Differential Diagnosis of Small Amounts of Subdiaphragmatic

The predominant extraperitoneal gas within the posterior pararenal fat may seek the immediate subdiaphrag matic tissue planes. While occasionally extraperitoneal air may be distinguished by its outlining of individual diaphragmatic muscle bundles,274 I have observed two further characteristics at this site on erect films that are particularly useful in differentiating even small amounts of extraperitoneal gas from free intraperitoneal air:

1. Free intraperitoneal air always conforms to the highest curvature of the dome of the diaphragm and may have a flat lower border. Gas in the subphrenic extraperitoneal tissues often parallels a lower plane of the diaphragmatic curvature, medial or lateral to its apex, and invariably demonstrates a crescentic outline (Figs. 8-211 and 8-212).

Crescentic Osteotomy

Fig. 8—218. Extraperitoneal gas following tracheostomy.

(a) Preferential spread into the flank fat (open arrows). A minimal amount outlines the left psoas muscle and suprarenal area (solid arrows).

(b) Lateral chest film demonstrates the continuous channel of gas from the chest to the abdomen deep to the endothoracic fascia and transversalis fascia (arrows).

(Reproduced from Meyers.9)

Fig. 8—218. Extraperitoneal gas following tracheostomy.

(a) Preferential spread into the flank fat (open arrows). A minimal amount outlines the left psoas muscle and suprarenal area (solid arrows).

(b) Lateral chest film demonstrates the continuous channel of gas from the chest to the abdomen deep to the endothoracic fascia and transversalis fascia (arrows).

(Reproduced from Meyers.9)

2. The amount of free intraperitoneal subdiaphrag-matic air increases on inspiration and decreases on expiration, presumably reflecting the influence of the greater negative intraabdominal pressure beneath the diaphragm during inspiration. In contrast, extraperitoneal subdiaphragmatic gas appears to increase on expiration and decrease on inspiration on erect frontal films (Fig. 8-219). The extraperitoneal tissues are not affected by respiratory variations in intraperitoneal pressure so that with descent of the diaphragm, extraperitoneal gas is simply compressed more diffusely, resulting in a thinner crescentic collection.

The anatomic boundaries of the three extraperitoneal spaces and the dynamics of the spread of extraperitoneal gas clearly explain its distribution and localization. The radiologic criteria that allow identification of the likely primary source of extraperitoneal gas are summarized in Table 8-3.

Extraperitoneal Tissue

Fig. 8-219. Subdiaphragmatic extraperitoneal gas.

Erect films demonstrate a greater accumulation of subdiaphragmatic extraperitoneal gas within the posterior pararenal spaces during expiration. (Reproduced from Meyers.9)

Fig. 8-219. Subdiaphragmatic extraperitoneal gas.

Erect films demonstrate a greater accumulation of subdiaphragmatic extraperitoneal gas within the posterior pararenal spaces during expiration. (Reproduced from Meyers.9)

Table 8-3. Spread and localization of extraperitoneal gas

Extraperitoneal Side of compartment Localizing roentgen features abdomen Most likely sources

Table 8-3. Spread and localization of extraperitoneal gas

Extraperitoneal Side of compartment Localizing roentgen features abdomen Most likely sources

Anterior pararenal

Medially: gas extends beyond the lateral border of

Right

Perforation of the descending

the psoas muscle toward tbe spine; on oblique

duodenum

projections the outline of the muscle may be

Left

Perforated sigmoid diverticulitis

preserved.

Laterally; there is no extension into the flank stripe

Bilateral

Sigmoid perforation into

except possibly inferiorly, below the cone of renal

mesocolon; fulminating

fascia.

pancreatitis

Superiorly: the renal outline may be preserved.

Perirenal

Gas collection presents an inferiorly convex border

Right

Renal infection

overlying the iliac crest.

Occasionally, perforation of the

Most prominent within the rich far posterior to the

descending duodenum

kidney.

Renal outline is enhanced.

Left

Renal infection

Inflammatory thickening and displacement of the

renal fascia.

Posterior pararenal

Medially: gas is limited by and parallels the margin

Left

Sigmoid diverticulitis

of the psoas muscle.

Laterally: gas extends into the flank stripe.

Bilateral

Rectal perforation

Superiorly: gas outlines the suprarenal area,

Supradiaphragmatic origin

diaphragm, and posterior aspects of the liver and

spleen.

Extension above the diaphragm leads to

pneumomediastinum and cervical subcutaneous

emphysema.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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