Guiando a estratégia cirúrgica das fístulas perianais com ressonância magnética: da imagem convencional ao modelo 3D

Neves, Priscilla Ornellas; Silva, André Araújo de Medeiros

doi:10.1590/0100-3984.2025.0064-en

Figuras (5)
Tabelas (2)

Figure 1 24-year-old male. Simple posterior intersphincteric fistula, with its internal opening at the mid-anal canal, without collections or secondary tracts. Fistulotomy is a safe procedure for this type of fistula.

A. Sagittal T2-weighted image showing the posterior intersphincteric fistula (white arrow). B. Axial T2-weighted image showing the inflammatory activity demonstrated by high T2 signal (white arrow).

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Figure 2 59-year-old male. High posterior transsphincteric fistula. The ascending tract within the intersphincteric space transfixes the puborectalis muscle, potentially misclassified as a lower fistula on anal inspection. Complex bifid extension to the left ischioanal fossa.

A,B: Oblique coronal T2-weighted MRI sequence showing the internal opening (*) and ascending transsphincteric tract within the intersphincteric space (white arrow; red line in B), with the EAS in blue and the IAS in green. C: Image segmentation of the fistulous tract (red), IAS (green), and EAS (dark blue), performed on axial T2-weighted turbo spin-echo images. D: 3D model in a posterior view, showing the internal opening and the ascending tract crossing the puborectalis muscle (light blue).

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Figure 3 29-year-old male. Posterior intersphincteric horseshoe fistula at the mid-anal canal, extending between 1 and 11 o’clock and perianal abscess. Horseshoe tracts are typically unsuitable for minimally invasive techniques such as LIFT, VAAFT, and FiLaC.

A: Axial contrast-enhanced image showing the horseshoe fistula (white arrow). B: Image segmentation performed on axial contrast-enhanced T1-weighted images. Note that the intersphincteric tract (red) terminates in an abscess in the posterior perianal region. C: 3D model in a top view, showing the intersphincteric horseshoe fistulous tract crossing the midline.

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Figure 4 64-year-old male. High posterior transsphinctericfistula with active inflammatory signs and a predominance of granulation tissue. Left intersphincteric tract with seton placement, without inflammatory activity or abscess. Transsphinctericfistula treated with VAAFT, with signs of healing on follow-up.

A. Axial contrast-enhanced T1-weighted image: seton (white arrow) and transsphincterictract (*). B. 3D model in a lateral view, showing the intersphincteric seton located along the left lateral wall (yellow) and the active fistulous tract with posterior origin (red), not drained by the seton, together with a collection in the posterior perianal margin.

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Figure 5 24-year-old male. Crohn’s disease with perianal involvement. Complex anal fistula with multiple origins, including four at the level of the puborectalis muscle. A posterior intersphincteric horseshoe tract communicates with these fistulas at the mid-anal canal. There is a collection extending into the left ischioanal fossa, as well as multiple anterior tracts reaching the scrotal margin. Initial management included drainage and seton placement, followed by biologic therapy.

A,B: Coronal 3D isotropic images showing a complex fistula (red lines in A; EAS in blue and IAS in green). C: Image segmentation on the coronal isotropic T2-weighted volumetric sequence. The fistulous tract with multiple origins was segmented in the axial plane. D: 3D model in a frontolateral view, showing anterior extension of multiple tracts. E: 3D model in a superior (top) view, showing the posterior left tract forming a collection in the topography of the ischioanal fossa.

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Chart 1 Characteristics of perianal fistulas and radiological predictors of the suitability of surgical techniques.

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Chart 2 Primary descriptors in anorectal fistula MRI.

Characteristic	Radiological considerations	Suitable techniques	Unsuitable techniques
Tract height	Distance from tract to anorectal junction; amount of EAS/IAS above the fistula	Fistulotomy (if low); FiLaC (if transsphincteric)	Fistulotomy (if high); LIFT (if very high intersphincteric)
Tract direction (obliquity)	Oblique cephalic tracts cross more sphincter and may affect continence	FiLaC; VAAFT	Fistulotomy if significant cephalic obliquity
Internal opening position	High openings near the anorectal junction, which are harder to access or close	VAAFT; FiLaC (some cases); RAF	LIFT (if high); plug (if deep or inaccessible)
Tract complexity (branches/extensions)	Horseshoe, supralevator, or intersphincteric extensions on MRI	RAF; VAAFT (selective); drainage with setons	FiLaC; plug; LIFT
Tract diameter	Narrow: may hinder VAAFT/ FiLaC access; Wide: risk of failure with laser/plug	FiLaC (if 3-5 mm); VAAFT (if > 3.3 mm)	FiLaC (for cavities); plug (for very wide tracts)
Intersphincteric space involvement	Oblique, wide tracts, or horseshoe extensions, which complicate ligation	LIFT (if straight, narrow, and intersphincteric)	LIFT (if horseshoe or with IAS defect)
Abscesses/collections	Hyperintense foci on T2/STIR MRI; require complete drainage	VAAFT (visualization); setons prior to surgery	Plug or FiLaC if collections untreated
IAS integrity	Defects or thinning, which may predict incontinence risk or flap failure	VAAFT; RAF (if IAS intact)	LIFT (if IAS disrupted); RAF (if damaged)
Previous surgery/ scarring	Altered anatomy on MRI, which affects flap mobility and sphincter function	Partial-thickness RAF; VAAFT	LIFT (if space distorted); full-thickness RAF
Inflammatory activity (Crohn's)	Proctitis or active inflammation on MRI, either of which impedes flap healing	Seton; VAAFT (for symptom control)	RAF (in active proctitis); plug

Descriptor category	Essential features to report
Classification	Relationship with sphincteric complex: intersphincteric, transsphincteric, extrasphincteric, or suprasphincteric
Internal opening	Radial (clock-face) position; height (upper/middle/lower third of the anal canal); number and size
Tract pathway	Direction (cephalic/caudal); angulation; extensions if present (horseshoe, blind)
External opening	Radial position; anatomical site (e.g., gluteal, labial, perianal skin, etc.)
Collections/abscesses	Presence; diameter (<10mm = small, 11-20mm = medium, and >20mm = large); location; and connection to primary tract
Fistula activity	Active vs. inactive (based on T2/ STIR hyperintensity); and presence of granulation or fibrotic tissue

[1] RAF, rectal advancement flap; STIR, short-tau inversion recovery.