NEUROPATHOLOGY FOR MEDICAL STUDENTS
Presented by William I. Rosenblum, MD
CHAPTER 5--PATHOLOGY OF CNS INFECTIONS
This chapter contains four interrelated sections. The other three sections are:
PRETEST: Answers can be found in the text of this section or click on link at end of questions
THE PURULENT REACTION
The purulent reaction (image below) is characterized by polymorphonuclear cells mixed with fibrin and bacteria. The great abundance of neutrophils imparts a characteristic creamy, yellow-white appearance to the pus which forms the center of an abscess or fills the subarachnoid space in meningitis.
A brain with severe meningitis is shown in the image BELOW. Note that the creamy pus completely obscures the underlying cortex in some areas. Pus also tends to collect in the cisterns at the base of the brain. Purulent meningitis is by far the most common CNS infection. Though antibiotics have not materially decreased its incidence, they have markedly increased survival and reduced the complications of this disease.
COMPLICATIONS OF MENINGITIS
The purulent reaction caused by bacteria may involve the vessels of the subarachnoid space and cause thrombosis resulting in small, cortical infarcts. Thrombosis of the superior sagittal sinus may occur. The underlying brain may also be infected by direct spread from the meningitis and abscesses may form. If the meningitis is allowed to smolder, there will be fibrosis of the subarachnoid space which will impede flow of CSF and cause hydrocephalus.
This fibrosis may also cause CSF to loculate into arachnoid cysts which may produce pressure effects like a tumor. The cranial nerves may be involved with infection or strangled by reactive connective tissue.
The incidence of brain abscess has been markedly lowered since the antibiotic era. As would be expected, they are found more frequently in individuals who are susceptible to general infection such as diabetics, alcoholics, debilitated and immunosuppressed patients, and the elderly. They are also noted with increased frequency in infants with cyanotic heart disease.
Brain abscesses are most frequently caused by staphylococcus, streptococcus, and pneumococcus and the primary infections are usually located in the sinuses (ear), lungs, or heart valves.
The image above shows a large abscess in the brain. The purulent center is surrounded by a capsule. Often a zone of hyperemia is present adjacent to the wall and there is marked swelling of the adjacent brain tissue.
The evolution of the abscess is as follows:
Antibiotic therapy greatly decelerates the growth of an abscess, and may allow time for a complete capsule to form after which the abscess may be removed surgically.
The image below reviews the basic structural features of a brain abscess from the histologic point of view. It illustrates (A) the purulent, necrotic center, (B) the thin zone of granulation tissue, and (C) the collagenous connective tissue capsule.