NEUROPATHOLOGY FOR MEDICAL STUDENTS
Presented by William I. Rosenblum, MD
CHAPTER 5--PATHOLOGY OF CNS INFECTION
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
STRUCTURES PROTECTING THE BRAIN
Over the surface of the brain and spinal cord, there are three protective coats or meninges. The thin (or "lepto") meninges are the innermost coverings and consist of two distinct membranes.
The first is the pia mater, which is tightly applied to the surface of the brain and spinal cord.
The second component of the leptomeninges is the arachnoid membrane. This membrane is external to the pia mater and connected with it by delicate trabeculae. The space between the pia mater and the arachnoid is called the subarachnoid space. This space is filled with cerebrospinal fluid, and it is this fluid which is sampled when a spinal puncture is performed. The surface blood vessels course in this space. The term "leptomeningitis," or simply "meningitis," refers to an infection within this space.
The outer most membrane covering the brain is much thicker than the leptomeninges. It is known as the dura mater and is tightly applied to the bones of the skull. Over the spinal cord, the dura is separated from the vertebral column by a space which contains adipose tissue and blood vessels. For this reason, epidural abscesses occur more readily here than inside the calvarium. In addition to the leptomeninges and the dura, the bony coverings of the brain and spinal cord and the skin form the outermost defenses of the central nervous system.
RESPONSE OF CELLS WITHIN THE BRAIN OR CORD
The astrocytes and the reticulo-endothelial elements of the brain or microglia, are the two types of brain or cord cells which may respond in a non-specific manner to a wide variety of noxious stimuli including infectious organisms. The microglia may simply proliferate while retaining their basic rod-like shape. In addition, monocytes enter the wound or lesion and form macrophages which carry away the necrotic debris.
The astrocytes may increase in number, may become larger, and may increase the length and numbers of their processes. However, astrocytic processes do not act well to wall off or impede the advance of infectious organisms. Instead, fibroblasts in the walls of cerebral blood vessels may proliferate, and lay down collagen to form a wall around bacterial invaders. Thus, abscesses within the brain can be walled off like abscesses anywhere in the body.
Unfortunately, since fibroblasts are not diffusely scattered throughout the brain, but are only present in vessel walls, and since the only other reservoir of fibroblasts is the meninges, the wall of a cerebral abscess may be less sturdy than that of abscesses outside the brain.
Leptomeningeal fibroblasts may also proliferate in response to smoldering subacute or chronic infections of the subarachnoid space and can actually impede or block flow of CSF to the point of producing hydrocephalus. Hydrocephalus may also be produced by a ventriculitis, which may cause inflammation, necrosis, and desquamation of the ependyma (the cells lining the ventricular system) at the level of the aqueduct of Sylvius.
In such cases, the inflammation produces hydrocephalus by causing aqueductal stenosis, thereby reducing drainage of CSF from the lateral ventricles and third ventricle, and increasing the cerebrospinal fluid pressure in these ventricles. Since the communication between the anterior and posterior portions of the ventricular system is compromised by aqueductal stenosis, the rise in pressure may not be detected by a spinal puncture because such a puncture enters the subarachnoid space below the point of blockade.
ROUTES OF ENTRY AND POTENTIAL SOURCES OF CNS INFECTION
Sinusitis, otitis media, and mastoiditis are still important sources of CNS infection. Otitis media may still be the leading cause of brain abscess. Spread of infection from the linings of the sinuses occurs through the bone (osteomyelitis) which is tissue paper thin or along veins in a retrograde manner (thrombophlebitis). Spread of infection through the calvarium from the scalp may also occur via the emissary veins.
Another common direct source of infections results from trauma such as bullet wounds, skull fractures, and surgery. Basilar skull fractures produce defects in the bony sinuses that will allow the flora of the upper respiratory tract to enter the CNS.