Let us discuss viral encephalitis in this post. First what do we mean when we say a patient has encephalitis and how does encephalitis differ from meningitis.
Meningitis is inflammation of the meninges of the brain. The meninges are the thin layers which cover the brain and include from inwards outwards pia mater, arachnoid mater and dura mater. When they get inflammed usually due to an infectious etiology a patient develops meningitis. There are numerous causes of meningits and they can be classified as bacterial causes (common bacteria which cause meningits include Haemophilus Influenzae type B, Nesseria meningitidis, Streptococcus pneumoniae, Listeria), viral causes (most of the viruses which cause meningitis are seasonal viruses and usually it is hard to isolate them. We do not try too hard to find out the particular virus causing the meningits as viral meningitis is usually self limited and has a good outcome with most patients recovering completely), fungal causes, parasitic causes and so on. So how does meningits present clinically? Patients usually have complaint of fever, headache and stiff neck. They feel tired and listless and may also have nausea and vomiting. Patients who have meningitis are bothered by bright light (we call this photophobia and hence they prefer to lie in a quiet dark room), loud sounds are also bothersome (we call this phonophobia). When we assess such patients clinically we look for tell-tale signs of meningitis: namely we check for neck stiffness (neck stiffness in a febrile patient is quite sensitive for meningitis). Patients with meningitis at least initially in their disease course are awake and alert and do not present with seizures (remember it is the meninges which are involved not the brain itself at least initally). Later on if the meningitis remains untreated patients become stuporous and comatosed. Bacterial meningitis as compared to viral meningitis is more fulminant and if untreated can prove fatal (bacterial meningitis patients usually look more “sick” as compared to those with viral meningitis). Once the diagnosis of meningitis is entertained we usually admit these patients to the hospital (some patients with viral meningitis may be managed on an outpatient basis if close supervision can be ensured). A CT scan is usually done next followed by a spinal tap to get some cerebrospinal fluid. This fluid is then sent for various tests. The diagnosis is confirmed if the spinal fluid shows inflammatory cells. On the basis of tests on the spinal fluid we can determine whether the patient has bacterial, viral, fungal or tubercular meningitis (each type has its own spinal fluid characteristic. Moreover we examine the fluid under the microscope and further culture it to isolate the organism). As you can imagine, it takes time for some of these tests to come back, so patients may be empirically started on antibiotics pending CSF culture results. Once the CSF results come back, the antibiotics can be changed depending upon which organism is isolated from the CSF.
So then what is encephalitis and how does it differ from meningitis. Encephalitis simply put is inflammation of the brain itself (not the meninges). Usually the term is used in connection with a viral etiology. The common causes of encephalitis include viruses like herpes simplex, cytomegalovirus, West Nile, flaviviruses to name a few and the common encephalitis are: herpes simplex encephalitis, Japanese encephalitis, Eastern Equine and Western Equine encephalitis, California encephalitis and tick-borne and arthropod borne encephalitis (such as Lyme encephalitis). Since in encephalitis the brain parenchyma is involved, these patients may present with depressed level of consciousness, altered personality, abnormal behavior and seizures. Usually in encephalitis the brain MRI is abnormal and gives us a hint to the extent of brain parenchymal involvement. The spinal fluid may show inflammatory cells (some patients have a component of meningitis along with their encephalitis: MENINGOENCEPHALITIS). Doctors may also order other tests like EEG to rule out seizures. The treatment of encephalitis varies. In the case of herpes encephalitis we use anti-viral drugs like acyclovir. In other encephalitis the treatment is more supportive. Some patients recover (those detected and treated in time), others are left behind with devastating neurological sequelae
Nitin Sethi, MD