Meningitis

 Infection within the subarachnoid space or throughout the leptomeninges is called meningitis. Based on the host’s response to the invading microorganism, meningitis is divided into two major categories: purulent and aseptic meningitis.

Purulent Meningitis. A patient with purulent meningitis typically has a marked, acute inflammatory exudative cerebral spinal fluid containing large numbers of poly morphonuclear cells (PMNs). Frequently, the underlying CNS tissue, in particular the ventricles, may be involved. If the ventricles become involved, this process is referred to as ventriculitis. Bacterial organisms are usually the cause of these infections.

Pathogenesis. The outcome of a host-microbe inter action depends on the characteristics of both the host and the microorganism. As previously indicated, an important host defense mechanism within the CNS is the blood-brain barrier; this barrier involves the choroid plexus, arachnoid membrane, and the cerebral microvascular endothelium. The unique structural properties of the vascular endothelium, such as the continuous inter cellular tight junctions, provide a barrier minimizing the passage of infectious agents into the CSF. The normal function of the vascular endothelium includes regulating the transport of nutrients in and out of the CSF, including low-molecular-weight plasma proteins, glucose, and electrolytes.

The host’s age and other underlying factors contribute to whether an individual is predisposed to the development of infectious meningitis. Neonates have the highest infection rate for meningitis, because of the immature neonatal immune system, the increased permeability of the blood-brain barrier in newborns, and the presence of colonizing bacteria in the female vaginal tract that can pass to the infant during childbirth. The most common bacterial pathogens responsible for meningitis in newborns are group B streptococci, Escherichia coli, and Listeria monocytogenes. Prior to the advent of the Hib vaccine in the United States in 1985, Haemophilus influenza type b (Hib) was a common cause of meningitis in children 4 months to 5 years of age. Because of the incorporation of Hib into childhood immunization programs, childhood Hib disease has dramatically declined.

Among young adults, Neisseria meningitidis is typically the agent that is associated with meningitis. N. meningitidis has been identified in epidemics among young adults in crowded conditions (e.g., military recruits and college dormitory mates). There are two meningococcal vaccines (vaccines for N. meningitidis) available in the United States. The meningococcal polysaccharide vaccine (MPSV4) is used for individuals older than 55 years of age, and the meningococcal conjugate vaccine (MCV4) is used for adolescents. Streptococcus pneumoniae is frequently the cause of meningitis in young children and the elderly; often this meningitis develops from bacteremia or from infection of the sinuses or middle ear. There are two pneumococcal vaccines (vaccines for S. pneumoniae) that are recommended currently in the United States. The pneumococcal conjugate vaccine (PCV13) protects against infection from 13 different serotypes of S. pneumoniae and is used for vaccination of children and adults. The second vaccine, pneumococcal polysaccharide vaccine (PPSV), provides protection from 23 serotypes of S. pneumoniae, including those associated with serious life-threatening infections. This vaccine is recommended for adults 65 years of age and older or anyone over the age of 2 who has long-term health problems or is immunocompromised.

Because the respiratory tract is the primary portal of entry for many etiologic agents of meningitis, factors that predispose adults to meningitis are often the same factors that increase the likelihood for the development of pneumonia or other respiratory tract colonization or infection. Alcoholism, splenectomy, diabetes mellitus, prosthetic devices, and immunosuppression contribute to increased risk. Finally, patients with prosthetic devices, particularly CNS and ventriculoperitoneal shunts, are at increased risk for developing meningitis.

For organisms to reach the CNS (primarily by the blood-borne route), host defense mechanisms must be overcome. Most cases of meningitis are a result of bacteria that share a similar pathogenesis. The successful meningeal pathogen must first sequentially colonize and cross host mucosal epithelium, then enter and thrive within the bloodstream. The most common causes of meningitis possess the ability to evade host defenses at each of these levels. For example, clinical isolates of Streptococcus pneumoniae and N. meningitidis secrete IgA proteases capable of destroying the host’s secretory IgA, thereby facilitating bacterial attachment to the epithelium. In addition, all of the most common etiologic agents of bacterial meningitis possess an antiphagocytic capsule that allows the organisms to evade destruction by the host immune system.

Organisms appear to enter the CNS by interacting and subsequently breaking down the blood-brain barrier at the level of microvascular endothelium. One of the least understood processes in the pathogenesis of meningitis is how organisms cross this barrier into the subarachnoid space. Nevertheless, there appear to be specific bacterial surface components, such as pili, polysaccharide capsules, and lipoteichoic acids, that facilitate adhesion of the organisms to the microvascular endothelial cells and subsequent penetration into the CSF. Organisms can enter (1) through loss of capillary integrity by disrupting tight junctions of the blood-brain barrier, (2) through transport within circulating phagocytic cells, or (3) by crossing the endothelial cell lining within endothelial cell vacuoles. After gaining access, the organism multi plies within the CSF, a site initially free of antimicrobial antibodies or phagocytic cells.

Clinical Manifestations. Meningitis can be classified as either an acute or a chronic disease in the onset and overall progression within the host.

Acute. Symptoms of acute meningitis include fever, stiff neck, headache, nausea and vomiting, neurologic abnormalities, and change in mental status.

In acute bacterial meningitis, the CSF usually contains large numbers of inflammatory cells (>1000/mm3), primarily polymorphonuclear cells (PMNs). The CSF shows a decreased glucose level relative to the serum glucose level and an increase in protein concentration. In a healthy individual, the normal CSF glucose level is 0.6 of the serum glucose level and ranges from 45 to 100 mg/ dL; the CSF protein range in an adult is 15 to 50 mg/dL; newborn CSF protein ranges run as high as 170 mg/dL with an average of 90 mg/dL.

The sequelae of acute bacterial meningitis in children are frequent and serious. Seizures can occur in 20% to 30% of patients, and other neurologic changes are common. Acute sequelae include cerebral edema, hydro cephalus, cerebral herniation, and focal neurologic changes. Permanent deafness can occur in 10% of children who recover from bacterial meningitis. Other subtle physiologic and psychological sequelae may also follow an episode of acute bacterial meningitis.

Chronic. Chronic meningitis can often occur in patients who are immunocompromised, although this is not always the case. Patients experience an insidious onset of disease, with some or all of the following symptoms: fever, headache, stiff neck, nausea and vomiting, lethargy, confusion, and mental deterioration. Symptoms may persist for a month or longer before treatment is sought. The CSF usually manifests an abnormal number of white blood cells (usually lymphocytic), elevated protein, and decrease in glucose content (Table 1). The pathogenesis of chronic meningitis is similar to that of acute disease.

Table1. Guidelines for Interpretation of Results Following Hematologic and Chemical Analysis of Cerebrospinal Fluid (CSF) from  Children and Adults (Excluding Neonates)

Epidemiology/Etiologic Agents-Acute Meningitis. The etiology of acute meningitis depends on the age of the patient. Most cases in the United States occur in children younger than 5 years of age. Before 1985, H. influenzae type b H. influenzae type b was the most common infectious agent in children between 1 month and 6 years of age within the United States. Ninety-five percent of all cases were due to H. influenzae type b, Neisseria meningitidis, and Streptococcus pneumoniae. In 1985, the first Hib vaccine, a polysaccharide vaccine, was licensed for use in children 18 months of age or older but was not efficacious in children younger than 18 months. However, the widespread use of conjugate vaccine, Hib polysaccharide protein conjugate, in children as young as 2 months of age has significantly affected the incidence of invasive H. influenzae type b disease; the total number of annual cases of H. influenzae disease in the United States have been reduced by 55% and the number of cases of H. influenzae meningitis by 94%. However, the risks for meningococcal and pneumococcal diseases resulting from agents other than H. influenzae have remained level. Children older than 6 years of age are less likely to develop meningitis, but the risk for meningitis infection increases when the child reaches early adulthood. As previously mentioned, neonates have the highest incidence of acute meningitis, with a concomitant increased mortality rate (as high as 20%). Organisms causing disease in the newborn are different from those that affect other age groups; many of them are acquired by the newborn during passage through the mother’s vaginal vault. Neonates are likely to be infected with, in order of incidence, group B streptococci, Escherichia coli, other gram-negative bacilli, and Listeria monocytogenes; occasionally other organisms may be involved. For example, Elizabethkingia meningoseptica has been associated with nursery outbreaks of meningitis. This organism is a normal inhabitant of water in the environment and is presumably acquired as a nosocomial infection.

Important causes of meningitis in the adult, in addition to the meningococcus in young adults, include pneumococci, Listeria monocytogenes, and, less commonly, Staphylococcus aureus and various gram-negative bacilli. Meningitis caused by the latter organisms results from hematogenous seeding from various sources, including urinary tract infections. The percentage of adults with nosocomial bacterial meningitis at large urban hospitals has been increasing. The various etiologic agents of chronic meningitis are listed in Box 1.

Box1. Etiologic Agents of Chronic Meningitis

Aseptic Meningitis. Aseptic meningitis is usually viral and characterized by an increase of lymphocytes and other mononuclear cells (pleocytosis) in the CSF; bacterial and fungal cultures are negative. (This is in contrast to bacterial meningitis, which is characterized by purulence and the polymorphonuclear [PMN] cell response in the CSF.) Aseptic meningitis is usually self-limiting with symptoms that may include fever, headache, stiff neck, nausea, and vomiting.

In addition to the increase of lymphocytes and other mononuclear cells in the CSF, the glucose level remains normal, whereas the protein CSF level may remain normal or be slightly elevated. Aseptic meningitis can also be a symptom for syphilis and some other spirochete diseases (e.g., leptospirosis and Lyme borreliosis). Stiff neck and CSF pleocytosis may also be associated with other disease processes, such as malignancy.

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اخبار العتبة العباسية المقدسة

جامعة العميد تقيم الملتقى العلمي الثاني لخريجي كلية طب الأسنان

ضمن دورتها العلمية (في رحاب العقيدة) الهيأة العليا لإحياء التراث تقدم محاضرة عن مفهوم الأسبقية في الإسلام

معهد القرآن الكريم النسوي ينهي استعداداته لإقامة مهرجان السقاء القرآني الخامس

قسم الشؤون الفكرية يقيم ورشة تطويرية عن مهارات الإلقاء والخطابة في ذي قار

المزيد

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اللحظات الأخيرة قبل الموت.. هذا هو تسلسل "فقدان الحواس"

وخز الأطراف وضعف الذاكرة.. علامات على نقص فيتامين هام

عادة صباحية بسيطة.. تأثيرها قد يحسن ضغط الدم

عادات يومية تدمر قلبك بصمت.. خبراء يطلقون التحذير

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الآخبار التكنلوجية

الذكاء الاصطناعي يفك لغز آثار أقدام الديناصورات

القمر أولا.."سبيس إكس" تؤجل خططها الخاصة بالرحلات إلى المريخ

الذكور أم الإناث؟ دراسة "تنسف الشائع" بشأن التوحد

خبير يكشف عن اقتراب العلماء من فك شفرات "لغات الحيوانات" !

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مواضيع ذات صلة

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Pneumonia

Bronchitis

Pathogenesis of the respiratory tract: Microorganism Factors