Infections of the Central Nervous System


Chapter 197

Infections of the Central Nervous System



Daniel W. O’Neill, Robyn M. Jennings



Definition and Epidemiology


Infections of the central nervous system (CNS) consist primarily of meningitis (inflammation of the meninges) and encephalitis (inflammation of the brain) and are caused by a variety of pathologic microorganisms. The high morbidity and mortality rates of bacterial meningitis make diagnosis and early treatment a high priority in the primary care setting. Bacterial meningitis is most common in children younger than 2 years, with a peak incidence at 3 to 8 months of age; however, it does occur throughout the life span, with a second peak incidence after 60 years of age. In the United States, the annual overall incidence rate is 1.3 to 2 per 100,000 persons.1 Despite the use of effective antimicrobial therapy, annual mortality rates remain at 17% to 30%, with up to 50% of survivors having some long-term neurologic sequelae. 2


Encephalitis is caused primarily by herpesviruses (most common in the United States), arboviruses (transmitted by insects), and enteroviruses.3 An increase in encephalitis caused by cytomegalovirus, Epstein-Barr virus (EBV), and human herpesvirus is occurring because of an increase in immunocompromised states, including human immunodeficiency virus (HIV) infection, organ transplantation, and chemotherapy. Viruses cause CNS infection by direct spread of cranial nerve or olfactory tract infections, reactivation of a latent virus within the CNS, or viremia followed by spread across the blood-brain barrier.3


Meningitis is defined as either aseptic or septic, depending on the identification of bacteria on the Gram stain or culture. Aseptic meningitis is caused mostly by enteroviruses, for which there is a good prognosis and no specific therapy. Bacterial meningitis is usually spread hematogenously from another primary source (predominantly the respiratory tract) or by contiguous spread from sinusitis, mastoiditis, or otitis media. The pathogens in meningitis are age specific: group B streptococci and Escherichia coli are most common in children younger than 1 month; Listeria monocytogenes is more common in the very young (younger than 1 month) and adults older than 50 years; and Streptococcus pneumoniae and Neisseria meningitidis are the most common causes in children and adults, with the latter seen in epidemics involving young adults.4 Haemophilus influenzae used to be the leading cause of meningitis in young children until the advent of universal vaccination.1 Because of the widespread overuse of oral antibiotics, there has been a dramatic rise in multidrug-resistant S. pneumoniae.4 N. meningitidis can occur in epidemic outbreaks in young adults. Elderly adults have a notably higher percentage of infections with L. monocytogenes. Staphylococci and gram-negative bacilli are seen in meningitis associated with neurosurgery and trauma.1 In 2012, 158 cases of iatrogenic fungal meningitis were reported in the United States in patients who received contaminated epidural steroid injections.5


Risk factors for bacterial meningitis are previous basilar skull fracture or neurosurgery, sickle cell disease, complement deficiency, asplenia, alcoholism, immunodeficiency (HIV infection or organ transplant recipient), recent travel to an endemic area, and exposure to a community outbreak. Once the pathogen gains access to the cerebrospinal fluid (CSF), where there is little natural host defense, it replicates and releases bacterial cell wall proteins, which stimulate cytokine release and capillary leak. This leads to the accumulation of protein and leukocytes, cerebral edema, microvascular thrombosis, and, ultimately, cerebral ischemia and hypoxia.



Clinical Presentation


The onset of symptoms of CNS infection can be either acute or subacute, with progression during several days. The classic adult presentation of bacterial meningitis is fever, headache, and stiff neck (meningismus); however, this is seen in only 44% of cases.6 Altered levels of consciousness, seizures, and hypotension predict a poor outcome. Nausea, vomiting, and photophobia are more common but can also be seen with migraine. Ear, sinus, or lung infections may precede pneumococcal meningitis. In fact, older adult patients on presentation may lack fever or meningismus but may be confused or even obtunded, often after an antecedent infection such as bronchitis, pneumonia, sinusitis, or urinary tract infection.7 Encephalitis manifests with signs and symptoms similar to those of meningitis but with more prevalent alterations in consciousness, focal neurologic signs, seizures, and autonomic and hypothalamic disturbances.3



Physical Examination


Nuchal rigidity, Kernig sign, and Brudzinski sign have low sensitivity but a moderate positive predictive value for meningitis in adults.8 In older adults, nuchal rigidity has an even lower sensitivity and specificity. Kernig sign is present if a patient in the supine position resists passive knee extension when the hip is fully flexed on the abdomen. Brudzinski sign is present if a patient in the supine position actively flexes the hips when the neck is passively flexed. Jolt sensitivity (observing a worsening of headache when a patient moves his or her head twice horizontally) is another test for meningitis with low sensitivity but may be used as an adjunctive test.8,9 Purpura and petechiae are often associated with rapidly progressing meningococcemia but can be seen with other infections or can be a sign of disseminated intravascular coagulopathy. In patients with meningitis, a careful neurologic examination may reveal focal deficits suggestive of brain abscess, cranial nerve inflammation, thrombosis, ischemia, or cerebral edema.1 Meningitis can lead to increased intracranial pressure (ICP), which manifests as depressed consciousness, sluggishly reactive or dilated pupils, ophthalmoplegia, respiratory depression, bradycardia, hypertension, posturing, hyperreflexia, and spasticity. With clinical presentation alone, it is difficult to distinguish aseptic meningitis from bacterial meningitis or encephalitis.



Diagnostics



• Blood cultures (positive in 19% to 70% of patients with bacterial meningitis6), complete blood count (CBC) with differential, and serum glucose concentration should be obtained immediately. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) can be useful for following the course of the illness.


• Lumbar puncture (LP) must be performed in all patients with suspected meningitis or encephalitis.


• The following are contraindications for LP: cardiorespiratory compromise, shock, evidence of increased ICP, or cellulitis over the LP site. Thrombocytopenia, coagulopathies, and use of anticoagulants are relative contraindications.6


• Opening CSF pressures should be measured.


• A sample of the CSF should be sent for protein level, glucose concentration, Gram stain, culture, and cell count with differential. A positive Gram stain examination of CSF has nearly 100% specificity, but a negative Gram stain cannot rule out bacterial disease.1 Rapid testing of the CSF for antigens of several common pathogens is widely available but not routinely used except in cases of prior antibiotic therapy.


• Extra tubes of CSF should be held for special studies, if indicated.


• Interpretation of CSF values is helpful in distinguishing viral from bacterial infections (Table 197-1). Further testing of the CSF with viral cultures, polymerase chain reaction, specialized stains, and cultures may be indicated.


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Oct 12, 2016 | Posted by in CRITICAL CARE | Comments Off on Infections of the Central Nervous System

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