Perianal abscess

Aetiology

Typically results from infection of anal glands secondary to obstruction of the draining duct by faecal debris. This leads to stasis, bacterial overgrowth, and ultimately inter-sphincteric abscess.
The abscess can then expand caudally to the anoderm (perianal abscess) or across the external sphincter into the ischiorectal fossa (ischiorectal abscess), or less commonly, into the supralevator or submucosal spaces (see below).
About 10% of abscesses occur due to other processes

Indolent onset of constant, throbbing anal pain with localised swelling, erythema and fluctuance
Even if no fluctuance is palpable, most patients with erythema and pain have some sort of deeper abscess
If the diagnosis is uncertain (pain and sepsis but no visible or palpable abscess), a pelvic CT or EUA is best

Perianal (60%)
- small, tender, red fluctuant swelling near anus
Ischiorectal (20%) - larger, deeper, diffuse, tender, indurated area within buttocks. Penetrate through external anal sphincter. Can extend bilaterally to become a horseshoe abscess - if untreated can spread to lower abdominal wall, scrotum, and perineum. Deep post-anal space infections leading to horseshoe abscess typically have a posterior midline internal opening.
- Gluteal induration, tenderness and fluctuance without tenderness on DRE
Intersphincteric (5%) - only about 5% of perianal abscesses. No skin changes, just a fluctuant mass penetrating into lumen.
- Exquisite tenderness on DRE, but no external findings
Supralevator (4%) - forms EITHER BY an obstructed gland process tracking superiorly, OR spread from pelvis (Crohn's?). Difficult to diagnose clinically - feel for induration above anorectal ring, but may be a/w severe pain, fevers, urinary retention. May require CT to diagnose.
- May have no anorectal exam findings
- Often treated by IR or abdominal drainage rather than EUA and I+D
Submucosal/'gluteal' (1%)
- No anal/rectal involvement
Postanal space
- Localised tenderness posterior to the anal verge, often without induration or fluctuance
- Can extend into bilateral ischiorectal fossas to become a horseshoe abscess.

Anal fissure - usually posterior tear below dentate line - this pain is more likely to be only a/w passing stool
Thrombosed haemorrhoid
Anal fistula
Prolapsed internal haemorrhoid
Pilonidal sinus
Buttock skin abscess eg carbuncle
Bartholin cyst
Hidradenitis suppuritiva
Be concerned for Crohn's with large, thick skin tags, fissures off the midline, or complex fistulae

Immunocompromised neutropaenic - if they don't have enough neutrophils to mount a suppurative response, there won't be a target for drainage. In this case, long-term Abx is warranted. Imaging or EUA may help to identify a target.
- Broad-spectrum antibiotics
- Bone marrow stimulating growth factors
- Sitz baths
- Stool management
Necrotising infections - rapid onset of disproportionate pain, especially in obese smokers with diabetes and renal impairment or neurologic conditions.