Bacterial meningitis

What it is

The causes of meningitis are infection, malignancy and autoimmune disease. The most common cause is viral meningitis.

Causes and risk

Streptococcus pneumoniae, gram positive diplococci, is the most common across ages; the patients more vulnerable to a repeated and severe course are those with CSF leak and nasal or basal of skull fractures. Haemophilus influenzae, gram negative coccobacilli, is seen in all age groups, in patients with chronic lung disease and colonized plus humoral-deficit patients. Listeria monocytogenes is seen at the extremes of age, neonates and elderly, in the immunocompromized, and with alcohol and chronic liver disease. Staphylococcus aureus, gram positive cocci in clusters, is seen in all age groups with a pre-existing source for bacteremia: endocarditis, skin soft tissue infections, foreign bodies. Neisseria meningitidis, gram negative diplococci, is seen in all age groups, in splenectomized and complement-deficient patients, and in college students in dorms and military recruits. E. coli, gram negative bacilli, is seen in all age groups but the extremes of age are vulnerable, with pre-existing medical co-morbidities and post neurosurgery.

A clue table sets the same organisms out by what gives them away: S. pneumoniae most common in adults, the elderly and after splenectomy; N. meningitidis in young adults, with a petechial rash and close contacts, with droplet precautions and rifampin prophylaxis; H. influenzae in unvaccinated children, common before the Hib vaccine; Listeria in the elderly, neonates, pregnant and immunocompromised, for whom ampicillin is added; Group B Streptococcus in neonates under 1 month, from maternal transmission during birth; E. coli in neonates, often from the birth canal; Cryptococcus neoformans in HIV or AIDS with CD4 under 100 and a subacute onset, diagnosed by India ink stain and treated with amphotericin; and Mycobacterium tuberculosis with basal meningeal enhancement and cranial nerve palsy, with prolonged therapy, to be suspected in endemic areas.

Recurrence is its own clue. A child with a second episode of meningococcal meningitis, two siblings dead of the same condition and no other infection of any kind is asked about immune deficiency, and the answer given is complement, with the mode of inheritance given as AR; the two plans given to prevent future infection are a prophylactic antibiotic and meningococcal vaccination.