At the emergency department, 17-year old Mike is brought in by his parents. He has a fever, headache, and photophobia. On clinical examination, Mike has neck stiffness and Kernig and Brudzinski signs are positive. Lumbar puncture reveals a predominantly neutrophilic pleocytosis, elevated protein, and low glucose. Next to Mike is 27-year old Helen, who came in with a fever, headache, and confusion for the past 72 hours. Kernig and Brudzinski signs are negative. Head CT reveals patchy temporal lobe enhancements. Finally, there’s 60-year Lucia, who is brought by her family because of a seizure. She also complains of headache and has a mild fever. There’s no neck stiffness and Kernig and Brudzinski signs are negative. Two weeks ago Lucia had an episode of otitis media for which she was treated with antipyretics and analgesics. She has no history of epilepsy. Head CT reveals a single ring-enhanced lesion in the right temporal lobe.
Okay, so all 3 people suffer from a central nervous system infection. CNS infections include meningitis, which is when pathogens infect the meningeal layers; encephalitis, when the pathogens infect the brain parenchyma; meningoencephalitis, where the infection starts in the meninges and then spreads into the brain parenchyma; and abscess which is when pathogens wall themselves off in the brain.
Okay, so let’s take a closer look at the various forms of CNS infections, starting with meningitis. Meningitis can be caused by any pathogen that infects the meninges. When it’s caused by viruses, mycobacteria, fungi, or parasites, it’s called aseptic meningitis, because routine bacterial cultures of the cerebrospinal fluid are negative. The most common cause of aseptic meningitis is a group of viruses called enteroviruses, like echovirus and coxsackie virus. Despite viral meningitis being much more common, acute bacterial meningitis is more life-threatening. The bacteria that is most likely to be responsible will depend on the individual’s age. For example, in infants less than 3 months, the most common causes in descending order are: group B Streptococci, Escherichia coli, and Listeria monocytogenes. In adolescents 13 to 17 years old, the most common causes are Neisseria meningitidis, followed by Streptococcus pneumoniae, and Haemophilus influenzae. In non-adolescent children 3 months to 12 years and in adults, Streptococcus pneumoniae is the most common cause, followed by Neisseria meningitidis and Haemophilus influenzae. It’s also important to consider Listeria monocytogenes in adults over the age of 50 or those who are immunocompromised. Also, Staphylococcus aureus is more common in individuals with a history of neurosurgical procedures or trauma to the head. Other rare but extremely important bacterial causes include Mycobacterium tuberculosis, lyme meningitis, rocky mountain spotted fever, and neurosyphilis. These are separated from the other causes because they’re usually associated with a specific risk factor. For example, tuberculous meningitis’s biggest factor is traveling to an endemic area or working with a high risk population. Lyme meningitis is caused by the spirochete Borrelia burgdorferi, and individuals can have a history of travel to an endemic region like the Northeast of the United States. Finally, Rocky Mountain Spotted Fever is caused by Rickettsia rickettsii and is carried by the Dermacentor dog tick. It is endemic to Central and South America as well as the Southeast United States. Neurosyphilis is rare, but it can arise in individuals with a history of untreated syphilis, especially in immunocompromised individuals.