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Infectious Disease Emergencies: Part 3, CNS Infections

Infectious Disease Emergencies: Part 3, CNS Infections

ABSTRACT: Bacterial meningitis and herpes simplex virus type 1 (HSV-1) encephalitis are both associated with significant morbidity and mortality. Thus, hospitalization and prompt initiation of therapy are essential. Patients with bacterial meningitis are usually extremely ill and commonly present with severe and generalized headache and at least 1 symptom from the following classic triad: nuchal rigidity, fever, and mental status changes. Analysis of cerebrospinal fluid (CSF) helps distinguish bacterial from viral meningitis by Gram staining, culture, and composition. Imaging studies such as CT are typically used only when the diagnosis is uncertain or if there is concern about increased intracranial pressure. Patients with HSV-1 encephalitis may complain of headache and fever of rapid onset; they may also exhibit 1 or more focal neurological findings. CSF analysis typically shows a lymphocytic pleocytosis with an increased number of erythrocytes and an elevated protein level.


Key words: community-acquired bacterial meningitis, herpes simplex type 1 encephalitis


Potentially life-threatening infectious diseases need to be recognized early so that intervention can be started promptly. The goal of this 3-part series is to help you quickly narrow the diagnostic possibilities and assess the likelihood of serious illness.

In the first 2 articles in this series, we addressed Rocky Mountain spotted fever, meningococcemia, and necrotizing fasciitis (CONSULTANT, November 2010, page 473) and staphylococcal toxic shock syndrome (TSS) and streptococcal TSS (CONSULTANT, December 2010, page 507). Here we focus on CNS infections (Table).

COMMUNITY-ACQUIRED BACTERIAL MENINGITIS
Overview.
Bacterial meningitis is primarily caused by Streptococcus pneumoniae, Neisseria meningitidis, and Listeria monocytogenes (especially in patients older than 60 years). Each year an estimated 1.2 million cases occur worldwide.1 It is among the most common infectious causes of death, responsible for about 135,000 deaths annually throughout the world. Despite proper treatment, survivors often have neurological sequelae (eg, hearing loss).

Clinical features. Patients are usually extremely ill and commonly present with at least 1 symptom from the following classic triad2-5:

• Nuchal rigidity.
• Fever.
• Mental status change.

Patients also typically complain of a severe and generalized headache. Other findings or manifestations of illness may include2-5:

• Photophobia.
• Seizures.
• Focal neurological deficits.
• Petechiae or palpable purpura (especially in cases of meningitis caused by N meningitidis).

Patients usually have an abnormal temperature (either hypothermic or febrile)6 and may even be hypotensive, depending on the severity of illness. Nuchal rigidity can be easily demonstrated by either the Brudzinski sign (ie, spontaneous flexion of hips during attempted passive flexion of the neck) or the Kernig sign (ie, inability or reluctance to allow full extension of the knee when the hip is flexed 90 degrees); both are effective in illustrating nuchal rigidity, even if patients do not specifically complain of neck stiffness. The presence of hypotension, altered mental status, and/or seizures was found to correlate with adverse outcome (ie, death or neurological deficit) in several cohort studies.7

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Diagnostic studies. Results of a complete blood cell count with differential may include leukocytosis with left shift or leukopenia. More useful laboratory studies are blood cultures and analysis of cerebrospinal fluid (CSF) obtained by lumbar puncture (LP). Analysis of CSF helps distinguish bacterial from viral meningitis by Gram staining, culture, and composition. CSF chemistry and cytological findings highly suggestive of bacterial meningitis include2,3:

• Protein concentration greater than 500 mg/dL.
• Glucose concentration less than
45 mg/dL.
• White blood cell count greater than 1000/¨L.

Imaging studies such as CT have a limited role in the diagnosis of bacterial meningitis. Instead, they are typically used when the diagnosis is uncertain or if there is concern about increased intracranial pressure. The Infectious Diseases Society of America (IDSA) has published recommended criteria for adult patients with suspected bacterial meningitis who should undergo CT scanning before LP; these include8:

• Immunocompromised state.
• History of CNS disease (eg, mass lesion, stroke, or focal infection).
• New-onset seizure within 1 week of presentation.
• Papilledema.
• Abnormal level of consciousness.
• Focal neurological deficit.

For patients in whom CT scanning is warranted before LP, blood cultures should be obtained and empiric antibiotic therapy (with the assumption that antimicrobial resistance is likely) started without delay.

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