inflammation: morphology and consequences

Chapter 4 Acute inflammation: morphology and consequences

How may acute inflammation be manifest in a clinical setting?

What are the sequelae of acute inflammation?

A number of morphological patterns of acute inflammation occur depending on the anatomical structures involved, and many descriptive terms have been coined for them.

Morphological appearance

Suppurative (purulent) inflammation

The terms suppurative and purulent imply the production of pus, which consists of dead and dying neutrophils, liquefied necrotic tissue and infecting microorganisms. Pus is sticky fluid, typically yellow or grey in colour, although haemorrhage may turn it red or brown. Certain bacteria such as Staphylococcus aureus are particularly associated with the production of pus and are, therefore, termed pyogenic. Pus is classically described as having a solid component (the neutrophils, necrotic debris and organisms) and a liquid component (the inflammatory exudate and dissolved substances, including nucleic acids, which give pus its sticky characteristics).

Abscesses, sinuses and fistulae

An abscess (Fig. 3.4.1) is a collection of pus within tissue. If it persists, it becomes walled off by a surrounding rim of granulation tissue. Sometimes, an abscess will spontaneously discharge via the skin or into a body cavity; it is said to ‘point’ as it erodes towards the surface of the tissue. When it has burst and discharged its contents, it may be obliterated by granulation tissue and healing will occur by fibrosis. Alternatively, the route along which the pus was discharged may persist as a sinus tract, which is defined as a connection between the tissue and a surface (epidermal or epithelial); the tract may be lined by granulation tissue and/or by epithelium derived from the surface. For example, osteomyelitis is often complicated by chronic sinuses connecting the infected bone to the surface of the skin, allowing pus from the site of inflammation to drain. Sometimes, an analogous process will produce a fistula, which is an abnormal connection between two epithelial surfaces or an epithelial surface and the skin. For example, a segment of ileum that is inflamed in Crohn’s disease may become adherent to the bladder, and if the inflammation spreads into the bladder wall a tract connecting the ileum and the bladder (an ileovesical fistula) may form. A similar process can produce fistulae between the ileum and another segment of bowel, the vagina or the skin. If a hollow organ (e.g. gallbladder or appendix) or anatomical space (e.g. pleural cavity) fills with pus, the term empyema may be used.