Anorectal Abscess

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Anorectal abscess

Anorectal abscess

An anorectal abscess is a pus-filled cavity that most commonly develops from an infected anal crypt gland (cryptoglandular infection) following obstruction and bacterial overgrowth**.** The infection begins in the intersphincteris space and may spread upward, downward, and laterally to involve other spaces around the anus and lower rectum. About 50% of anorectal abscesses heal within several weeks after drainage and do not recur. The remainder can form a fistula in ano. If anorectal abscesses remain undrained, necrotizing soft-tissue infection can occur. These carry a 50% mortality.

More common in men than women. Mean age of onset is 40 years (20 – 60 years range). 50% of abscesses develop into fistulas.

Anatomical locations of anorectal abscess

Anorectal abscess	Location
Ischiorectal abscess (most common)	Abscess beneath the levator ani, traverses the external sphincter into the ischiorectal space. Spreads laterally from the crypts, through the external sphincter and into fatty tissue of ischiorectal space
Perianal abscess	Abscess beneath the perianal skin, does not traverse the external sphincter. Extends distally from the crypts to skin surrounding anal verge
Intersphincteric abscess	Abscess between the internal and external sphincter. Confined to crypts and glands.
Supralevator abscess (least common)	Abscess above the levator ani, extension of intersphincteric abscess OR suppurative abdominal condition (appendicitis, complicated diverticulitis). Extends proximally from crypts past the levator ani muscles into the supralevator space.

Risk factors for anorectal abscess
- Flow obstruction and infection of anal crypt glands (MCC, 90%)
- Inflammatory bowel disease (chron’s, ulcerative colitis)
- Acute appendicitis, Complicated diverticulitis
- Radiation-induced proctitis (secondary to radiotherapy for ca prostate)
- Iatrogenic (post-hemorrhoidectomy via iatrogenic sinuses or fistulas)
- Foreign bodies (ingested toothpick or bones; stuff in anus >.<)
- Colorectal cancer
- Immunosuppression (may present with leukopenia or perianal pain without noticeable swelling or signs of infection)
Pathophysiology
- Obstruction of anal glands by debris → stasis and bacterial overgrowth → Abscess
- Extension of abscess into perirectal spaces (intersphincteric, transphincteric, suprasphincteric, extrasphincteric) → fistula formation, bacteremia, sepsis
Signs and symptoms
- Constant anorectal pain not associated or worsened with defecation
- Swelling at or around the rectum/anus
- Fever and chills
- Apparent cellulitis or abscess on buttock
- Area of focal tenderness on DRE
- Erythematous, subcutaneous mass near the anus = perianal abscess
- Rectal or perirectal drainage (bloody, purulent, mucoid) = perirectal abscess
- Purulent drainage from the anal canal or anoderm = Fistula
DRE findings for anorectal abscess
- Fluctuant, indurated mass
- Pain with pressure
Investigations
- CT or MRI scan: confirm location of deep abscess
- Anal ultrasound: confirm deep abscess
- Proctosigmoidoscopy or anoscopy: rule out other comorbidities (IBD, Malignancy)
- Fistula probe with methylene blue: identify possible fistulae and comorbidities (IBD, Malignancy)
Treatment
- Incision and drainage: stops further infection and tissue damage (can lead to sphincter damage, promote fistula formation, and lead to sepsis)
- Sitz bath
- Analgesics and stool softeners
- Post-op antibiotics: ciprofloxaxin and metronidazole to cover gram-negatives and anerobes.
Indications for post-operative antibiotics
- Immunocompromised patients
- Diabetic patients
- Patients with valvular heart disease

Reference Intervals ›

Biochemistry

ACTH	P: <80 ng/L
ALT	P: 5–35 U/L
Albumin	P: 35–50 g/L
Aldosterone	P: 100–500 pmol/L
Alk. phosphatase	P: 30–130 U/L
α-Amylase	P: 0–180 IU/dL
α-Fetoprotein	S: <10 kU/L
Angiotensin II	P: 5–35 pmol/L
ADH	P: 0.9–4.6 pmol/L
AST	P: 5–35 U/L
Bicarbonate	P: 24–30 mmol/L
Bilirubin	P: 3–17 μmol/L
BNP	P: <50 ng/L
CRP	P: <10 mg/L
Calcitonin	P: <0.1 mcg/L
Calcium (ionized)	P: 1.0–1.25 mmol/L
Calcium (total)	P: 2.12–2.60 mmol/L
Chloride	P: 95–105 mmol/L
Cholesterol	P: <5.0 mmol/L
VLDL	P: 0.128–0.645 mmol/L
LDL	P: <2.0 mmol/L
HDL	P: 0.9–1.93 mmol/L
Cortisol AM	P: 450–700 nmol/L
Cortisol Midnight	P: 80–280 nmol/L
CK ♂	P: 25–195 U/L
CK ♀	P: 25–170 U/L
Creatinine	P: 70–100 μmol/L
Ferritin	P: 12–200 mcg/L
Folate	S: 2.1 mcg/L
FSH	P: 2–8 U/L ♂; >25 menopause
GGT ♂	P: 11–51 U/L
GGT ♀	P: 7–33 U/L
Glucose (fasting)	P: 3.5–5.5 mmol/L
Growth hormone	P: <20 mu/L
HbA1C (DCCT)	B: 4–6%
HbA1C (IFCC)	B: 20–42 mmol/mol
Iron ♂	S: 14–31 μmol/L
Iron ♀	S: 11–30 μmol/L
Lactate (venous)	P: 0.6–2.4 mmol/L
Lactate (arterial)	P: 0.6–1.8 mmol/L
LDH	P: 70–250 U/L
LH	P: 3–16 U/L
Magnesium	P: 0.75–1.05 mmol/L
Osmolality	P: 278–305 mosmol/kg
PTH	P: 0.8–8.5 pmol/L
Potassium	P: 3.5–5.3 mmol/L
Prolactin ♂	P: <450 U/L
Prolactin ♀	P: <600 U/L
PSA	P: 0–4 mcg/mL
Protein (total)	P: 60–80 g/L
Red cell folate	B: 0.36–1.44 μmol/L
Renin (erect)	P: 2.8–4.5 pmol/mL/h
Renin (recumbent)	P: 1.1–2.7 pmol/mL/h
Sodium	P: 135–145 mmol/L
TBG	P: 7–17 mg/L
TSH	P: 0.5–4.2 mU/L
T4	P: 70–140 nmol/L
Free T4	P: 9–22 pmol/L
TIBC	S: 54–75 μmol/L
Triglycerides	P: 0.50–2.3 mmol/L
T3	P: 1.2–3.0 nmol/L
Troponin T	P: <0.1 mcg/L
Urate ♂	P: 210–480 μmol/L
Urate ♀	P: 150–390 μmol/L
Urea	P: 2.5–6.7 mmol/L
Vitamin B12	S: 0.13–0.68 nmol/L
Vitamin D	S: 50 nmol/L

Arterial Blood Gases

pH	7.35–7.45
PaCO₂	4.7–6.0 kPa
PaO₂	>10.6 kPa
Base excess	±2 mmol/L

Urine

Cortisol (free)	<280 nmol/24h
Hydroxyindole acetic acid	16–73 μmol/24h
Hydroxymethylmandelic acid	16–48 μmol/24h
Metanephrines	0.03–0.69 μmol/mmol cr.
Osmolality	350–1000 mosmol/kg
17-Oxogenic steroids ♂	28–30 μmol/24h
17-Oxogenic steroids ♀	21–66 μmol/24h
17-Oxosteroids ♂	17–76 μmol/24h
17-Oxosteroids ♀	14–59 μmol/24h
Phosphate (inorganic)	15–50 mmol/24h
Potassium	14–120 mmol/24h
Protein	<150 mg/24h
Protein/creatinine ratio	<3 mg/mmol
Sodium	100–250 mmol/24h

Haematology

WCC	4.0–11.0 ×10⁹/L
RBC ♂	4.5–6.5 ×10¹²/L
RBC ♀	3.9–5.6 ×10¹²/L
Hb ♂	130–180 g/L
Hb ♀	115–160 g/L
PCV ♂	0.4–0.54 L/L
PCV ♀	0.37–0.47 L/L
MCV	76–96 fL
MCH	27–32 pg
MCHC	300–360 g/L
RDW	11.6–14.6%
Neutrophils	2.0–7.5 ×10⁹/L (40–75%)
Lymphocytes	1.0–4.5 ×10⁹/L (20–45%)
Eosinophils	0.04–0.44 ×10⁹/L (1–6%)
Basophils	0–0.10 ×10⁹/L (0–1%)
Monocytes	0.2–0.8 ×10⁹/L (2–10%)
Platelets	150–400 ×10⁹/L
Reticulocytes	0.8–2.0% / 25–100 ×10⁹/L
Prothrombin time	10–14 s
APTT	35–45 s

Paediatric

Pulse Rate (bpm)
Neonate	140–160
Infant <1yr	120–140
1–5 years	110–130
5–12 years	80–120
>12 years	70–100
Respiratory Rate (tachypnoea)
0–2 months	≥60/min
2–12 months	≥50/min
1–5 years	≥40/min
>5 years	≥30/min
Blood Pressure (mmHg)
Term	65/45
1 year	75/50
4 years	85/60
8 years	95/65
10 years	100/70
Weight Formulas
3–12 months	(a + 9)/2 kg
1–6 years	2a + 8 kg
>6 years	(7a − 5)/2 kg
Haemoglobin (g/dL)
Term newborn	13–20
1 month	11–18
2 months	10–15
1–2 years	10–13
>2 years	11–14
MUAC (6 months–5 years)
Obese	>17.5 cm
Normal	13.5–17.4 cm
At risk	12.5–13.4 cm
Moderate malnutrition	11.5–12.4 cm
Severe malnutrition	<11.5 cm
Developmental Milestones
Social smile	1.5 months
Head control	4 months
Sits unsupported	7 months
Crawls	10 months
Stands unsupported	10–12 months
Walks	12–13 months
Talks	18 months
CSF WBC (/mm³)
Term newborn	0–25
>2 weeks	0–5

Calculator ›

Anorectal abscess

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