What causes anal fissures in adults?

1. What a fissure actually is — the basic mechanics

An anal fissure is a longitudinal tear of the anoderm (the lining of the anal canal) usually located distal to the dentate line; when the thin mucosa is stretched or cut it exposes muscle beneath and can trigger severe pain and bleeding ^{[2] [4]}. The exposed internal sphincter often goes into spasm after the tear, which increases pain and can pull the wound edges apart, slowing or preventing healing ^{[2] [3]}.

2. The common, obvious causes: constipation, hard stools, and diarrhea

Clinical overviews repeatedly identify passage of large, hard stools and straining from constipation as the leading immediate causes in adults — the mechanical stretching or tearing during defecation is the direct trauma that creates most fissures ^{[1] [3] [5]}. Conversely, prolonged or recurrent diarrhea can also irritate and shear the anoderm repeatedly, producing fissures through persistent irritation rather than a single hard stool event ^{[4] [6]}.

3. Why some fissures become chronic: sphincter tone and blood flow

Authors note that many fissures are initially acute and self-limited, but chronic fissures result when internal anal sphincter hypertonicity causes reduced perfusion to the tear, sustaining ischemia and a non-healing ulcer; pain-induced sphincter spasm therefore both follows and perpetuates the problem ^{[2] [3]}. This pathophysiology is why treatments sometimes target muscle relaxation (topical nitrates, calcium-channel blockers, botulinum toxin, or surgical sphincterotomy) to restore blood flow and allow healing ^[7].

4. Secondary and less common causes clinicians look for

When fissures are atypical (multiple, off-midline, large, or recurrent), clinicians consider secondary causes: inflammatory bowel disease (Crohn’s), sexually transmitted infections, HIV, tuberculosis, anorectal cancer, prior anorectal surgery, obstetric trauma, and anoreceptive intercourse — each can produce lesions by inflammation, infection, or direct trauma ^{[2] [6] [8]}. StatPearls and specialty centers emphasize investigating these when the fissure’s pattern or clinical course is unusual ^{[2] [8]}.

5. Epidemiology and age patterns — who gets them and when

Sources indicate fissures occur at any age but are commonly seen in young adults and are very common in infants; susceptibility tends to decline with age though older adults may be at risk from poorer blood flow or other comorbidities ^{[9] [10] [5]}. Reliable precise incidence figures are limited in the general literature, and some sources give differing context or estimates ^{[7] [5]}.

6. Practical implications: prevention, red flags, and when to investigate further

Because mechanical trauma from stool consistency is central, prevention focuses on avoiding constipation and hard stools (dietary fiber, fluids, stool softeners as needed) and addressing chronic diarrhea or infections ^{[11] [4]}. Red flags prompting further workup include atypical fissure features (off-midline, multiple, or non-healing), systemic symptoms, or risk factors for IBD or malignancy — in those cases clinicians may order endoscopy or tests for infections ^{[2] [8]}.

7. Competing perspectives and limits of current reporting

Most sources agree on trauma from hard stools and sphincter spasm as the central mechanism ^{[1] [2] [3]}. Variation appears in emphasis: some patient-focused resources highlight constipation and bowel-habit changes for prevention ^{[11] [10]}, while surgical and specialty reviews stress sphincter physiology and the role of secondary causes when fissures are chronic ^{[2] [7]}. Available sources do not mention precise population-wide incidence rates with consistent methodology — estimates and phrasing differ across summaries ^{[7] [5]}.

If you want, I can summarize typical self-care steps and the main medical treatments tied to these causes, or outline when to seek a colorectal specialist versus primary care follow‑up (sources: ^[1]; ^[7]; ^[1]3).