Approach to encephalitis: Clinical sciences

Encephalitis refers to the inflammation of the brain parenchyma, which can occur due to infections, including herpes simplex virus-, varicella-zoster virus-, West Nile virus-, and Toxoplasma gondii infection; as well as autoimmune conditions, such as acute disseminated encephalomyelitis, anti-NMDA receptor encephalitis and anti-LGI1 encephalitis.

Alright, if your patient presents with a chief concern suggestive of encephalitis, first, perform an ABCDE assessment to determine if they are stable or unstable. If unstable, stabilize the airway, breathing, and circulation. At this point, you might need to intubate the patient and provide mechanical ventilation. Next, obtain IV access, consider intravenous fluids, and don’t forget to put your patient on continuous vital signs monitoring, including heart rate, blood pressure, and pulse oximetry. Finally, if needed, be sure to manage increased intracranial pressure.

Now, let’s go back to the ABCDE assessment and take a look at stable individuals. In this case, you should obtain a focused history and physical examination. Your patient or their loved ones will report a new onset of progressive confusion, behavioral or personality changes, or a decreased level of consciousness. History might also reveal fever, headache, and seizures.

Depending on the location of the injury, there might be additional neurologic symptoms like weakness, numbness, changes in vision, language impairment, or incoordination. On physical exam, you will notice altered mental status, sometimes in combination with focal neurologic deficits. With these findings, consider encephalitis and assess for current signs and symptoms of infection, such as fever, leukocytosis, or a viral exanthem.

If present, consider infectious encephalitis. Next, obtain a brain MRI and perform a lumbar puncture for CSF analysis. First, let’s discuss herpes simplex virus, or HSV encephalitis for short. Both HSV type 1 and type 2 can cause encephalitis. In this case, your patient will likely present with prodromal symptoms consistent with an upper respiratory tract infection. Sometimes, they might have olfactory hallucinations.

If the brain MRI reveals edema and hemorrhage in the temporal lobes and orbitofrontal cortex; and the CSF PCR testing is positive for HSV type 1 or 2, diagnose HSV encephalitis. Moreover, HSV is the most common cause of encephalitis, and if left untreated, it can be fatal. In other words, if your clinical suspicion is high, always start intravenous acyclovir, even if your PCR results are still pending.

Let’s switch gears and move on to varicella-zoster virus encephalitis. These individuals will report prior or current chickenpox or shingles rash, which are both clinical manifestations of the varicella-zoster virus infection. Chickenpox occurs as a result of primary infection, while shingles, also known as herpes zoster, occur due to the reactivation of a virus that has been dormant in the dorsal root ganglia after the initial infection. Additionally, the varicella-zoster virus can affect the cerebellum, causing cerebellitis and symptoms like loss of coordination, dizziness or vertigo, and gastrointestinal symptoms, such as nausea and vomiting.

Next, the virus can cause vasculopathy, leading to ischemic or hemorrhagic strokes, so be aware that your patient might also present with sudden onset of symptoms suggestive of stroke. Lastly, the patient is likely immunocompromised or elderly, which puts them at higher risk of virus reactivation.

Now, the brain MRI might show ischemic strokes or hemorrhages in the deep brain structures, or edema and hyperintensities in the cerebellum, which is consistent with cerebellitis. However, regardless of the brain imaging findings, if you identify positive VZV PCR or VZV antibodies in the CSF, diagnose VZV encephalitis.

Next, let’s discuss West Nile virus, which is an arbovirus, meaning a virus causing human disease via mosquito or tick vectors. West Nile is typically transmitted during the summer and fall seasons. These patients will report a recent mosquito bite and non-specific symptoms, including fever and muscle aches. They might also have a rash.

In addition to symptoms of encephalitis, involvement of the basal ganglia and substantia nigra could result in tremors or other abnormal involuntary movements; and the involvement of the cells in the anterior horn could cause weakness in all extremities, which is also known as flaccid paralysis. Again, these patients are probably immunocompromised, elderly, or have diabetes.

Next, the brain MRI will show edema and hyperintensities in the basal ganglia and thalami, while the CSF analysis will reveal positive IgM antibodies for the West Nile virus. With these findings, diagnose West Nile encephalitis.

Next up is Toxoplasma gondii encephalitis. These individuals might be immunocompromised, such as having an HIV infection with a CD4 count of less than 200 cells per microliter, or they might be immunosuppressed, such as after an organ transplantation. In these situations, encephalitis occurs as a result of reactivation of latent T. gondii infection. However, if the immune system is not compromised, the likely cause is a primary infection from eating undercooked meat or raw shellfish or ingesting oocytes that were shed in cat feces.

In this case, the brain MRI will show ring-enhancing lesions in the cortex or basal ganglia, while the CSF analysis will be positive for Toxoplasma gondii on PCR. These findings are suggestive of encephalitis from Toxoplasma gondii.

Finally, let’s discuss infectious encephalitis due to a helminth, Taenia solium. The larval stage of T. solium can cause cysticercosis, which consists of cystic deposits that can affect different body organs. If the helminth affects the brain, we are talking about neurocysticercosis, which is rarely associated with encephalitis.