Anorectal Structure and Function Flashcards
Describe the internal anal sphincter
- Consists of smooth muscle continuous with the inner circular muscle of the bowel.
- It is NOT under voluntary control
- It has a resting tone (contracted at rest)
Describe the external anal sphincter
- Consists of skeletal muscle continuous with the pelvic floor
- It is under voluntary control
- It has longitudinal muscle fibres
- It is lined by anoderm
The epithelial lining of the anus (external sphincter)
Anoderm
Where is the dentate line?
It bisects the anal canal
The dentate canal marks the separation between..
the hindgut (endoderm, GI tissue) and ectoderm (skin)
Which part of the anal canal is sensate? Which is insensate?
Sensate: ectoderm (skin)
Insensate: endoderm (hindgut)
Function of anal crypts and glands
Secrete mucus to lubricate the anus
Note can get infected and causes abscesses and fistulas
What is a hemorrhoid?
It is a normal anatomic structure - a sinusoidal cushion supplied by arterial blood (inferior rectal artery)
What is the purpose of hemorrhoids?
They engorge when abdominal pressure increases (e.g. when we sneeze or laugh) to prevent us from excreting feces.
May also prevent trauma (prevent injury during constipation, diarrhea)
Internal vs external hemorrhoids
Internal: Above the dentate line, visceral innervation, insensate
External: Near the anal verge, somatic innervation, sensate
Internal hemorrhoids are supplied by…
They are derived from….
Supplied by branches of the superior/middle rectal arteries.
Derived from endoderm
External hemorrhoids are supplied by…
They are derived from…
Supplied by inferior rectal arteries
Derived from ectoderm
What nerve innervates the external hemorrhoids
Pudendal nerve (somatic innervation of the anoderm, sensate)
Which hemorrhoids tend to be painful?
External hemorrhoids (they are sensate)
Common differential diagnoses for hemorrhoids (4)
Rectal prolapse
Anal fissures
Neoplasms
Anal warts (condylomas)
Internal hemorrhoids can occur 3 anatomic sites
Left lateral
Right antero-lateral
Right postero-lateral
External hemorrhoids do not tend to prolapse, but to…
thrombose (bleed, painful, purple)
Most common hemorrhoid symptoms
Painless bleeding (bright red blood during defecation)
often in association with hard stools, constipation, straining
Other than painless bleeding, 4 other common symptoms of hemorrhoids
- Anal pain (due to irritation of the anoderm)
- Tenesmus (sensation of incomplete evacuation, need to pass stools when rectum is empty)
- Itch (pruritus ani)
- Urgency
Describe the 4 grades of hemorrhoids
I: Prominent vessels, no prolapse
II: Prolapse with spontaneous reduction
III: Prolapse requiring manual reduction
IV: Chronically prolapsed, cannot be reduced
How do we treat internal hemorrhoids?
Grade 2,3 vs Grade 3,4
Stool bulking/softeners/warm baths (best treatment)
Do NOT use creams/suppositories
Avoid straining, prolonged pressures (like reading on the toilet)
Grade 2,3: Rubber band ligation, sclerotherapy, infrared coagulation
Grade 3,4: Surgical excision
How do we treat external hemorrhoids?
If thrombosed <48h: Surgical excision (do NOT incise them)
If >48h: Warm baths, stool softeners/bulkers (clot will reabsorb with time)
Excess removal (during surgical excision of hemorrhoids) can cause…
ana stenosis
How can we prevent recurrence of hemorrhoids? (2)
- High fibre diet and adequate hydration
- Proper toilet habits
What is an anal fissure?
A linear tear in the anoderm, distal to the dentate line
Symptoms of anal fissures (3)
- Extremely painful (burning, tearing, ripping).
- Lasting pain after bowel movement
- Bright red bleeding
So painful patients will usually refuse a DRE
Main cause of anal fissures
Trauma (from hard stools, chronic diarrhea)
Acute vs chronic anal fissures
Acute: simple tear in anoderm, will go away after a few days
Chronic: never heals, edges scar after 8-12 weeks but fissure stays open, inflamed, edematous
Signs of a chronically inflamed anal fissure (2)
- hypertrophied anal papillus
- sentinel piles (skin tag)
Where are anal fissures typically located?
In the anterior or posterior midline.
- 75% posterior
- 25% anterior (usually women)
- sometimes both
If not on the midline, you need to think of other diagnoses
What age category is usually affected by anal fissures?
Young patients (20-30 years)
How do anal fissures develop (pathophysiology)?
Patients who develop fissures have a hypertonic internal anal sphincter that spasms (stays tightly contracted for long periods instead of relaxing properly).
This can cause relative ischemia in anterior and posterior midlines.
When these patients pass hard stools or have diarrhea, they can tear the anal mucosa. The ischemia will then prevent healing.
The subsequent pain will discourage patients from passing stools, causing more constipation and more pain (vicious cycle).
If fissures occur outside of the midline, what are the potential diagnoses?
- Crohn’s disease/ulcerative colitis
- Anorectal trauma
- Infections (HIV/AIDS, syphilis, etc)
- Neoplasms (leukemia, lymphoma)
How do we treat anal fissures?
The goal is to break the cycle of pain/constipation:
* Stool bulking agents (fiber, psyllium, water)
* Warm sitz baths
* Topical anesthetics
This works for 80% of patients!
How do we treat anal fissures in patients for whom stool bulking, warm sitz baths and topical anesthetics do not work?
Topical nifedipine:
A calcium channel blocker (promotes relaxation of smooth muscle and internal sphincter)
True or false: Botox can be used to treat anal fissures
True: provides immediate and persistent muscle paralysis (3-6 months) - allows muscle to relax, improves blood flow and promotes healing
More invasive potential treatment of anal fissures
Surgical sphincterotomy (cutting part of the internal sphincter to release tension and promote blood flow)
May lead to continence issues (esp. pregnant women)
What is an anorectal abscess?
Accumulation of pus under the skin of the anus and rectum following an infection of the anorectal crypts/glands.
Most common cause of infection and abscess formation in the rectum
Cryptoglandular obstruction
Most common anorectal abscess
Perianal abscess
4 anorectal abscesses other than perianal
- Ischioanal
- Intersphincteric
- Supralevator
- Submucosal
How do we treat an abscess?
Abscesses must be drained (incision and drainage).
If an abscess is left untreated, it may become…
a fistula
Ascending and transverse colon (main function)
Churning/mixing of stool
Descending colon (main function)
Water absorption/stool delivery
Function of the rectum
To hold stool until the (socially) acceptable moment (this is called capacitance)
Function of the anus
“Cork” at the bottom of the rectum, which prevents involuntary stool loss
Do parasympathetic pathways generally…
a) promote GI motility
b) reduce GI motility
a) Parasympathetic pathways generally PROMOTE GI motility
(rest & digest)
How do sympathetic pathways affect GI motility?
Sympathetic pathways generally slow colonic (GI) motility
(fight or flight)
What types of contractions transport/transform stool in the colon? (2)
High amplitude propagated contractions (HAC): transport stool over long distances
* Occur after waking
* Occur after meals
Low amplitude propagated contractions (LAC): transport stools over shorter distances, but more frequently
Segmental contractions: non-propagating
What is the gastro-colic reflex?
When food enters the mouth, it stimulates colonic motility and segmental contractions
What is the recto-anal inhibitory reflex (RAIR)?
In response to rectal distension, the internal anal sphincter relaxes
What is the recto-anal excitatory reflex (RAER)?
In response to rectal distension, the external anal sphincter contracts
How do the RAIR and RAER interact to prevent inappropriate defecation
They work together to regulate defecation and maintain continence.
RAIR: When the rectum fills with stool, the internal anal sphincter (IAS) relaxes reflexively, reducing pressure in the anal canal. This allows for sampling of rectal contents (solid? air?).
RAER: At the same time, the external anal sphincter (EAS) and puborectalis muscle contract, maintaining continence by preventing involuntary stool passage.
These reflexes interact to allow voluntary control of defecation: If it is socially appropriate, voluntary relaxation of the EAS and puborectalis enables defecation. If not, the stool is pushed back into the rectum via contraction of the EAS and increased rectal compliance.
When a person chooses to defecate, what sequence of maneuvers/reflexes occur? (4)
- Valsalva maneuver (increased abdominal pressure and glottis closure)
- Puborectalis muscle (pelvic floor) relaxes
- EAS relaxes
- Rectal contents are emptied
Why is it easier to defecate when we squat down than when we sit?
A squatting position optimizes the angle at which the puborectalis muscle can fully relax.
Pro-defecatory stimuli (5)
- Exercise
- Distension (bulky stools, fibre)
- Waking up
- Eating
- Drugs (laxatives)
What is constipation?
Infrequent bowel movements, straining and hard stools.
Not the same as mechanical obstruction!
Who is more affected by defecatory disorders (constipation)?
a) women
b) men
a) women
Risk factors for constipation (3)
- Increasing age
- Low-fibre Western diet
- Decreased physical activity
In short, LIFESTYLE NEGLECT.
Some medical conditions that can cause constipation
IBS
Hypothyroidism
Medication use (narcotics, etc)
Before treating constipation, we need to rule out…
mechanical obstruction
How do we treat constipation? Is surgery indicated?
Colonoscopy strongly suggested
Lifestyle and dietary changes
Surgery is not indicated EXCEPT for true slow-transit constipation (neurologic disorder).
What is a true slow-transit constipation?
A neurologic disorder where the colon fails to pass stools.
Must be treated by surgery.
What type of laxatives are best to recommend?
Bulking laxatives, fibre-based laxatives (however, they require a lot of fluid intake)!
What is fecal incontinence?
Having insufficient voluntary control of gas or stool.
Note: This is NOT a diagnosis, but a symptom!
What is a major reason for admission to a nursing home?
Fecal incontinence (50% of nursing home residents are affected)
When a patient presents with fecal incontinence, it is important to define 3 key points:
- the cause of incontinence (diagnosis)
- degree of incontinence
- degree to which the patient is affected (impact)
Before treating fecal incontinence, we need to rule out…
“Pseudo”-incontinence
* Urgency and stool loss from poor rectal compliance (inflammatory disorders: Crohn’s, UC)
* Overflow incontinence from stool impaction/severe constipation
* Poor hygiene
* Anorectal STD
* Anorectal neoplasms
* Prolapse, hemorrhoids
1 cause of fecal incontinence
Obstetric:
1/3 of women will have occult or overt injury to their sphincters during delivery. This will manifest as fecal incontinence later in life (50s, 60s).
Other than obstetric, what are some other causes of fecal incontinence (3)
- Iatrogenic
- Congenital
- Rectal prolapse
Best test to assess fecal incontinence
Endoanal ultrasound (allows the evaluation of both the internal and external sphincters)
How do we treat fecal incontinence?
By treating the underlying condition!
Then normalize stool consistency, use biofeedback and physical therapy.
Main treatment for obstetric sphincter damage?
Overlapping sphincteroplasty (surgical repair of damaged sphincters).
However only 50% remain continent to both solid and liquid stool at 5 years
Current mainstay of treatment for fecal incontinence
Sacral nerve stimulation
(no surgery required, persists up to 10 years)
