Pyogenic meningitis

Bacterial meningitis, also referred as pyogenic meningitis, is a life-threatening CNS infectious disease affecting the meninges, with an elevated mortality and disability rates. Three bacterias (Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis) account for the majority of cases ^4,5.

Epidemiology

The epidemiological spectrum of pyogenic meningitis has changed in the last two decades in some countries due routine vaccination. The largely use of the H. influenzae type b (Hib) conjugate vaccine for infants and the heptavalent protein-polysaccharide pneumococcal conjugate vaccine (PCV7) are good examples that explain the significant reduction on H. influenza and pneumococcal disease incidences: near elimination of serogroup C meningococcal meningitis and H. influenzae meningitis has been documented in wealthy nations ^3,4. 

The median age at diagnosis of bacterial meningitis has increased in the last decades as a result of children vaccination, although infants under 2 months of age have not experienced this incidence reduction ³. 

It is important to note that chronic and immunocompromising conditions were common predisposing factor for bacterial meningitis among adults, such as ⁵:

• elderly patients (> 65 years) 
• splenectomy and hyposplenic state
• alcoholism 
• HIV/AIDS 
• diabetes mellitus 
• cancer 
• anatomical defect (related with recurrent meningitis)
• organ transplant recipients

Clinical presentation 

In older children and adults there are typical symptoms and signs, such as: fever, headache, stiff neck, committing and mental dysfunction hanging from lethargy to coma. The signs are less clear in infants, being related to non-specific signs of sepsis and seizures ².

The diagnosis is usually confirmed by lumbar puncture. 

Pathology

Bacteria may arise at the CNS as a result of direct implantation, contagious infection from a local septic process (e.g. sinusitis) or an infected foreign body (e.g. a shunting catheter), or by haematogenous spread ². 

Aetiology

• group B streptococcus (GBS): is the major cause of bacterial meningitis in infants under 2 months of age
• Neisseria meningitidis: is the major cause of bacterial meningitis in older children and young adults
• Streptococcus pneumoniae: is the the most common pathogen in adults

Radiographic features

As the response to these insults are limited and follows a stereotypical fashion, the imaging findings are mostly nonspecific with respect to the causative pathogen. Nevertheless, imaging findings are helpful in detecting an abnormality and making differential diagnosis with other noninfectious causes ¹. 

CT and MRI

• typically shows a thin and linear leptomeningeal enhancement 

Treatment and prognosis 

Empirical antimicrobial therapy for purulent meningitis is guided by the age of the patient ³. 

The adult case fatality has a straight correlation with increasing age, the overall rate is estimated around 16% in the USA, ranging from ~9% among patients 18 to 34 years of age vs. ~23% among those older than 65 years ³.

Differential diagnosis

• fungal meningitis: usually exhibit a thicker, lumpy or nodular enhancement ¹
• viral meningitis