Local causes of extraperitoneal emphysema (Fig. 13)
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1.
Hollow viscera rupture (e.g. rectal perforation by diverticulitis, perforated carcinoma, foreign body, post endoscopic evaluation).
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2.
Penetrating pelvic trauma.
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3.
Residual air from previous surgery (e.g. abdominoperineal amputation, hysterectomy, caesarean).
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4.
Emphysematous infection: e.g. necrotising fasciitis of perineal, genital or perianal regions (Fournier gangrene) is a polymicrobial infection complicated by thrombosis of small subcutaneous vessels and rapid gangrenous involvement of the surrounding skin and deep fascia; although the diagnosis is based on clinical examination, CT plays an important role not only in confirming the diagnosis, but essentially to assess the extent of disease to plan the surgical treatment, sometimes identifying a potential underlying cause [1].
Extraperitoneal abdominal and pelvic anatomy
As already stated, the abdominopelvic cavity is entirely lined by the endoabdominal (parietal abdominal) fascia, whose innermost layer is the transversalis fascia; this name is derived from its location between the inner surface of the transverse abdominal muscle and the parietal peritoneum in the anterior and lateral abdominal wall; nevertheless, it also undersurfaces the diaphragm, is the anterior surface of the anterior longitudinal ligament, paravertebral, iliopsoas and internal obturator muscles; moreover, it delineates the lateral walls and diaphragm of the pelvis [4, 18].
Anatomical and embryological differences dictate that the fascia transversalis should be considered differently from the supra-abdominal and infra-abdominal regions. The arcuate line, which is an horizontal line that demarcates the lower limit of the posterior layer of the rectus sheath, separates the cranial from the caudal part of the abdominal wall; below this line, the aponevrosis of internal oblique, external oblique and abdominal transversal muscles pass anterior to the rectus muscle, and the fascia transversalis becomes stronger and functions as an aponevrosis, attached to the abdominal rectus muscle [18].
In the infraumbilical properitoneal space are the urachus and the obliterated umbilical arteries, which are developmental remnants, forming the medial and the median umbilical ligaments, respectively [18].
The endopelvic fascia is the internal investing fascia of the pelvis and consists of two layers, the parietal layer (how the transversalis fascia is named at this level) and visceral layer; it courses along the lateral borders of the pelvic organs, and encircles the perirectal fat, at this point named mesorectal fascia. Both sleeves are continuous and attached to the diaphragmatic part of the pelvic fascia along the tendinous arch (a thickened band on the upper layer of the diaphragmatic part of the pelvic fascia at the level of a line extending from the lower part of the pubic symphysis to the spine of the ischium) [14, 29] (Fig. 12).
The umbilicovesical fascia runs inferiorly from the umbilicus, posterior to tranversalis fascia and anterior to parietal peritoneum, with a triangular configuration with its apex at the umbilicus. It surrounds the urachus and obliterated umbilical arteries. Courses on the pelvis below the peritoneal reflection and extends to the pelvis floor surrounding the urinary bladder, then blending with the endopelvic visceral fascia along the lateral aspects of the lower uterus or seminal vesicles and the rectum.
Pelvic cul-de-sacs between the rectum and bladder (the rectovesical space in males), and rectum and uterus (the Douglas cul-de-sac in females) are shaped by parietal peritoneum reflections. These reflections extend inferiorly, and their anterior and posterior sleeves are fused, originating septa that separate the urological or genital tract from the rectum: rectovesical and rectovaginal septum.
Thus, a compartmental pelvis may be described as follows [4, 30, 31]:
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1.
Prevesical space
Anterior and lateral to the umbilicovesical fascia, and posterior to the trasversalis fascia. Prevesical space laterally communicates with the properitoneal space of the abdominal wall and flanks; posteroinferiorly it surrounds the lateral walls of the urinary bladder, and anteroinferiorly it forms the retropubic space (the space of Retzius) [32] (Fig. 11).
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2.
Paravesical space
Medially is limited by the umbilicovesical fascia, which fuses inferiorly with the visceral sleeve of the endopelvic fascia. The lateral boundary is the parietal endopelvic fascia, and the superior one the peritoneum (Fig. 11).
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3.
Presacral space
Represents the posterior communication between both paravesical spaces. It is located between the rectal and parietal sleeves of the endopelvic fascia (Fig. 11).
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4.
Perivesical space
Circumscribed by the umbilicovesical fascia anterior and laterally, and rectovaginal/rectovesical septum posteriorly, it encounters the urachus, obliterated umbilical vessels, urinary bladder and the lower uterine segment or seminal vesicles (Fig. 11).
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5.
Perirectal space
Limited by the rectovaginal (in females) or rectovesical (in males) septum, and the visceral endopelvic fascia (rectal fascia) (Fig. 14).
In the abdomen and pelvis, there is another anatomical continuum, the subperitoneal space, deep to the surface lining of the visceral and parietal peritoneum, the omentum and the various peritoneal ligaments and mesenteries, which in turn is an extension of extraperitoneal space, allowing spread of air/gas, fluid and other disease processes [23] (Fig. 13a, b).
Extrapelvic air/gas spread
The extraperitoneal compartment in the pelvis communicates with the subperitoneal space in the anterior abdominal wall anteriorly and the retroperitoneum posteriorly, easily allowing the spreading of air/gas among them. Besides this, the extraperitoneal pelvis has various outlets for the transit of organs and neurovascular structures to the rest of the body [32] (Fig. 15).
The inferior epigastric vessels courses for a distance in the properitoneal space before entering the rectus sheath just below the arcuate line; thus, air/gas can easily extend along with them, through fascia transversalis, to become intramuscular and rapidly spread interstitially [4, 16, 30].
Superior and inferior gluteal branches of the internal iliac arteries perforate the pelvic fascia on their way to gluteal region, opening one more channel to air spread [3].
It is also important to recognise that the femoral (vascular) sheath partly consists of a downward prolongation of the transversalis fascia. Consequently, air may extend from the prevesical space (anteriorly limited by the transversalis fascia) to the femoral sheath [4, 30].
The inguinal canal is a small passage that leads to the lower abdominal wall. It has openings at either end: the deep and superficial inguinal rings. The deep inguinal ring is a defect in the transversalis fascia that lies laterally to the inferior epigastric vessels. Thus, air contained by this fascia may dissect away through it, alongside the spermatic cord in males and the round ligament in women, respectively until scrotum and grand labia [4, 33].
Fascial investments of some muscles and vessels provide anatomic pathways of spread from the pelvis to the buttocks, hips and thighs. Three pelvic muscles have extra-pelvic insertions: the iliopsoas originates from the transverse processes and bodies of the 12th thoracic and lumbar vertebrae, upper two-thirds of the iliac fossa and sacral ala, inserting on the lesser trochanter of the femur (forming potential pathway of communication between the mediastinum and the upper thigh); the piriformis muscle, which originates from the sacrum and the margin of the greater sciatic foramen and inserts on the greater trochanter of the femur; and the internal obturator, running from the margins of the obturator foramen to insert on the greater trochanter of the femur [3, 32].
Figure 16 summarises the main connections between spaces reviewed.