Fistulography and Sinography

Fistulography and sinography are fluoroscopic or radiographic studies using water soluble iodinated contrast. It is injected through the external opening of a sinus or a fistula to outline its course, its length, its branches and any internal opening.

A sinus is a blind tract opening on one surface. A fistula is an abnormal communication between two epithelial surfaces, or between an organ and the skin.

A soft catheter (6 to 10 French) or a blunt cannula is placed in the external opening, and a balloon or a gentle seal prevents leakage. Then 5 to 20 mL (sometimes up to 50 mL for a large cavity) of contrast is injected slowly under fluoroscopy, with spot films in anteroposterior, lateral and oblique projections. Overlying skin markers or a metal probe show the position of the opening.

CT fistulography combines the injection with CT for three dimensional detail. MR fistulography or contrast free MR (the usual test for perianal fistula) shows the tract and the surrounding muscle without radiation. The effective dose is low, about 0.5 to 3 mSv, depending on the region.

Chronic discharging tracts are common after surgery, trauma and infection. Among the commonest are chronic osteomyelitis, post-operative wound sinuses, perianal fistula (about 10 in each 100 000 people each year), enterocutaneous fistula after abdominal surgery or in Crohn disease, branchial and thyroglossal tracts, pilonidal sinus, and tuberculous sinuses.

The study answers four questions: where does the tract go, does it connect to a bone, joint, bowel, bladder or other organ, are there side branches or abscess cavities, and is there a foreign body or sequestrum? The result decides excision, drainage or repair.

Indications. Chronic discharging sinus or fistula of uncertain origin. Suspected chronic osteomyelitis with sinus (to find a sequestrum or the bone involved). Enterocutaneous fistula (origin in small bowel or colon, length and any distal obstruction). Post-operative sinus (anastomotic leak, retained foreign body such as a suture or mesh).

Perianal fistula when MR is not available (the course relative to the sphincters). Neck sinuses (branchial, thyroglossal, pharyngeal pouch remnants). Fistula after trauma or radiotherapy. Pilonidal and pelvic sinuses. Empyema sinus after chest drainage (bronchopleural fistula, chest wall sinus). Biliary and pancreatic fistulae after surgery or drainage.

Urinary fistula (vesicocutaneous, urethrocutaneous). Not for an acute abscess before drainage, and not when a simple clinical probe or MR gives the answer.

Contraindications and precautions. Acute abscess with cellulitis (risk of spreading infection; drain first). Allergy to iodinated contrast (a small amount is absorbed, but reactions occur). Suspected communication with the cranial or spinal cavity, the pericardium or the peritoneum with a risk of contamination: inject with extreme care and only low pressure.

High pressure injection can force bacteria into the bloodstream (bacteraemia and sepsis), so antibiotic cover is considered. Pregnancy: avoid unless essential. Wear gloves, masks and eye protection for discharge from infected tracts, including tuberculous and HIV-related sinuses. Patients with a pneumothorax or empyema need chest assessment first. Oil based contrast is avoided.

Preparation and technique. A control film is taken. The skin is cleaned and the opening is cannulated with a catheter or a cannula tip (a Foley catheter with a small inflated balloon for a large opening).

Contrast is injected under fluoroscopy while spot films are taken as the tract fills; the injection stops when contrast reaches the internal opening, a cavity, or when resistance or pain occurs. Patients are turned to fill all branches and to show the tract in two planes. Films after removal of the catheter show the contrast retained in the tract.

Normal appearances. There is no normal sinus or fistula tract, so the interpretation depends on the expected anatomy of the region. A simple tract fills as a narrow, smooth channel of uniform calibre (1 to 5 mm) that runs in a straight or gently curved course to a defined end, with no side branches, and contrast passes out of the opening.

Normal structures that contrast may enter include the intestinal lumen (in a fistula to bowel), the bladder, a bronchus, or the skin. No collections, cavities, sequestra or foreign bodies are present.

Interpretation.

• Simple sinus: a single tract ending blindly, often with a small cavity at the depth, the commonest form after wound infection, with a retained suture or necrotic tissue as the cause.
• Complex fistula or sinus: multiple branches, loops or horseshoe extensions, cavities (abscesses) with contrast pooling and irregular walls, and several external openings.
• Osteomyelitis sinus: a tract that leads to a bone, with contrast tracking along the cortex and into a cavity in the medulla. A sequestrum is a filling defect with sharp edges inside the contrast filled cavity, and a cloaca is an opening in the cortex. Involucrum is seen as new bone around the cavity.
• Enterocutaneous fistula: contrast tracks from the skin to a loop of small or large bowel, filling the lumen.
• Note the level of the enterocutaneous fistula (proximal, mid or distal), its length (a long tract under 2 cm in the wall closes less often than a long tract of over 2 cm in some series), any associated abscess, distal obstruction, mucosal disease or malignancy at the fistula, and the length of normal bowel beyond it.
• Fistulae with a distal obstruction, a foreign body, epithelialisation, radiation damage, infection (abscess), inflammatory bowel disease or malignancy rarely close.
• Perianal fistula: classified by relation to the sphincter complex into intersphincteric, transsphincteric, suprasphincteric and extrasphincteric, with the internal opening usually at the dentate line. Contrast shows the course and the opening in the anal canal (internal opening) and any horseshoe or supralevator extension. MR gives better soft tissue detail.
• Neck and head sinuses: branchial sinus (a tract from the lateral neck toward the tonsillar fossa or the pharyngeal wall); thyroglossal duct sinus (a midline tract to the hyoid bone and the foramen caecum).
• Pyriform sinus fistula: a tract from the pyriform fossa through the thyroid gland, causing recurrent left sided neck abscess and thyroiditis. Dental sinus: a tract from the cheek or chin to a root apex.
• Empyema and bronchopleural fistula: contrast fills the pleural cavity and shows the cavity's size and any connection to a bronchus (contrast in the bronchial tree, with cough on injection).
• Pilonidal sinus: multiple tracts that extend from the natal cleft to the sacrum and coccyx, with an occasional cavity.
• Biliary and pancreatic fistula: contrast tracks to the bile duct or pancreatic duct, with a leak after surgery.
• Urinary fistula: contrast passes into the bladder, ureter or urethra.
• Tuberculous sinus: long, branching tracts with cavities near the spine (Pott disease with a paravertebral abscess and psoas tract), a joint, or lymph nodes, often with calcification.
• Foreign body: a filling defect or a radio opaque body (suture, mesh, gauze with a marker, bullet fragment, wood or glass) in the tract or cavity.
• Malignant change: an irregular, destructive tract with a mass (Marjolin ulcer in a chronic sinus, squamous cell carcinoma), best shown by cross-sectional imaging and biopsy.

Critical values and findings needing immediate action.

• Contrast entering the bloodstream (intravasation) or the peritoneum, pleura, pericardium or cranial cavity unexpectedly: stop the injection, observe, treat sepsis if fever or rigors occur, and refer.
• Sepsis after injection (rigors, fever, hypotension): blood cultures, intravenous antibiotics and fluids.
• Abscess cavity with undrained pus: drainage by percutaneous or surgical route.
• Enterocutaneous fistula with high output, dehydration or electrolyte loss: fluid and electrolyte replacement, nutrition, skin protection and surgical review.
• Fistula to a hollow organ with peritonitis: resuscitation and urgent surgery.
• Bronchopleural fistula with tension or aspiration: chest drain and respiratory support.
• Tract with a malignant appearance: biopsy and oncology referral.
• Anaphylaxis to contrast: stop, adrenaline, oxygen and fluids.

Limitations and artefacts.

• Incomplete filling is the commonest error: a tract may close up, be blocked by granulation or debris, or fill only as far as the first obstruction, and branches or the internal opening may be missed. Injection pressure that is too low fails to fill the tract, and pressure that is too high ruptures it and creates false passages, spreading infection.
• A tract that leaks around the catheter dilutes the study; a balloon or a tight seal is needed.
• Air bubbles mimic cavities or stones, and contrast pooling in skin folds mimics a tract.
• The study shows the lumen only and not the surrounding tissues, muscles or abscesses that do not communicate with the tract; CT and MR give this information.
• A negative study does not exclude a fistula, particularly in Crohn disease.
• A tract with no internal opening seen can still be connected, as spasm or mucosal folds may block the opening.
• Radiation dose is low but should be justified; MR is the choice for perianal fistula and for young patients.
• Infection can spread after injection, and cultures should be taken before.

Special groups.

• Children: congenital tracts (branchial, thyroglossal, urachal, dermal sinus of the spine) are imaged with ultrasound or MR first; a dermal sinus in the midline over the spine is not injected because of the risk of meningitis, and MR is used.
• Patients with Crohn disease: perianal and enterocutaneous fistulae are common; MR and endoanal ultrasound are preferred.
• Patients with HIV or immunosuppression: tuberculous and fungal sinuses, and perianal disease.
• Patients after radiotherapy: fistulae that do not heal and may need diversion.
• Pregnancy: avoid.
• Regions: chronic osteomyelitis, tuberculous sinuses, mycetoma (grains in the discharge, with multiple sinuses in the foot), post-operative sinuses and obstetric fistulae (vesicovaginal and rectovaginal) are common, and a simple sinogram is often the only imaging available before surgery.

Follow up tests.

• MR of the region (perianal MR, MR enterography).
• CT of the abdomen, pelvis or chest with contrast.
• Ultrasound (including endoanal ultrasound).
• Plain radiographs of the bone (osteomyelitis, sequestrum).
• Barium or water soluble contrast study of the bowel, and cystoscopy.
• Culture and sensitivity of the discharge, acid fast bacilli stains and tuberculosis tests.
• Fungal and mycetoma grain microscopy.
• Full blood count, erythrocyte sedimentation rate, C-reactive protein, albumin, electrolytes and HIV test.
• Biopsy of any suspicious tract, and referral to surgery, orthopaedics or infectious disease services.