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Pathophysiology
Clinical meaning
An anal fissure is a longitudinal tear or ulceration in the anoderm (squamous epithelium lining the anal canal) that extends from the anal verge proximally toward, but not beyond, the dentate line. The condition is one of the most common causes of anorectal pain and rectal bleeding, yet it is frequently underreported because patients may be embarrassed to discuss anal symptoms. Understanding the anatomy of the anal canal and the pathophysiology of fissure formation is essential for the practical nurse to provide effective assessment, patient education, and symptom management.
The anal canal is approximately 3-4 centimeters long and extends from the anorectal junction (dentate line) to the anal verge (where the canal meets the perianal skin). It is lined by two types of epithelium: columnar epithelium above the dentate line and squamous epithelium below. The squamous epithelium below the dentate line is exquisitely sensitive to pain because it is innervated by somatic nerves (inferior rectal nerve), which explains why anal fissures are so painful. The internal anal sphincter (IAS) is a smooth muscle ring that provides 70-85% of resting anal canal pressure and is under involuntary control. The external anal sphincter (EAS) is a striated muscle under voluntary control that provides squeeze pressure during continence.
Approximately 90% of anal fissures occur in the posterior midline of the anal canal. This location is vulnerable because the posterior commissure has the poorest blood supply of any area in the anal canal. The inferior rectal artery branches in the posterior midline are sparse and must traverse the internal sphincter muscle to reach the anoderm, creating a relative watershed zone of ischemia. When a hard, large stool passes through the anal canal, the mechanical trauma tears the anoderm, particularly in this poorly perfused posterior region.
The initial tear triggers a pathological cycle that perpetuates the fissure and prevents healing. The exposed internal sphincter fibers beneath the torn anoderm go into spasm (hypertonicity), which further reduces blood flow to the posterior midline by compressing the already sparse arterial supply. Reduced blood flow impairs tissue repair and healing. Additionally, the sphincter spasm increases resting anal pressure, which makes subsequent bowel movements more traumatic to the fissure. Each bowel movement reopens the healing wound, perpetuating the cycle of injury, spasm, ischemia, and impaired healing.
Anal fissures are classified as acute or chronic based on duration and morphological features. Acute fissures appear as fresh, superficial tears in the anoderm with sharp, well-defined edges and a red base. They have been present for fewer than 6-8 weeks and most heal with conservative management. Chronic fissures have been present for more than 6-8 weeks and develop characteristic features: a sentinel tag (a skin tag at the external aspect of the fissure), hypertrophied anal papilla at the internal aspect, and exposed internal sphincter fibers at the base of the ulcer. The sentinel tag is a fibrotic skin fold that forms as a result of chronic inflammation and lymphatic drainage impairment.
Secondary anal fissures occur in atypical locations (lateral, anterior, or multiple) and suggest underlying pathology. Conditions that cause secondary fissures include Crohn disease (which can cause fissures in any location with characteristic deep, undermined edges), sexually transmitted infections (syphilis, herpes simplex, HIV), tuberculosis, leukemia, and prior anorectal surgery. Any fissure that is not in the posterior midline, does not heal with standard therapy, or recurs frequently should prompt investigation for underlying causes.
The mainstay of fissure treatment is to break the cycle of sphincter spasm, ischemia, and impaired healing. Conservative measures include increasing dietary fiber and fluid intake to soften stools, sitz baths in warm water for 10-15 minutes three to four times daily to promote sphincter relaxation and blood flow, and topical analgesics for pain relief. Chemical sphincterotomy using topical agents (nitroglycerin ointment or diltiazem cream) reduces internal sphincter pressure by 30-50%, improving blood flow to the fissure and promoting healing. Surgical lateral internal sphincterotomy is reserved for chronic fissures that fail medical management and involves dividing a portion of the internal sphincter muscle to permanently reduce sphincter pressure.
The practical nurse is responsible for patient education regarding bowel habits, sitz bath technique, proper medication application, and recognition of complications. Pain management is a priority because the severe pain associated with bowel movements can lead to stool withholding behavior, which paradoxically worsens constipation and creates harder stools that cause more trauma. Breaking this behavioral cycle through education, reassurance, and effective analgesia is a key nursing intervention.
Exam Focus
Exam relevance
Risk factors:
- Chronic constipation with straining at stool (most common cause -- passage of hard, large stools traumatizes the anoderm)
- Low dietary fiber intake (leads to hard stools and increased straining during defecation)
- Chronic diarrhea (frequent liquid stools cause chemical irritation and maceration of the anal mucosa)
- Vaginal delivery (especially with prolonged second stage of labor or large birth weight, causes perineal and anal trauma)
- Crohn disease (causes secondary fissures that are often multiple, lateral, deep, and slow to heal)
- Prior anorectal surgery (scar tissue, altered sphincter mechanics, and reduced blood supply)
- Hypertonic internal anal sphincter (elevated resting sphincter pressure reduces blood flow to the posterior commissure)
Diagnostics:
- Visual inspection of the perianal area: gently separate the buttocks to visualize the fissure; posterior midline location is most common; note presence of sentinel tag indicating chronicity
- Digital rectal examination: may need to be deferred initially due to severe pain and sphincter spasm; when performed, assess for sphincter tone, masses, and tenderness localization
- Anoscopy: direct visualization of the anal canal to assess fissure depth, length, and features (chronic vs acute); may require topical anesthesia due to pain
- Complete blood count (CBC): if significant rectal bleeding is reported, to assess for anemia from chronic blood loss
- Fecal occult blood test: confirms presence of blood in stool when gross bleeding is not evident
- Colonoscopy: indicated for fissures in atypical locations, multiple fissures, recurrent fissures not responding to treatment, or when Crohn disease or malignancy is suspected
Core concept
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Clinical scenario
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Takeaways
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