Introduction:
Perianal fistulas are one of the many complications of Crohn’s disease and are often difficult to manage if not diagnosed correctly.
This may be due to occult sepsis not identified on probing or examination under anaesthesia by the surgeon.
For example,
they may be located within the pelvis from supralevator abscess/fistula formation.
Hence if treated incorrectly,
they can cause significant morbidity to the patient and decrease their quality of life(1).
It is important for radiologists to understand the pathophysiology of perianal fistulas and the relevant anatomy of the pelvis to be able to interpret pelvic MRI studies and contribute useful information to the clinician.
Aetiology of perianal fistulas:
The crytoglandular hypothesis(2) for fistula formation postulates sepsis of the intramuscular anal glands which are found at the level of the dentate line.
The dentate line is the transition between anal squamous epithelium and rectal columnar epithelium(3) and lies approximately 2 cm proximal to the anal verge.
It is believed that the majority of these glands are located in the intersphincteric space and infection often results in abscess formation(4).
These abscess can self resolve or if they persist,
can result in fistula formation.
Thus,
any inflammatory condition,
including Crohn's disease,
will increase the incidence of fistula in ano formation.
Anatomy:
The anatomy of the perineum is described as an “anal clock” by surgeons with the patient in lithotomy position(1).
The natal cleft is at the 6 o’clock position and the anterior aspect of the perineum (in the same position as the pubic symphysis) is described as 12 o’clock.
The left lateral aspect is 3 o’clock and similarly,
the right lateral aspect is 9 o’clock.
This description coincidentally correlates with the axial images obtained on MRI.
The anal sphincter complex is divided into internal and external sphincters(6).
The internal sphincter is comprised of smooth muscle and is a continuation of the circular smooth muscle of the rectum.
It is involuntary and contributes approximately 85% to resting anal tone(1).
The external sphincter is the direct extension of the levator ani muscle and is also continuous with the puborectalis muscle.
It contributes only 15% of the resting anal tone however,
because it is a voluntary muscle,
it is a strong contributor to faecal continence.
Division of the external sphincter can therefore result in incontinence(7).
Fistula classification:
A lot of work on perianal fistulas originated from St Mark's hospital in London,
England.
Goodsall first described the cutaneous opening of a fistula and correlated it to the expected site of enteric opening(8).
It states that if a perianal skin opening is anterior to the transverse anal line,
the fistulous tract is usually radial to the anal canal.
If the perianal opening is posterior to the transverse anal line,
the fistula will open into the anal canal in the midline posteriorly i.e 6 o'clock position(8),
sometimes with a curvilinear tract (Fig.
6).
Park’s classification(9) divides fistulas into intersphincteric,
transphincteric,
suprasphincteric and extrasphincteric fistulas and is widely used by surgeons in practice (Fig.
7).
The St James University grading system(1) improves on the Park's classification by correlating the location of the fistulas to anatomy seen on MR imaging in the axial and coronal planes.
This system divides fistulas into 5 grades:
1 – intersphincteric
2 – intersphincteric with ramification or abscess formation
3 – transphincteric
4 – transphincteric with ramification or abscess formation
5 – supralevator
MRI sequences:
Usually T1 and T2 water and fat sensitive sequences with post contrast sequences in the relevant anatomical planes to identify fistulas and its complications.
At the Royal Hobart Hospital,
our pelvic fistula MRI protocol sequences include (Fig.
8):
-sagittal T2
-axial oblique T2
-axial oblique T2FS
-coronal oblique T2FS (fat saturated)
-axial oblique and coronal oblique T1FS pre contrast
-axial oblique and coronal oblique T2FS post contrast sequences
The oblique plane is relative to the position and orientation of the anal canal,
rather than relative to the anatomical sagittal plane of the patient.