The anatomical compartments and their connections as demonstrated by ectopic air

Local causes of cervicofacial emphysema

Cervical anatomy

Two major “fascial” layers enclose infrahyoid neck structures: the former superficial fascia, according to Terminologia Anatomica (1998) now known as subcutaneous layer (tela subcutanea) and the (deep) cervical fascia. The tela subcutanea is a fat-filled layer of connective tissue that surrounds the neck and contains the platysma, superficial lymph nodes, nerves and vessels. It allows the skin to easily glide over deeper structures and extends all over the body [12, 13].

Although some controversy exists in the description/nomenclature of the (deep) cervical fascia, for an easy understanding, it is commonly accepted that it consists of three layers: superficial, middle and deep layers, which divide the deep neck into multiple spaces (Fig. 3).

The superficial (or investing) layer lies between the superficial cervical fascia and the muscles of the neck; it splits to encircle the sternocleidomastoid and trapezius muscles, to blend posteriorly with the ligamentum nuchae, and splits again between the angle of mandible and the mastoid process to enclose the parotid gland. The middle fascia extends from the skull base superiorly to the mediastinum inferiorly and anteriorly from the hyoid bone to thoracic inlet; it is divided into muscular and visceral divisions (also called pretracheal and buccopharyngeal, in the suprahyoid neck), surrounding the strap muscles and the visceral structures of the neck, respectively. The deep layer of cervical fascia surrounds the deep muscles of the neck and the cervical vertebrae; it extends from the skull base into the mediastinum and has two divisions: the alar and the prevertebral layers; the alar layer forms the posterior and lateral wall of the retropharyngeal space, and bridges the transverse processes of the vertebrae; the prevertebral layer encloses the paraspinal muscles. The alar layer is attached to the prevertebral layer by loose connective tissue only, and thus an easily distended space is found between them. This space is limited above by the base of the skull, while below it extends behind the oesophagus into the posterior mediastinum to the diaphragm, creating a conduit for free movement of air/gas between mediastinum and neck. This space has been named “dangerous space”, as it allows the spread of the infection between the neck and mediastinum. The prevertebral layer laterally forms a sheath for the brachial nerves and subclavian vessels in the posterior triangle of the neck, and it is continued under the clavicle as the axillary sheath [6, 12, 14, 15] (Fig. 3c and d).

Air/gas spread to the upper limbs

The superficial cervical fascia laterally merges with subcutaneous tissues of the neck.

The cervicoaxillary canal with its fibrous neurovascular sheath, enclosing the first part of the axillary artery, the axillary vein and the brachial plexus, forms a passageway between the neck and the upper limb, also allowing air/gas to go through.

The muscular fascia also enables air/gas spread between the trunk and the upper limbs, as happens with the upper limb muscular insertions of scapular girdle, and the extensions of fascia in between (e.g. the floor of the axilla is formed by the fascia extending among the pectoralis major and the latissimus dorsi) [16].

Where is the connection between neck and chest?

There are no barriers on the subcutaneous tissues all over the body. So, free movement of air/gas is possible just under the skin. An equivalent free air/gas flow is possible in the perivertebral space. The visceral and retropharyngeal spaces directly communicate with the mediastinum, creating a conduit for free movement of air/gas between mediastinum and neck [2] (Fig. 4).

Local causes of pneumomediastinum

Mediastinal anatomy

According to Terminologia Anatomica the fasciae of the trunk are listed as parietal, extraserosal and visceral. The fascia parietalis is defined as a sheet or band of fibrous tissue which lies outside the parietal layer of a serosa and lines the wall of a body cavity; the parietal fascia of the thorax is the endothoracic fascia and that of the abdomen is the endoabdominal fascia. The endothoracic fascia is firmly attached to the costal pleura, and posteriorly blends with the prevertebral fascia [18].

The mediastinum lies between the right and left pleura; it extends anteriorly from the sternum (undersurfaced by the endothoracic fascia), to the vertebral column posteriorly (prevertebral fascia), and contains all the thoracic viscera except for the lungs. Despite being academically divided into upper, anterior, middle and posterior compartments, it represents a single space and thus air/gas freely moves inside (Fig. 6).

Where is the link between chest and abdominal cavity?

During the early embryonic life, the coelomic cavity is formed, lined by a serous membrane, and deep to it lies the subserosal space; the integrity of this space is maintained during the division of the coelomic cavity, forming a thoracoabdominal continuum, which crosses the diaphragmatic hiatuses connecting the subperitoneal and subpleural portions of the subserous space [19, 21] (Fig. 7).

The parietal pleura is attached to the thoracic wall by the endothoracic fascia. This fascia lines the diaphragm superiorly, while the transversalis fascia, which is the innermost layer of the endoabdominal fascia, lines its inferior surface [18]. The costal insertions of the diaphragm on the inner surfaces of the lower six ribs and their cartilages provide a separation between theses sleeves. However, three anterior defects in the diaphragmatic attachments provide blending of the two fasciae: [1] one in the midline, between the two slips of sterna origin of the diaphragm that arise from the back of xiphoid process and [2] two parasagittal gaps between the sternal and costal origins of the diaphragm—the sternocostal triangles or foramen of Morgagni, through which run internal mammary vessels that continue in the abdomen as the superior epigastric vessels (Fig. 7a and b). Therefore, air/disease processes that follow these diaphragmatic defects may enter a space between the transversalis fascia and the parietal peritoneum, the properitoneal (or preperitoneal) space, and spread through it laterally to the flank, arriving into the retroperitoneum[18, 20, 21].

Posteriorly, endothoracic and transversalis fasciae are continuous behind the diaphragm, at lumbocostal arches and the aortic and oesophageal hiatus, providing communication point between mediastinum and retroperitoneum (Fig. 7b) [21].

Besides these fully accepted congenital communications, there are also areas of structural weakness of embryologic origin, as tiny focal defects on the tendinous part of the diaphragm or the rare septum transversum lacunar aplasia, which allows transphrenic migration of air/gas [19].

The vena caval foramen is not considered part of the thoracoabdominal continuum because the wall of this vessel sticks to the margins of the foramen and thus interrupts continuity of the subserous space [20].

Thus, subserous space provides not only avenues through which vital structures come from thorax to abdomen and vice versa but also pathways for bidirectional spread of disease processes [20].

Local causes of pneumoretroperitoneum

Retroperitoneal anatomy

The retroperitoneum is the space between the posterior parietal peritoneum and the transversalis fascia, and extends from the diaphragm superiorly to the pelvis inferiorly. Although traditionally divided in three distinct compartments, the anterior pararenal, the perirenal and the posterior renal spaces, by the anterior (Gerota or Toldt) and the posterior (Zuckerlandl) renal fasciae, more recent work has demonstrated that the renal fascia is a single multilaminated structure, which contains potential space, named the interfascial plane; these potential spaces are represented by the retromesenteric, retrorenal, lateroconal and combined interfascial planes [22–24] (Fig. 9).

The anterior pararenal space (APS) is bounded anteriorly by the posterior parietal peritoneum, laterally by the lateroconal fascia and posteriorly by the anterior renal fascia; it contains the pancreatic gland, retroperitoneal segments of the duodenum, ascending and descending colon and the mesenteric root, as well as adipose tissue. This space is continuous across the midline [25].

The posterior pararenal space (PPS) is bound anteriorly by the posterior renal fascia and posteriorly by the transversalis fascia, and it only contains adipose tissue, lymphatics and blood vessels. Medially, it is limited by the margin of the psoas muscle. The posterolateral portion of PPS has anatomic continuity with the properitoneal flank “stripe” (a layer of areolar tissue and variable amount of fat in between the transversalis fascia and parietal peritoneum) Therefore, air arising within or entering this space may spread to the anterior abdominal wall. PPS also continues superiorly as a thin subdiaphragmatic layer of extraperitoneal fat [18, 19, 25, 26].

The perirenal space (PS) lies between the anterior and posterior renal fascia; it is a paired inverted cone shaped of tissue containing the kidney, the proximal portion of the collecting system, renal and perirenal vasculature, perirenal lymphatics, adrenal glands and variable amount of fat. The space is divided into multiple compartments by thin fibrous lamellae, which form bridging septa crossing the perinephric fat and connect the renal capsule and the anterior and posterior renal fascia, providing a bidirectional channel between spaces [24] (Fig. 10a).

There is a considerable disagreement with the exact arrangement of the renal fascia. In Meyers’ concept the renal fasciae are closed above and below the kidney and adrenal glands; however, clinical studies have shown fluid collections extending into the adjacent spaces, bare area of the liver on the right, superior extraperitoneal space on the left, towards the midline from each perirenal space and into the lower retroperitoneal space of the abdomen and pelvis [25–27]. So, the concept of a multilayered structure of the fascia, helps to understand these findings.

The retroperitoneal interfascial planes are potential expandable planes; the retromesenteric plane stands between the APS and PS, and is continuous across the midline and in front of the major vessels; the retrorenal plane between the PS and PRS communicates with the vascular compartment of the retroperitoneum, including the aorta and inferior vena cava; the lateroconal interfascial plane is between the layers of the lateroconal fascia, and communicates with the retromesenteric and the retrorenal planes; the combined interfascial plane is formed by the (lower) blending of the retromesenteric and retrorenal planes, inferiorly to the cone of perirenal fat, and posteriorly to the ascending and descending mesocolon, and continues along the surface of the psoas muscle to the pelvis [22].

How does retroperitoneal air/gas reach the extraperitoneal abdominal and pelvic spaces?

Superiorly, the left retromesenteric plane and the PS extend to the dome of the diaphragm, where they connect with the left subdiaphramatic extraperitoneal space [22, 27] (Fig. 10b). On the right the retromesenteric plane extends at the level of the right inferior coronary ligament and the bare area of the liver, and communicates with the liver hilum through the subperitoneal space of the hepatoduodenal ligament (Fig. 11). Inferiorly, the combined interfascial plane continues in the pelvis along the anterolateral margins of the psoas muscle, contiguous with the pelvic retroperitoneal perivesical and presacral spaces [22, 24]. The PPS is in continuity with properitoneal space (Fig. 9).

These paths are also based on the continuity of the transversalis fascia (as the outer border of the retroperitoneal and peritoneal cavities) with the parietal endopelvic fascia, lining the abdominopelvic cavity and outlining a conduit for air/gas spread [2, 24, 28] (Fig. 12).

Thus, air in the retroperitoneum can spread across the midline in the APS and via the anterior interfascial retromesenteric plane, and get off to the pelvic extraperitoneal space through the properitoneal fat and by the combined interfascial plane [24].

Local causes of extraperitoneal emphysema (Fig. 13)

1. 1.

   Hollow viscera rupture (e.g. rectal perforation by diverticulitis, perforated carcinoma, foreign body, post endoscopic evaluation).

    
2. 2.

   Penetrating pelvic trauma.

    
3. 3.

   Residual air from previous surgery (e.g. abdominoperineal amputation, hysterectomy, caesarean).

    
4. 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.

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.