Meningitis.

  • Meningitis is an inflammation of the adrachnoid, the pia mater, and the intervening cerebrospinal fluid (CSF). The inflammatory process extends throughout the subarachnoid space about the brain and spinal cord and regularly involves the ventricles.

 ETIOLOGY/ PATHOGENESIS

  • Three major pathways exist by which an infectious agent (ie, bacteria, virus, fungus, parasite) gains access to the central nervous system (CNS) and causes disease.
  • Infectious agent colonizes or establishes a localized infection in the host. This may be in the form of colonization or infection of the skin, nasopharynx, respiratory tract, gastrointestinal tract, or genitourinary tract.
  • Meningitis occurs in people of all age groups, but very young individuals (infants and young children) and elderly individuals (>60 y) are more predisposed to the infection.
  • Depending on their ages, individuals are also predisposed to certain etiologic agents. See table 1
  •  Escherichia coli K1 and S agalactiae meningitis are common among the neonatal group, and L monocytogenes meningitis is common among neonates and elderly individuals. See table below for bacterial agents common among different age groups.

        Table 1: The Most Common Bacterial Pathogens Based on Age and Predisposing Risks

Risk and/or Predisposing Factor Bacterial Pathogen
Age 0-4 weeks S agalactiae (group B streptococci)
E coli K1
L monocytogenes
Age 4-12 weeks S agalactiae
E coli
H influenzae
S pneumoniae
N meningitidis
Age 3 months to 18 years N meningitidis
S pneumoniae
H influenzae
Age 18-50 years S pneumoniae
N meningitidis
H influenzae
Age older than 50 years S pneumoniae
N meningitidis
L monocytogenes
Aerobic gram-negative bacilli
Immunocompromised state S pneumoniae
N meningitidis
L monocytogenes
Aerobic gram-negative bacilli
Intracranial manipulation, including neurosurgery Staphylococcus aureus
Coagulase-negative staphylococci
Aerobic gram-negative bacilli, including
Pseudomonas aeruginosa
Basilar skull fracture S pneumoniae
H influenzae
Group A streptococci
CSF shunts Coagulase-negative staphylococci
S aureus
Aerobic gram-negative bacilli
Propionibacterium acnes

Table adapted from: Razonable, R.R., &  Keating,, M.R. (2007). Meningitis. Retrieved: 4-8-2008.  http://www.emedicine.com/med/TOPIC2613.HTM#table2

  • Viruses are the major causes of aseptic meningitis syndrome, an illness that is reported to occur with an incidence rate of 10.9 cases per 100,000 person-years. Enteroviruses account for 90% of cases. HSV accounts for 0.5-3% of cases of aseptic meningitis and is most commonly associated with primary genital infection and is less likely during recurrences.
  • Aseptic meningitis syndrome: Aseptic meningitis is the most common infectious syndrome affecting the CNS. Most episodes are caused by a viral pathogen, but they can also be caused by bacteria, fungi, or parasites.
  • Acute viral meningitis: Viral meningitis comprises most aseptic meningitis syndromes. The viral agents for aseptic meningitis include the following: Enterovirus, Herpesvirus, Arthropod-borne viruses, HIV
  • Patients with meningitis caused by the mumps virus usually present with the triad of fever, vomiting, and headache. It follows the onset of parotitis (salivary gland enlargement occurs in 50% of patients), which clinically resolves in 7-10 days.
  • Chronic meningitis is a constellation of signs and symptoms of meningeal irritation associated with CSF pleocytosis that persists for longer than 4 weeks. It can be caused by bacteria (M tuberculosis, B burgdorferi, T pallidum, Brucella species), Fungi (C neoformans, C immitis, B dermatitidis, H capsulatum, Candida albicans, Aspergillus species) or parasites (Acanthamoeba, species, N fowleri, Angiostrongylus cantonensis, G spinigerum, B procyonis)

PERTINENT HISTORICAL FINDINGS/ CLINICAL SYMPTOMS

  • The classic presentation of meningitis includes fever, headache, neck stiffness, photophobia, nausea, vomiting, and signs of cerebral dysfunction (eg, lethargy, confusion, coma).
  • The triad of fever, nuchal rigidity, and change in mental status is found in only two thirds of patients. Fever is the most common manifestation (95%), while stiff neck and headache are less common.
  • The classic presentation of acute meningitis is the onset of symptoms within hours to a few days, compared to weeks for chronic meningitis.

PERTINENT PHYSICAL EXAM FINDINGS

  • Evidence of meningeal irritation (drowsiness and decreased mentation, stiff neck, Kernig’s and Brudzinski’s signs) is usually present. Although the classic triad of fever, stiff neck, and change in mental status is present in only 44% of episodes, a combination of two of four symptoms (headache, fever, stiff neck, and altered mental status) is found in 95% of patients.
  • The findings of meningitis may be easily overlooked in infants, obtunded patients, or elderly patients with heart failure or pneumonia, who may have meningitis without prominent meningeal signs; their lethargy should be investigated carefully, meningeal signs should be sought, and examination of the CSF is indicated if any doubt exists.
  • The presence of a petechial, purpuric, or ecchymotic rash in a patient with meningeal findings almost always indicates meningococcal infection and requires prompt treatment because of the rapidity with which this infection can progress.
  • Cranial nerve abnormalities, involving principally the third, fourth, sixth, or seventh nerve, occur in 5 to 10% of adults with community-acquired meningitis nd usually disappear shortly after recovery.
  • Seizures (focal or generalized) occur in 20 to 30% of patients
  • Brain swelling and increased CSF pressure are associated with seizures, sixth- and third-nerve dysfunction, abnormal reflexes, reduced consciousness or coma, dilated and poorly reactive pupils, decerebrate posturing, hypertension, bradycardia, and irregular respirations.
  • Focal cerebral signs (principally hemiparesis, dysphasia, visual field defects, and gaze preference) occur in approximately one third of patients.

DIFFERENTIAL DIAGNOSIS

  • HSV-1 encephalitis
  • Rocky Mountain spotted fever
  • Acute subarachnoid hemorrhage
  • Parameningeal infections
  • Brain abscess
  • Stroke

DIAGNOSTIC EVALUATIONS

  • Blood cultures should be obtained prior to beginning therapy
  • CSF examined and cultured: Measure the opening pressure and send the fluid for cell count (and differential count), chemistry (ie, CSF glucose and protein), and microbiology (ie, Gram stain and cultures). See table 2
  • The use of nucleic acid amplification (eg, PCR) has revolutionized the diagnosis of herpes simplex meningitis.
  • CT scan of the brain may be performed prior to lumbar puncture in some patient groups with a higher risk of herniation. These groups include those with newly onset seizures, an immunocompromised state, signs suspicious for space-occupying lesions (such as papilledema and focal neurologic signs), and moderate-to-severe impairment in consciousness.
  • Special studies, such as serology and nucleic acid amplification, may also be performed depending on clinical suspicion.

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Dr. Zachary Lahlou

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