Abscesses, Sinuses & Fistulae
Surgery · General Surgery · lean revision notes
Abscesses, Sinuses & Fistulae
These three entities are the bread-and-butter of basic surgical principles and are tested every year through definitions, Goodsall's rule, and "drug/management of choice" stems. Master the precise definitions first — most marks are lost by confusing a sinus with a fistula.
Core definitions
| Term | Definition | Number of openings | Classic example |
|---|---|---|---|
| Abscess | Localised collection of pus walled off by a pyogenic membrane | Closed cavity | Breast abscess, perianal abscess |
| Sinus | A blind track lined by granulation tissue, opening on an epithelial (usually skin) surface | One (external) opening | Pilonidal sinus, sinus over osteomyelitis |
| Fistula | An abnormal communication between two epithelial-lined surfaces (two hollow organs, or a hollow organ and the skin) | Two openings | Fistula-in-ano, enterocutaneous fistula |
High-yield: Sinus = ONE opening (blind track). Fistula = TWO openings (connects two surfaces). This single fact answers a disproportionate number of one-liner MCQs.
A useful etymological anchor: sinus is Latin for "a hollow/bay" (one mouth), while fistula means "a pipe/reed" (two open ends).
Part 1 — Abscess
Definition & pathophysiology
An abscess is a localised collection of pus (dead neutrophils, bacteria, liquefied necrotic tissue) within a cavity walled off by a pyogenic membrane — a layer of granulation tissue that limits spread but also prevents antibiotic penetration. The commonest organism is Staphylococcus aureus (coagulase-positive, produces coagulase which walls off the lesion).
The natural sequence is: inflammation → cellulitis → suppuration → abscess → pointing → rupture/discharge.
Clinical features
The five cardinal signs of inflammation apply — calor (heat), rubor (redness), dolor (pain), tumor (swelling), functio laesa (loss of function). Specific abscess signs:
- Throbbing pain (worse at night, classically the night before pointing)
- Fluctuation — the cardinal sign of a fluid collection; elicited in two perpendicular planes (Paget's test)
- Pointing — thinning of overlying tissue where the abscess is about to discharge
- A deep abscess (e.g. pelvic, psoas, subphrenic) may show NO fluctuation; instead suspect it from swinging pyrexia, tachycardia and localised tenderness.
High-yield: Fluctuation needs at least ~5 mL of fluid and is unreliable in deep abscesses. A swinging (hectic) fever is the systemic hallmark of pus collection — "where there is pus, let it out."
Investigations
- Mostly clinical. Ultrasound is the investigation of choice for a superficial/soft-tissue collection and to differentiate cellulitis (no drainable collection) from abscess.
- CT for deep collections (subphrenic, pelvic, retroperitoneal, intra-abdominal).
- Pus for Gram stain + culture and sensitivity; raised CRP/WBC.
Management — Incision & Drainage (I&D)
Drainage is the definitive treatment of an abscess; antibiotics alone will NOT cure it because the pyogenic membrane blocks drug entry.
Stepwise approach to a superficial abscess: Confirm collection (clinical ± USG) → adequate anaesthesia → incision at the most dependent/pointing part → break loculi with a finger or sinus forceps → drain pus + send for C/S → wash out cavity → leave open to heal by secondary intention (loose packing/wick), NOT primary closure.
- Hilton's method — for an abscess near vital structures (e.g. parapharyngeal, axillary, deep neck): incise skin and superficial fascia only, then insert closed sinus forceps and open them to break loculi, avoiding injury to nerves/vessels.
- Antibiotics are adjuncts, indicated when there is surrounding cellulitis, systemic sepsis, immunocompromise, diabetes, prosthetic material, or high-risk site. First choice empirically covers S. aureus; for MRSA consider clindamycin/vancomycin/linezolid.
- Cavity is left open to heal by secondary intention — primary closure risks reaccumulation.
High-yield: For an uncomplicated, small, drained skin abscess in a healthy patient, antibiotics are not mandatory after adequate I&D. The exam-correct stem is "incision and drainage" as the single best answer.
Special abscesses worth remembering
- Antibioma — a chronic, sterile, thick-walled, hard mass produced when an abscess is inadequately treated with antibiotics without drainage. Mimics a tumour. Treat by drainage.
- Cold abscess — tuberculous abscess without the signs of acute inflammation (no calor/rubor); seen in TB spine (psoas abscess), tuberculous lymphadenitis. Treat with aspiration through non-dependent site + ATT; incision risks a non-healing sinus.
- Pyaemic/metastatic abscess, Brodie's abscess (subacute osteomyelitis, classically upper tibia in young males).
Part 2 — Sinus
Definition
A sinus is a blind, granulation-tissue-lined track connecting a deep cavity, abscess, or focus of chronic infection to an epithelial surface, with a single opening. (Note: this is the pathological sinus, distinct from anatomical sinuses such as paranasal air sinuses.)
Causes of a persistent (non-healing) sinus — high-yield list
A sinus/fistula fails to heal when something keeps it open. Mnemonic "FRIEND":
- F — Foreign body / Foreign material (suture, sequestrum)
- R — Radiation
- I — Infection / Inflammation (specific: TB, actinomycosis)
- E — Epithelialisation of the track
- N — Neoplasm (malignancy in the track)
- D — Distal obstruction / Discharge (e.g. urine, faeces, recurrent foreign-body material)
Additional reasons: ischaemia, presence of a chronic abscess cavity, specific granulomatous disease (TB, actinomycosis), and Crohn's disease.
High-yield: The single commonest cause of a non-healing sinus/fistula is a retained foreign body (including sequestrum or non-absorbable suture). Always exclude it before re-operating.
Common clinical examples
- Pilonidal sinus — in the natal cleft (sacrococcygeal region), contains a tuft of hair; classically affects hairy young men, drivers ("jeep disease"), barbers (web of fingers). Caused by hair penetrating skin and inciting a foreign-body reaction. Treated by excision (e.g. Karydakis flap, Bascom procedure, Limberg/rhomboid flap) — midline closure has high recurrence.
- Median mental sinus — from a chronic periapical dental abscess of a lower incisor.
- Sinus over chronic osteomyelitis discharging through a cloaca, with sequestrum within.
- Tuberculous sinus, actinomycosis (sulphur granules, multiple sinuses around jaw/neck — "wooden" induration).
Investigation
- Sinogram (contrast injected into the track) defines depth, extent and any deep cavity.
- MRI for soft-tissue extent; X-ray for underlying bone sequestrum.
- Biopsy of the edge/track if malignancy (Marjolin's ulcer/squamous change) suspected.
Management principle
Eliminate the underlying cause (remove foreign body/sequestrum, treat TB, treat Crohn's) and excise the track. A sinus will not heal while the maintaining factor persists.
Part 3 — Fistula
Definition
An abnormal communication between two epithelial- (or endothelial-) lined surfaces — i.e. two hollow viscera, or a hollow viscus and the skin. Because both ends are lined by epithelium, fistulae are persistent and tend not to close spontaneously when epithelialised.
Classification
- Congenital — branchial fistula, tracheo-oesophageal fistula, urachal fistula.
- Acquired — post-operative, inflammatory (Crohn's, diverticulitis, TB), malignant, traumatic, post-radiation.
- By output (enterocutaneous) — high output > 500 mL/day, low output < 200 mL/day (some texts: high >500, moderate 200–500, low <200).
- Internal (organ-to-organ, e.g. colovesical) vs external (organ-to-skin).
Factors preventing fistula closure
Same mnemonic family — "FRIEND" (Foreign body, Radiation, Infection/IBD, Epithelialisation, Neoplasm, Distal obstruction) plus the surgical favourite "HIS FRIEND" adding High output, IBD/Short tract / Steroid use.
High-yield: A fistula with distal obstruction will never close until the obstruction is relieved — classic stem answer (e.g. colovesical fistula with distal stricture).
General management of an enterocutaneous fistula — the SNAP approach
- S — Sepsis control and Skin protection (drain collections, barrier creams/stoma appliance)
- N — Nutrition (often TPN/parenteral for high-output; correct fluid & electrolytes)
- A — Anatomy definition (fistulogram / CT / contrast study)
- P — Procedure / Plan (conservative trial; surgery if FRIEND factors present or no closure in ~4–6 weeks)
Most low-output fistulae without FRIEND factors close spontaneously. Somatostatin/octreotide reduces output. Definitive surgery (resection with primary anastomosis) is reserved for those that fail conservative management.
Fistula-in-ano (the most tested fistula)
Definition & cause
An abnormal track between the anal canal (internal opening, usually at the dentate line) and the perianal skin (external opening). Most arise from an infected anal gland (cryptoglandular infection) in the intersphincteric space → perianal abscess → fistula. So a fistula-in-ano is the chronic sequel of an anorectal abscess.
Park's classification
| Type | Track relation to sphincters | Frequency |
|---|---|---|
| Intersphincteric | Between internal & external sphincter | Commonest (~45%) |
| Trans-sphincteric | Crosses external sphincter into ischiorectal fossa | ~30% |
| Suprasphincteric | Over the top of puborectalis | ~20% (less) |
| Extrasphincteric | Outside sphincters, through levator ani | Rare; often trauma/Crohn's |
A track below the anorectal ring is a low fistula (safe to lay open); above it is a high fistula (laying open risks incontinence).
Goodsall's rule — exam favourite
Draw an imaginary transverse line across the anus (the anal transverse line):
- Anterior external opening → track runs in a straight, radial line to an internal opening at the corresponding point.
- Posterior external opening → track is curved and opens into the midline posteriorly (6 o'clock) in the anal canal.
- Exception: an anterior opening more than 3 cm from the anal margin may curve to the posterior midline (behaves like a posterior fistula).
High-yield: Posterior fistulae curve to the 6 o'clock midline; anterior ones run straight/radially. This is among the single most repeated surgery one-liners.
Investigation of choice
- MRI pelvis is the investigation of choice for delineating complex/high/recurrent fistulae and identifying secondary tracks.
- Endoanal ultrasound is also useful (operator-dependent).
- Examination under anaesthesia (EUA) with probing remains the practical gold standard intra-operatively.
Management — drug/procedure of choice
- Low fistula → fistulotomy (laying open the track) is the treatment of choice; high cure, acceptable continence.
- High/complex fistula → seton placement (loose draining seton or cutting seton), LIFT procedure (Ligation of Intersphincteric Fistula Tract), advancement flap, or fibrin glue/fistula plug (lower success).
- Crohn's-related fistulae — medical therapy with infliximab plus draining seton; avoid aggressive fistulotomy (poor healing, incontinence risk).
Comparison: Abscess vs Sinus vs Fistula
| Feature | Abscess | Sinus | Fistula |
|---|---|---|---|
| Nature | Pus collection in a cavity | Blind track | Track joining two surfaces |
| Openings | None (closed) | One | Two |
| Lining | Pyogenic membrane | Granulation tissue | Epithelium/granulation |
| Tends to heal? | Yes after drainage | May persist (FRIEND) | Persists, esp. if epithelialised |
| Treatment | Incision & drainage | Excise track + treat cause | Treat cause + excise/SNAP |
Complications
- Of abscess: spreading cellulitis, bacteraemia/sepsis & septic shock, metastatic abscesses, sinus/fistula formation, antibioma, chronic discharging sinus, necrotising fasciitis (esp. diabetics/perianal — Fournier's gangrene).
- Of sinus: persistent discharge, secondary infection, malignant change (squamous cell carcinoma — Marjolin's ulcer) in long-standing sinuses, recurrence.
- Of fistula: fluid & electrolyte loss, malnutrition (high-output ECF), skin excoriation, sepsis, malignancy in chronic tracts, anal incontinence after aggressive fistula surgery.
High-yield: Long-standing discharging sinus/scar → think Marjolin's ulcer (well-differentiated SCC, slow-growing, painless because of destroyed lymphatics/nerves, spreads late).
Key differentials
- A perianal abscess vs a fistula-in-ano — abscess is the acute phase, fistula the chronic sequel.
- Pilonidal sinus vs perianal fistula vs hidradenitis suppurativa (apocrine gland inflammation, multiple sinuses in axilla/groin).
- Cold abscess (TB) vs pyogenic abscess — cold abscess lacks acute signs; do NOT incise (risk of non-healing sinus).
- Cellulitis (no drainable collection) vs abscess (drainable) — USG settles it and changes management.
Recently asked / exam angle
- "Number of openings in a fistula?" → Two. (Sinus = one.)
- Goodsall's rule — direction/curvature of the track based on external opening position; the 3 cm anterior exception is a favourite trap.
- "Commonest cause of a non-healing sinus/fistula?" → Foreign body (FRIEND mnemonic asked directly).
- "Treatment of choice for a low anal fistula?" → Fistulotomy; for high fistula → seton.
- "Investigation of choice for complex fistula-in-ano?" → MRI pelvis.
- "Sterile firm mass after partially treated abscess?" → Antibioma.
- Hilton's method of abscess drainage (near vital structures) — definition-based recall.
- Enterocutaneous fistula management → SNAP; high-output cut-off >500 mL/day; octreotide reduces output.
- Commonest organism in pyogenic abscess → Staphylococcus aureus.
- Park's classification — commonest type is intersphincteric.
Rapid revision
- Abscess = pus in a cavity walled by a pyogenic membrane; commonest organism S. aureus.
- Definitive treatment of an abscess is incision & drainage — antibiotics alone fail (pyogenic membrane).
- Leave the drained cavity open to heal by secondary intention; use Hilton's method near vital structures.
- Antibioma = sterile hard mass from giving antibiotics without draining.
- Cold abscess (TB) — aspirate via a non-dependent site + ATT; do NOT incise.
- Sinus = one opening (blind track); Fistula = two openings (joins two epithelial surfaces).
- Causes of non-healing sinus/fistula = FRIEND (Foreign body, Radiation, Infection, Epithelialisation, Neoplasm, Distal obstruction); foreign body is commonest.
- Pilonidal sinus — hair-bearing natal cleft; treat with flap excision (Limberg/Karydakis), not midline closure.
- Fistula-in-ano arises from cryptoglandular anal gland infection; internal opening at the dentate line.
- Goodsall's rule — anterior opening = straight radial track; posterior = curved to 6 o'clock midline; anterior >3 cm is the exception.
- MRI pelvis is the investigation of choice for complex fistula-in-ano; fistulotomy for low, seton/LIFT for high.
- Enterocutaneous fistula → manage by SNAP (Sepsis, Nutrition, Anatomy, Procedure); high output >500 mL/day; chronic sinus/scar → suspect Marjolin's ulcer (SCC).