanchnic nerves that pass to pre-aortic ganglia and give rise to post-ganglionic adrenergic fibers.
3) Non-adrenergic, non-cholinergic (NANC) also play a role |
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Term
| What is the intrinsic innervation of the colon and what myogenic elements regulate colonic motility? |
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Definition
1) Submucosal, Meisner's plexus for secretions and Myenteric plexus in between circular and longitudinal smooth muscle layers.
2) Electrical slow waves dictate contractions
- Short duration bursts are stationary motor patterns that facilitate water extraction (in phase with slow wave) in circular smooth muscle
- Long duration contrasting are important for segmentation and arise from contractile electrical complexes in longitudinal smooth muscle
- Giant migrating contractions (GMCs) are for mass fecal movement and arise from myenteric potential oscillations (induced by rectal distention and intra-colonic glycerol)
** Slow waves occur in longitudinal and circular smooth muscle, but are only coordinated by ICCS when required. |
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Term
| Why might you give atropine to a patient with IBD? What would you do if you wanted to treat constipation instead? |
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Definition
1) Stop the diarrhea!
Inhibit giant migrating contractions (GMCs) that arise from myenteric potential oscillations and cause mass movement of feces.
2) Glycerol activates these GMCs, so glycerol suppositories can be used! |
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Term
| What are the basic neurohumoral elements that regulate colonic motility? |
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Definition
Visceral afferents responding to wall tension increases colonic tone.
1) CCK, motilin, 5-HT and gastrin all excite contractions
- After meal, gastrocolonic response causes urge to defecate with GMCs (most important stimulant is fat)
2) Secretin, glucagon, VIP, neuropeptide Y and NO inhibit smooth muscle contractions |
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Term
| What is the functional anatomy of the anorectal canal and how does it maintain continence? |
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Definition
1) Internal anal sphincter (circular smooth muscle)
2) External anal sphincter (striated muscle innervated by pudendal nerve S2-4)
3) Puborectalis muscle (striated with efferent innervation by S2-4) maintains anorectal angle
Rectal sensation, storage capacity, resting IAS tone, contractile responses of PRM and EAS and motivation are all critical to maintain continence. |
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Term
| How is rectal and anal sensation mediated? |
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Definition
1) Mechanoreceptors in peri-rectal tissue
2) Sensory nerve endings for gas/liquid/solid distinctions in anal epithelium |
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Term
| What is "adaptive compliance" in relation to the rectum? |
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Definition
Determines rectal reservoir capacity
- Rectal pressures increase relatively little in response to increases in volume.
- Can be measured by rectal balloon (increased in Megarectum and Decreased in Proctitis) |
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Term
| What process is primarily disturbed in constipation? |
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Definition
Colonic or Anorectal motility
Can be classified as either
1) Normal transit
2) Colonic inertial (decreased motor activity after meals)
- transit proximal to rectum is delayed because of decreased propulsive activity (can occur in megacolon)
3) Outlet delay
- Delayed signmoid/rectum emptying because of disorder of anorectal function (Megarectum, Hirschsprung's, Dyssynergic defecation) |
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Term
| How should you treat a young women who has persistent difficulty following meals? |
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Definition
Most likely Colonic inertia constipation (common in women following meals)
Treat with osmotic laxative (Polyethylene glycol) + stimulant laxative (Anthraquinone or diphenylmethane) + Pro-kinetic agent. |
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Term
How can each of the following disorders cause outlet delay constipation?
1) Megarectum
2) Rectocele
3) Hischsprung's disease
4) Dyssynergic defecation |
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Definition
Anorecetal dysfunction.
1) Weak propulsion
2) Misdirection of propulsion
3) Failure of IAS relaxation
4) Failure of striated muscle relaxation |
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Term
A young boy presents with constipation, which he claims he has had since birth.
What tests would you run to diagnose/treat him? |
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Definition
Sounds like Hirschsprung's, which is congenital absence of intramural ganglion cells of submucosal and myenteric plexuses in the distal bowel, always involving the IAS and producing Megacolon.
**Requires surgery with excision (Swenson) or Bypassing diseases segment (Duhamel)**
1) Barium X-ray (Diagnostic in 80% of patients)
- demonstrate functional vs. mechanical obstruction
- demonstrate transition zone between ganglionic and aganglionic bowel.
2) Anorectal manometry
- Evaluate IAS tonic constriction
3) Rectal biopsy
- Normal # of submucosal ganglion cells excludes HD
- Stain for Acetylcholinesterase to distinguish heavily-staining nerves and ganglion cells |
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Term
| How does treatment for Desyinergic defecation differ from that used in Hirschsprung's disease? |
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Definition
1) HD is lack of innervation of IAS smooth muscle and requires surgery (bypass or excision or un-inervated tissue)
2) Dyssynergia is inability to relax PRM and/or EAS, an unconsciously learned behavior that can be modified in adults with biofeedback. |
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Term
A patient in the hospital who recently underwent a hip-replacment surgery presents with constipation, but a functional or mechanical obstruction cannot be found.
How do you treat? |
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Definition
Pseudo-obstruction from Acute Megacolon (imbalance between sympathetic and parasympathetic regulation of colonic motor activity).
1) Conservative
- Nothing by mouth
- Correct fluid and electrolytes
- Nasogastric suction
- Rectal tube decompression
- Stop offending medications
- Frequent position changes (ambulation is good)
2) Pharm
- if aggressive interventions are required, try Neostigmine (reversible AChE-i) to increase parasympathetic input.
3) Colonoscopic decompression (if 1-2 fail)
4) Surgical decompression (if ischemia or perforation). |
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Term
| What are the possible etiologies of Fecal Incontinence and how are they treated? |
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Definition
1) Overflow
- Fecal impaction/megarectum/blunting of rectal sensation
- Medical therapy only, with disimpaction, bowel cleansing and habit training.
2) Reservoir
- Decreased rectal compliance, or tumor
- Treat medically with reduced fiber, anti-inflammatories/Loperamide, or surgically with a colostomy.
3) Internal sphincter incontinence
- Weakness of IAS due to trauma, degeneration, autonomic
- Loperamide or cotton plug treatment.
4) Rectosphincteric incontinence |
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Term
| What systemic diseases are associated with rectosphincteric incontinence? |
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Definition
1) Diabetes (peripheral neuropathy)
- maintained rectal sensation, with decreased squeeze pressure and PRM contraction
2) Spinal chord injury
- Sacral chord lesions (no rectal sensation or tone)
- Suprasacral lesions (preserved reflex and prevented by planned defecation by rectal stimulation)
3) Systemic sclerosis (GI smooth muscle disorder)
- Atrophy of circular smooth muscle and replacement with collagen (EAS and PRM should be normal)
- lowered resting pressure of IAS only.
4) Multiple sclerosis (central neurological)
- Alterations of rectal sensation and EAS (pudendal nerve injury) |
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Term
What do each of the following tell you about the origin of rectrosphincteric fecal incontinence?
1) Lowered resting pressure only
2) Lowered squeezing pressure only
3) Lowered squeezing pressure and PRM contraction
4) Lowered squeezing pressure, PRM contraction and rectal sensation. |
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Definition
1) IAS weakness (Systemic sclerosis)
2) EAS trauma
3) Peripheral neurogenic (Diabetes)
4) Central neurogenic (MS) |
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