Meningitis

Meningitis is inflammation of the meninges of the brain and spinal cord. It is typically caused by a viral or bacterial infection.

Viral meningitis is more common than bacterial meningitis. Many of the cases of viral meningitis are caused by the enteroviruses (“entero” refers to the virus transmission route through the intestines). The three enteroviruses that most often cause viral meningitis are the coxsackievirus, polio virus, and the echo virus. Sometimes children will experience viral meningitis via the mumps or herpes virus infections as well. There are also several mosquito-borne viruses that cause a few cases of viral meningitis each year.

Symptoms of viral meningitis include fever, headache, stiff neck, and fatigue. There are really no specific medicines or antibiotics that can help a viral meningitis. The treatment is mostly supportive.

Bacterial meningitis is less common but more severe than viral meningitis. Bacterial meningitis comes on more abruptly and with more serious symptoms. It tends to be more lethal if medical treatment is not started quickly. Most cases of bacterial meningitis are caused by Streptococcus pneumoniae or Neisseria meningitidis, but Haemophilus influenzae can also cause it. Neonates with bacterial meningitis are mostly infected by group b Streptococcus. These various bacteria are somehow exposed to the cerebral spinal fluid (CSF) where they replicate and release endotoxins that induce inflammation. This inflammation can cause vascular congestion and ischemic infarction of brain tissue. Adhesions may also form in the meninges. These adhesions can affect cranial nerve function (causing cranial nerve palsies) and block the flow of CSF (causing hydrocephalus) which increases intracranial pressure.

The most common symptoms of acute bacterial meningitis are sudden onset of headache, fever, and nuchal rigidity. Other symptoms may include photophobia, nausea, vomiting, and altered mental state. Patients are also at increased risk for seizures.

While an antibiotic treatment can help to fight bacterial meningitis, it may also cause an increase in inflammation as the bacteria that die release a storm of endotoxins. To counteract this inflammation, corticosteroids to inhibit the inflammatory response are administered prior to the start of the antibiotic treatment.

Some of the risk factors for contracting bacterial meningitis include: head trauma, otitis media, sinusitis, neurosurgery, dermal sinus tracts, systemic sepsis, or being immunocompromised. The microbes that cause bacterial meningitis can be spread by coughing, sneezing, kissing, and sharing utensils that others have touched. Prevention involves avoiding sharing things that touch the mouth, practicing good hygiene by washing hands frequently, and covering the mouth when coughing or sneezing. Vaccines are available for prevention of meningitis against Streptococcus pneumoniae (pneumococcus), Haemophilus influenzae, and Neisseria meningitidis (meningococcus).

Two physically demonstrable signs of meningitis are the Brudzinski sign and the Kernig sign. The Brudzinski sign occurs when flexion of the neck while laying down induces flexion of the hip and knee. This flexion happens because with meningitis, patients will assume positions that decrease stretching of the meninges all the way down the spine and sciatic nerve. They are more sensitive to flexion because increased pressure from the CSF pushes on the meninges. The Kernig sign occurs when there is resistance to extension of the patient’s knee while they are lying down with their hip flexed at a right angle. Some resistance to extension is normal, especially with tight hamstrings. However, if the resistance is accompanied with increased symptoms of stiff neck and tingling or pain in the neck, back, or leg, then it is likely to involve inflammation of the meninges. If either of these signs are present, meningeal irritation and increased CSF pressure may be the cause. Although these tests are well known in literature, they are not the most reliable or valid diagnostic tool for meningitis. Other, more modern assessments are important to perform as well.

A common modern way to diagnose meningitis is with a cerebrospinal fluid sample obtained by a lumbar puncture. In an individual with bacterial meningitis, the CSF sample is cloudy and purulent with a higher than normal protein level. Protein levels can be mildly elevated in viral meningitis as well, but bacterial meningitis has a stronger likelihood of elevated protein. Reduced CSF sugar levels are also common in bacterial meningitis patients because the bacteria use the sugar for fuel. Sugar levels with viral meningitis are normal. In a CSF sample of viral meningitis, lymphocytes can be found rather than polymorphonuclear cells (granulocytes), which are found in bacterial meningitis.

Encephalitis

Encephalitis is a generalized infection of the parenchyma of the brain or the spinal cord. Parenchyma is the functional tissue of an organ that is distinguished from connective and supporting tissue. The parenchyma of the brain includes the neurons and glial cells.

Encephalitis is usually caused by viruses. In America, encephalitis is most often due to herpes viruses, some enteroviruses, mosquito borne viruses (like West Nile), measles, rabies, and some rare viruses carried by ticks. Poliovirus can also invade the parenchyma of the central nervous system, but it tends to selectively destroy cell bodies of the anterior horn of the gray matter.

Like with meningitis, patients with encephalitis experience fever, headache, and nuchal rigidity because the degeneration of nerve cell bodies results in inflammation. However, patients with encephalitis more often experience neurological disturbances such as disorientation, lethargy, delirium, and coma.