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ACUTE BACTERIAL MENINGITIS

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Introduction

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Bacterial meningitis remains the most dangerous and often rapidly fatal infection. Hence, a timely diagnosis and prompt treatment are key to preventing mortality from this disease. Annual incidence in the United States is 3/100,000 population.

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CASE 7-1

A 21-year-old man with no significant past medical history presents with a 1-day history of fever, headache, and rash. Twelve hours earlier he developed a headache and fever. He told his friends from the dormitory where he attends college that he did not feel well and was going to rest. His friends went to check on him a few hours later and found him confused and brought him to the Emergency Department (ED). On physical examination he was obtunded with a fever of 40ºC (104ºF), nuchal rigidity, and a purpuric (non-blanching) rash on his extremities. Fundoscopy showed no papilledema. Which anatomic site of the central nervous system (CNS) is involved?

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Anatomy of meninges

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The brain is surrounded and protected by three connective tissue layers called meninges. These meninges, from superficial to deep, are the dura mater, arachnoid mater, and pia mater (see Figure 7-1).1 The dura mater is made up of outer periosteal layer and the inner meningeal layer. The dural venous sinuses are venous channels located between the periosteal and the meningeal layers of the dura mater. The venous sinuses, in addition to receiving blood from the cerebral, diploic, and emissary veins, receive the cerebrospinal fluid (CSF), drained by the arachnoid granulations. Deeper to the dura is the arachnoid followed by pia mater. Between the arachnoid and the pia mater is the subarachnoid space in which the CSF circulates. CSF is the special ultrafiltrate of plasma that bathes and protects the brain. CSF is produced mainly by the choroid plexus located in the lateral ventricles and the fourth ventricle. The spinal cord is also enveloped in arachnoid, so that CSF covers its surface as well. A specimen of CSF is commonly obtained through a lumbar puncture (LP) performed between the fourth and the fifth lumbar space when meningitis is suspected. The arachnoid granulations around the longitudinal fissure reabsorb CSF into the dural sinuses. Obstruction of CSF flow causes hydrocephalus. The capillaries of the CNS differ from other anatomical areas due to presence of tight junctions linking the endothelial cells. The limited permeability forms a physiologic barrier referred to as the blood–brain barrier (BBB). The BBB protects the brain from toxic substances and pathogens but, on the other hand, also prevents entry of immunoglobulins, complements, and antibiotics.

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Figure 7-1

Anatomy of the meninges. Reproduced with permission from Waxman S: Clinical Neuroanatomy, 27th edition. New York: McGraw-Hill Professional; 2013.

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Definition

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Meningitis is an inflammation of the arachnoid membrane, the pia mater, and the intervening CSF. ...

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