Overview

Meningitis and encephalitis can also appear in mixed forms: a meningeal infection can spread to the brain (and/or spinal cord), or vice versa, causing meningo(myelo)encephalitis. The latter term is only used if the patient unequivocally manifests clinical signs of both meningeal and cerebral involvement.

Localization and Nomenclature

The sites and nomenclature of infectious diseases of the CNS are summarized in ▶ Fig. 6.43.

The Meningitic Syndrome

The general features of the meningitic syndrome are as follows:

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  Headache.

  
  
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  Fever (though elderly and immune-deficient patients are often afebrile).

  
  
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  Nausea and vomiting due to intracranial hypertension.

  
  
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  Meningismus, which, in severe cases, may be evident as a spontaneous extended posture of the neck, or opisthotonus.

  
  
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  Positive meningeal signs, with the patient examined in the supine position:

  
  
  
  
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    Lasègue sign: passive elevation of an extended leg to 45 degrees (at most) induces shooting pain into the leg.

    
    
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    Brudzinski sign: passive neck flexion induces reflexive knee flexion.

    
    
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    Kernig sign: passive elevation of the extended legs induces reflexive knee flexion.

The clinical aspects of individual types of meningitis depend on the pathogen and the immune state of the host.

Meningitis can be classified as shown in ▶ Table 6.19.

The Encephalitic Syndrome

The encephalitic, rather than meningitic, syndrome is characterized by focal neurologic and neuropsychological deficits as well as a variably severe impairment of consciousness. Encephalitis, like meningitis, can be of viral, bacterial, fungal, protozoal, or parasitic origin. Prion diseases are a special category of encephalitis. Encephalitis can also arise on an autoimmune or paraneoplastic basis; in such cases, it mainly affects the limbic system (see section ▶ 6.8.8).

The infectious processes that cause encephalitis often also involve other structures in the nervous system besides the brain (e.g., the peripheral nerves and plexuses, nerve roots, spinal cord, and meninges). In particular, the three important clinical varieties of spirochetal infection (syphilis, borreliosis, and leptospirosis) often present initially with meningitic or polyradiculitic/polyneuritic manifestations.

The general features of an encephalitic syndrome are as follows:

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  Fever.

  
  
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  Headache.

  
  
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  Impairment of consciousness.

  
  
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  Personality changes and neuropsychological abnormalities.

  
  
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  Epileptic seizures.

  
  
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  Focal neurologic deficits.

The diseases covered in this section are listed in ▶ Table 6.20.

General Diagnostic and Therapeutic Measures in Cases of Suspected CNS Infection

History The following should be specifically asked about:

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  Contact with animals (e.g., certain viruses, leptospira, Coxiella burnetii).

  
  
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  Tick bites, fleas, mites (borreliosis, early summer meningoencephalitis, rickettsiae).

  
  
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  Swimming in ponds (amebiasis).

  
  
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  Consumption of unpasteurized dairy products (listeriosis, brucellosis).

  
  
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  Past medical history (immune deficiency, diabetes, surgery, endocarditis, or recent pneumonia, mumps, or measles). The patient’s past illnesses may predispose to CNS infection with certain types of pathogen.

Physical examination

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  Meningismus (abnormal response to passive movement of the neck), neurologic examination.

  
  
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  Inspection: skin (petechiae, e.g., in meningococcal sepsis), oral cavity and throat (pharyngitis, tonsillitis, teeth, and gums).

  
  
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  Palpation: lymph nodes, trigeminal exit points (sinusitis as the source of CNS infection).

  
  
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  Auscultation of the heart and lungs.

Further procedure If the clinical signs warrant, blood cultures should be drawn.

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  No intracranial hypertension or focal deficit: CSF is obtained by LP and treatment is begun with antibiotics (directed against the most likely pathogen) and glucocorticoids (dexamethasone). The antibiotic treatment is tailored later based on the specific pathogen detected.

  
  
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  Presence of intracranial hypertension and/or focal deficits: immediate initiation of treatment with antibiotics and glucocorticoids (dexamethasone), then head MRI. An LP should not be performed until the signs of intracranial hypertension have subsided.

6.7.2 Acute Bacterial Meningitis

Pathogens and routes of infection The bacteria that cause bacterial meningitis can reach the meninges by any of three routes:

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  Hematogenous spread (e.g., from a focus of infection in the nasopharynx).

  
  
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  Continuous extension (e.g., from the middle ear or paranasal sinuses).

  
  
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  Direct contamination (through an open wound or CSF fistula).

The organisms that most commonly cause acute, purulent meningitis are:

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  In neonates, Escherichia coli, group B streptococci, and Listeria monocytogenes.

  
  
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  In children, Haemophilus influenzae (HIB), pneumococci, and meningococci (Neisseria meningitidis).

  
  
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  In adults, pneumococci, meningococci, and, less commonly, staphylococci and Gram-negative enterobacteria.

Clinical features

The course of purulent meningitis is characterized by the meningitic signs and symptoms listed earlier, as well as by:

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  Myalgia, back pain.

  
  
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  Photophobia.

  
  
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  Epileptic seizures (40%; these may occur if the infection is mainly located over the cerebral convexity with irritation of the underlying brain parenchyma).

  
  
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  Cranial nerve deficits (10–20%, sometimes permanent deafness, particularly after pneumococcal infection).

  
  
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  Variably severe impairment of consciousness.

  
  
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  In infection with N. meningitidis, there may be petechial cutaneous hemorrhages and hemorrhagic necrosis of the adrenal cortex due to endotoxic shock (Waterhouse–Friderichsen syndrome).

Diagnostic evaluation The most important and most urgent components of the diagnostic evaluation are blood culture (at least two sets of aerobic and anaerobic cultures from two different veins) and CSF culture after the CSF has been obtained by LP. Whenever acute meningitis is suspected, an LP should be performed at once, unless there is clinical evidence of intracranial hypertension.

Laboratory findings: the CRP and ESR are elevated, and the differential white blood count may reveal leukocytosis (with mainly segmented granulocytes).

CSF examination enables confirmation of the diagnosis of meningitis and, in two-thirds of patients, demonstration of bacteria by Gram stain and identification of the pathogen by CSF culture.

Treatment The treatment begins with antibiotic therapy, with a single drug or multiple drugs, chosen for their effectiveness against the most likely causative organism(s) in the given clinical setting (= empiric treatment).

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  Previously well children and adults with community-acquired meningitis are treated empirically at first with a third-generation cephalosporin (e.g., ceftriaxone) and ampicillin (which also covers Listeria).

  
  
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  Nosocomial infections and infections in parts of the world where penicillin-resistant pneumococci are common are treated, for example, with a combination of ceftriaxone and vancomycin.

Corticosteroids (dexamethasone) are given as well, as they have been shown to improve the clinical course. Once the pathogen has been identified in the blood or CSF culture (with antibiogram), the empirically chosen antibiotics can be replaced with specifically tailored ones.

Course and Prognosis Bacterial meningitis can have severe neurologic and general medical complications, including the following:

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  Malresorptive hydrocephalus.

  
  
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  Brain infarction and sinus vein thrombosis.

  
  
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  Cerebral edema.

  
  
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  Brain abscess.

  
  
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  Cochlear damage resulting in hearing loss.

  
  
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  Cranial nerve deficits.

  
  
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  General medical complications: pneumonia, septic shock, consumption coagulopathy.

The prognosis depends on the pathogenic organism. The mortality in meningitis due to Streptococcus pneumoniae is over 50%. Survivors often suffer from neurologic deficits including deafness.

Pneumococcal Meningitis

Pneumococci (S. pneumoniae) are Gram-positive diplococci; they are the commonest cause of bacterial meningitis in adults. The diagnosis is established by Gram stain, blood or CSF culture, or demonstration of the antigen in the blood or CSF. Vaccination is possible for infants as well as adults; it is recommended for persons with immune deficiency or chronic disease and for persons older than 60 years.

Meningococcal Meningitis

Meningococci (N. meningitidis) are Gram-negative diplococci; they are the commonest cause of bacterial meningitis in children and adolescents. The meningococcal antigen can be detected in the CSF or blood. In Germany, group B meningococci are the most common type (65–70%).

Patients must be isolated for 24 hours after the beginning of antibiotic treatment, and their contacts should be given antibiotics prophylactically (e.g., rifampicin). Vaccination against group C meningococci is recommended for all infants, and vaccination against groups A and C is recommended for travelers to Africa (recommendations of the German vaccination authority [STIKO]). There is as yet no vaccine against group B. Meningococcal meningitis is a reportable illness: all cases (even suspected ones) and fatalities must be reported. Nasopharyngeal colonization with meningococci without any sign of illness is not reportable.

6.7.3 Acute Viral Meningitis: Aseptic or Lymphocytic Meningitis

Pathogens and routes of entry Several viruses can cause so-called aseptic or lymphocytic meningitis. The more common ones are enteroviruses (polio- and Coxsackie viruses), arboviruses, and HIV; other, rarer ones include lymphocytic choriomeningitis virus, CMV, type II herpesvirus, and the mumps, Epstein–Barr, and influenza viruses.

Clinical features The illness begins acutely (less commonly, subacutely) after a nonspecific prodromal stage with flulike or gastrointestinal symptoms. The main clinical manifestations are headache, fever, meningismus (often mild), and general symptoms such as fatigue and myalgia.

Diagnostic evaluation The causative virus is identified by serologic testing.

Treatment and course The natural course of aseptic meningitis is usually favorable, provided the brain is not involved (i.e., provided there is no encephalitic component). Antiviral treatment is given if the causative virus is one for which an effective treatment exists (a virustatic agent, e.g., acyclovir for herpes simplex virus or varicella zoster virus). Residual neurologic deficits, such as deafness, are rare.

6.7.4 Chronic Meningitis

Chronic meningitis is caused by different organisms from the pus-forming bacteria that cause acute meningitis, and therefore takes a less acute and dramatic course, at least initially: the meningitic symptoms arise gradually, often fluctuate, and, depending on the causative organism, may progressively worsen over a long period of time. Fever and other clinical and laboratory signs of infection (elevated ESR and CRP, blood count abnormalities, general symptoms such as fatigue and myalgia) are common but may be absent. There may be variably severe neurologic deficits. The spectrum of causative organisms is very wide. By definition, chronic meningitis lasts longer than 4 months. Fortunately, it is a rare condition.

Tuberculous Meningitis

Etiology and route of infection Mycobacterium tuberculosis bacilli reach the meninges by hematogenous spread, either directly from a primary complex (early generalization) or from a focus of tuberculosis in an internal organ (late generalization). The site of origin may be clinically silent.

Clinical features and pathogenesis Meningitic symptoms usually develop gradually. Febrile bouts and general symptoms are often but not always present. Because the infectious process typically centers on the base of the brain (so-called basal meningitis; ▶ Fig. 6.44), in contrast to bacterial meningitis, which is typically located around the cerebral convexities), cranial nerve palsies are common, particularly of the nerves of eye movement and the facial nerve. Moreover, arteritis of the cerebral vasculature may result in focal brain infarction. The protein concentration in CSF is typically markedly elevated, and gelatinous exudates in the subarachnoid space, including the basal cisterns, cause progressive fibrinous coating of the meninges and malresorptive hydrocephalus.

Diagnostic evaluation The most important part of the evaluation is the detection of the pathogen in the CSF or other bodily fluids (sputum, tracheal secretions, gastric juice, urine). In the past, the detection of mycobacteria in the CSF often required weeks of culture; it can now be done relatively quickly with PCR (polymerase chain reaction). Occasionally, a Ziehl–Neelsen stain of the CSF will directly and immediately reveal acid-fast bacilli (mycobacteria).

Neuroimaging (CT, MRI) reveals pathologic lesions of multiple types (e.g., tuberculomas, infarcts, hydrocephalus, coated basal meninges).

Treatment and prognosis The treatment generally begins with a combination of four tuberculostatic drugs (isoniazid, rifampicin, pyrazinamide, and myambutol), followed by a combination of three drugs, and then of two, for at least 12 months. Untreated tuberculous meningitis is lethal.

Tuberculosis (confirmed or suspected) is a reportable illness.

Other Causes

Several other organisms can rarely cause chronic meningitis, usually accompanied by variably severe encephalitis. These include:

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  Fungi, mainly but not exclusively in immunodeficient patients; the causative species include Cryptococcus neoformans, Candida albicans, and Aspergillus.

  
  
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  Protozoa: T. gondii.

  
  
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  Parasites: Cysticercus, Echinococcus.

The noninfectious causes of the chronic meningitic syndrome include sarcoidosis, which, like tuberculous meningitis, is mainly found around the base of the brain ( ▶ Fig. 6.45), and seeding of the meninges with metastatic carcinoma or sarcoma (carcinomatous or sarcomatous meningitis).

6.7.5 Bacterial (Meningo)encephalitis: Spirochetal Infections

Neuroborreliosis (Lyme Disease)

Etiology Borreliosis is called “Lyme disease” in North America (after the town of Lyme, Connecticut, in which an outbreak was described) and is caused there by Borrelia burgdorferi, a spirochete transmitted by bites of the tick Ixodes ricinus. Borrelia afzelii and Borrelia garinii cause borreliosis in Europe.

Clinical manifestations Borreliosis can involve the nervous system, joints, cardiovascular system, liver, and skin. Its clinical manifestations are equally varied and run through three stages ( ▶ Table 6.21). After transfer of the organism by a tick bite, some patients locally develop erythema chronicum migrans, a red, annular rash that expands centrifugally around the site of the tick bite, clearing in the central area as it grows outward. If the spirochetes are then disseminated systemically, headache, fever, arthralgia, and sometimes generalized lymphadenopathy will follow.

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Metabolic Disorders and Systemic Diseases Affecting the Nervous System Cerebellar Diseases and Other Conditions Causing Ataxia Brain Tumors Epilepsy and Its Differential Diagnosis Cerebral Ischemia and Ischemic Stroke Diseases of the Spinal Cord

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