Encephalitis
B. Denise Hemby and Theresa M. Campo
Definition
A.Encephalitis is the inflammation of brain parenchyma caused by viral agents or other toxins. It primarily involves the brain, but meninges are frequently involved. It presents as focal or diffuse neuropsychological dysfunction. Most cases are viral, but it can be caused by bacterial, autoimmune, and fungal disorders. Encephalitis is different from meningitis but they share many clinical features—such as photophobia, headache, or stiff neck. There are several specific types of encephalitis, with the most common cause being herpes simplex encephalitis (HSE) and anthropod-borne viruses. The mortality depends on the etiologic agents of the encephalitis. Patients with either syndrome present with fever, headache, and altered states of consciousness. In most cases of encephalitis, there is some concomitant meningeal inflammation, in addition to the cephalitic component, a condition commonly referred to as meningoencephalitis.
Incidence
The true incidence of encephalitis can be difficult to track because there are no standardized reporting policies as well as lack of strict enforcement. In the United States, several thousand cases of viral encephalitis are reported to the Centers for Disease Control and Prevention (CDC) each year. In the United States, the incidence of encephalitis is between 3.5 and 7.4 per 100,000 per year. Estimates regarding an annual incidence of about 20,000 new cases has been reported by the CDC. There are numerous viruses known to cause encephalitis, but the most common is HSE with an overall incidence of 10% of all cases in the United States. The arbovirus group affects 150 to 3,000 people per year and the two most common are St. Louis encephalitis and California encephalitis (CE). St. Louis encephalitis is caused by a virus that is primarily found near the Mississippi River but can be found throughout the United States. CE can be found in the rural areas of the Upper Midwest and Northeast but is not common to California as its name depicts.
A.HSE—may occur in both healthy and immunocompromised adults and is potentially deadly if left untreated. Quick identification and treatment can be lifesaving. Herpes encephalitis is the most common form in the United States and has a mortality rate as high as 75% in untreated patients. Survival and recovery from neurologic sequelae are related to the mental status at the time of acyclovir initiation.
B.Arthropod-borne viruses—there are more than 20 arboviruses that can cause encephalitis. These arboviruses are enveloped RNA viruses from different families. Some common arboviral encephalitides are the following:
1.St. Louis encephalitis virus (SLEV)—has a mortality rate of 2% to 20% but is higher in patients 60 years and older. Long-term sequelae include behavioral disorders, memory loss, and seizures.
2.Eastern equine encephalitis (EEE) virus—rare and is the deadliest; it is in areas of New England and surrounding areas. EEE is one of the most severe mosquito transmitted diseases in the United States with about 33% mortality and significant brain damage in survivors. Death usually occurs in 2 to 10 days after onset of symptoms.
3.Western equine encephalitis (WEE) virus —milder disease, in rural communities west of Mississippi River, west of the Rocky Mountains. It is associated with fewer deaths, less than 5%, and less morbidity. It is a summertime disease, common between April and September, with peaks in July and August. Heavy rainfalls or snowy seasons can increase the vector population.
4.Japanese encephalitis (JE) virus —endemic in rural areas. In the United States, JE develops mostly among military personnel, expatriates, and returning travelers. Outbreaks are rare in the U.S. territories of Guam and Saipan. It is a seasonal disease, occurring mostly in temperate areas starting June to September. In subtropical areas, it can be seen as early as March and go until October.
Pathogenesis
A.HSE—it is believed to be the reactivation of herpes simplex virus (HSV) lying dormant in the trigeminal ganglia. The virus replicates outside the central nervous system (CNS) and enters by either hematogenous spread or by travel along neural pathways. Human herpesvirus (HHV)-6 is the causative agent.
B.SLEV—an enveloped, single-stranded, positive sense RNA virus from the subgroup of Flaviviridae. It is transmitted via mosquito from wild birds to humans. The mosquitoes include Culex pipiens, Culex tarsalis, and Culex quinquefasciatus.
C.EEE—is in the genus Alphavirus, family Togaviridae, and is transmitted to humans by the bite of Culiseta melanura mosquitos. The virus has a single-stranded, positive-sense RNA genome. The incubation period ranges from 4 to 10 days.
D.WEE—belongs to the genus Alphavirus. Causes encephalitis and viral symptoms without a rash. It is spread primarily by the vector mosquito C. tarsalis. It is transmitted into subcutaneous and cutaneous tissue from bites. It is usually subclinical and may mimic other inflammatory and viral syndromes.
E.JE—belongs to the genus Flavivirus. It is transmitted via the bite of infected Culex mosquitoes, especially Culex tritaeniorhynchus, Culex vishnui, Culex gelidus, and Culex fuscocephala. Incubation period averages 6 to 8 days but can range from 4 to 15 days.
Predisposing Factors
A.Exposure to vectors:
1.Mosquitoes.
2.Ticks.
3.Bats.
4.Raccoons.
5.Feral dogs/cats.
6.Sandflies.
B.HIV positive.
C.Herpes.
D.Occupational exposure:
1.Laboratory workers.
2.Healthcare workers.
3.Veterinarians.
E.Recreational activities (camping/hunting).
F.Recent travel into endemic areas.
G.Recent vaccination.
Common Complaints—May Be Focal or Diffuse Symptoms
A.Altered mental status.
B.Personality changes (very common).
C.Lethargy.
D.Neck pain, stiffness.
E.Ataxia.
F.Cranial nerve defects.
G.Photophobia.
H.Generalized or focal seizures.
I.Decreased level of consciousness (LOC).
Other Signs and Symptoms
A.CNS symptoms:
1.Nuchal rigidity.
2.Irritability.
3.Hemiparesis: Weakness or paralysis on one side of body.
4.Flaccid paralysis.
5.Seizures.
6.Exaggerated deep tendon reflexes.
7.Ataxia.
8.Nystagmus.
B.Photosensitivity.
C.Swollen or protruding eyes.
D.Malaise.
E.Nausea and/or vomiting.
F.Dysphagia with rabies.
Subjective Data
A.Review the onset, duration, and course of all symptoms.
B.Rule out recent history of chickenpox, rubeola, herpes, or other infections.
C.Ask the patient about recent travel.
D.Question the patient regarding recent mosquito or animal bites (rule out rabies).
E.Elicit a detailed sexual history.
F.Does the patient have HIV?
G.Ask about recent recreational activities, including camping, spelunking, or hunting.
Physical Examination—Signs May Be Diffuse or Focal
A.Check temperature, pulse, respirations, and blood pressure.
B.Inspect:
1.Observe general overall appearance.
2.Conduct an eye exam.
3.Examine the skin for rash, vesicles, or bites:
a.Maculopapular rash is seen in approximately half of patients with West Nile virus (WNV).
b.Grouped vesicles in a dermatomal pattern suggest varicella-zoster.
c.Classic herpetic skin lesions suggest herpes encephalitis.
4.Assess dehydration status.
5.Observe for seizure activity.
6.Observe for tremors of the eyelids, tongue, lips, and extremities, which may suggest the possibility of St. Louis encephalitis or West Nile encephalitis.
C.Auscultate: Auscultate the lungs and monitor breathing pattern. Auscultate the heart.
D.Palpate:
1.Palpate the lymph nodes: Preauricular, posterior auricular, submental and sublingual, anterior cervical chains, and supraclavicular nodes.
2.Palpate the mastoid bone.
E.Neurologic exam:
1.Assess LOC.
2.Assess the patient for personality changes.
3.Assess for meningeal signs:
a.Signs of meningeal irritation include nuchal rigidity.
b.Positive Brudzinski’s and Kernig’s signs (see Figures 19.1 and 19.2):
i.Brudzinski’s sign: Place the patient supine and flex the head upward. Resulting flexion of both hips, knees, and ankles with neck flexion indicates meningeal irritation.
ii.Kernig’s sign: Place the patient supine. Keeping one leg straight, flex the other hip and knee to a bent knee to form a 90-degree angle. Slowly extend the lower leg. This places a stretch on the meninges, resulting in pain and spasm for the hamstring muscle. Resistance to further extension can be felt.
4.Check deep tendon reflexes (exaggerated and/or pathologic reflexes).
Diagnostic Tests
A.Complete blood count (CBC) with differential.
B.Complete metabolic panel (CMP).
C.Urine electrolyte levels.
D.PCR tests for viruses.
E.Serology for arboviruses.
F.Lumbar puncture for the following: Cerebrospinal fluid (CSF) analysis:
1.Pressure.
2.Cell count.
3.Microorganisms.
4.Glucose.
5.Protein.
G.HSV cultures.
H.Viral cultures of CSF including HSV.
I.Blood cultures for bacterial pathogens.
J.Complement fixation antibodies to identify arbovirus.
K.Heterophile antibody and cold agglutinin testing for Epstein–Barr virus (EBV).
L.Serologic tests for Toxoplasma.
M.EEG.
N.CT scan: Useful to rule out space-occupying lesions or brain abscess.
O.MRI: Sensitive in detecting demyelination.
FIGURE 19.1 Brudzinski’s sign.