Bacterial meningitis: Description, Causes and Risk Factors:
Bacterial meningitis is a medical emergency requiring immediate diagnosis and immediate treatment. Streptococcus pneumoniae and Neisseria meningitidis are the most common and most aggressive pathogens of meningitis.
Bacterial meningitis is an inflammation of the meninges, in particular the arachnoid and the pia mater, associated with the invasion of bacteria into the subarachnoid space, principles known for more than 100 years. The pathogens take advantage of the specific features of the immune system in the CNS, replicate and induce inflammation. A hallmark of bacterial meningitis is the recruitment of highly activated leukocytes into the CSF. Beside bacteria, viruses, fungi and non-infectious causes as in systemic and neoplastic disease as well as certain drugs can induce meningeal inflammation.
Several cell types seem to be involved and as mentioned endothelial cells, perivascular macrophages and mast cells may play a crucial role. Heat killed bacteria and pathogen-associated molecular patterns (PAMP) of meningitis pathogens as lipoprotein (LP), lipotechoic acid (LTA), peptidoglycan (PG), and lipopolysaccharide (LPS) cause meningitis indistinguishable from living bacteria. Immune pattern recognition molecules as CD14 and LBP function as sensors in identifying PAMPs. Pneumococcal PG and LP are recognized by TLR2 whereas LPS, and interestingly the pneumococcal toxin pneumolysin, signal through TLR4. TLR signals are conveyed by the intracellular adapter protein MyD88 downstream to a multitude of inflammatory signaling cascades including NF-kB and MAP kinases leading to a rapid inflammatory response in meningitis.
During the last 20 years, the epidemiology of bacterial meningitis has dramatically changed. Haemophilus influenzae, formerly a major cause of meningitis, has disappeared in developed countries and serves as a remarkable example of a successful vaccination campaign. Nowadays, pneumococci are the most important cause of bacterial meningitis in children and adults in the US as well as in Europe.
With Haemophilus on the decline, Neisseria meningitides has become the leading meningitis pathogen in developing countries, but it continues to pose a major health problem in the US and Europe. In addition to classical meningitis, meningococci frequently cause systemic disease including fulminant gram-negative sepsis and disseminated intravascular coagulopathy.
The current assumption is that high-grade bacteremia precedes meningitis and that bacteria invade from the blood stream to the central nervous system (CNS). Alternatively, direct accesses to the CNS through dural defects or local infections are potential entrance routes. In the clinical setting, such defects should be identified by CCT (cranial computed tomography) or MRI scans.
Early clinical features of bacterial meningitis are nonspecific and include fever, malaise and headache; and later on, meningismus (neck stiffness), photophobia, phonophobia and vomiting develop as signs of meningeal irritation. Headache and meningismus indicate inflammatory activation of the trigeminal sensory nerve fibers in the meninges and can be blocked experimentally.Approximately 33% of patients develop focal neurological signs, such as epileptic seizures or paresis of a limb, and up to 69% present with impaired consciousness or 14% with coma.
The key to the diagnosis of bacterial meningitis is the proof of bacteria in the CSF by Gram-staining or a positive bacterial culture. Detection rates in the CSF may be as high as 90%, while about 50% positive results are observed in blood cultures. The diagnostic yield of CSF microscopy can be improved by centrifugation of a larger sample and experience. Polymerase chain reaction (PCR) may be attempted if microscopic and cultural identification of the pathogen fail but is not yet a routine test. PCR has an important role in strain identification mostly in meningococcal disease. Latex agglutination-based rapid tests are available for major meningitis pathogens, but imperfect sensitivity and specificity argue against routine clinical use at this time.
Inflammatory activation of endothelial cells seems to be a prerequisite for bacterial invasion but also results in the regulation of adhesion molecules as ICAM-1 (intercellular adhesion molecule-1). Subsequently, these molecules promote the multi-step process of leukocyte invasion. Leukocytes, in particular the presence of granulocytes in the CSF, are the diagnostic hallmark of meningitis.
Cerebral imaging and repeat lumbar puncture should be considered in patients who fail to improve clinically after 48 h of treatment to assess antibiotic failure.
Despite modern antibiotics and improved critical care, bacterial meningitis is still an unresolved problem in clinical medicine. Although highly effective antibiotics kill bacteria efficiently, mortality rates are still up to 34%.
Immediate antibiotic therapy is imperative and must not be postponed by diagnostic delays; for example, waiting for a CT scan. Pre-hospital antibiotic treatment is advised in cases of suspected meningococcal disease but depends on local resistance situation and the medical environment. Prior to treatment, a blood culture should be obtained. Since microbiological identification of the pathogen is not immediately available, the initial choice of antibiotics is usually empirical.
Corticosteroids reduce brain edema, intracranial hypertension and meningeal inflammation in experimental models of bacterial meningitis. Subsequent clinical studies have led to conflicting results concerning potential benefits of steroid use in patients with meningitis.
Severe headache requires generous analgesia, often including opioids. Antiepileptic treatment is indicated if seizures occur; prophylactic treatment is not recommended.
Local infections are especially frequent in pneumococcal meningitis and may require surgical treatment.
The most effective way to protect you and your child against certain types of bacterial meningitis is to complete the recommended vaccine schedule. There are vaccines for three types of bacteria that can cause meningitis: Neisseria meningitidis (meningococci), Streptococcus pneumoniae (pneumococci), and Haemophilus influenzae type b (HIB).
NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.