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Viral Infections of the Central Nervous System. Introduction. Anatomic considerations are important in infections of the CNS. - the brain and spinal cord are protected by bone and meningeal coverings that compartmentalize infection.
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Introduction • Anatomic considerations are important in infections of the CNS. - the brain and spinal cord are protected by bone and meningeal coverings that compartmentalize infection. - they are divided by barriers from the systemic circulation. - they lack an intrinsic immune system - they have a unique compact structure.
Natural Defenses of the CNS • Skull and vertebrae • Microglial cells and macrophages • Restricted entry into brain (blood-brain barrier) for: • microorganisms • medications, including antibiotics • immune system
The blood-brain barrier largely prevents macromolecules from entering the brain parenchyma. The space between cells in the brain parenchyma is too small to permit passage even of a virus. However, tetanus toxin and some viruses travel through the CNS by axoplasmic flow. Most agents invade from blood. Bacteria grow rapidly in cerebrospinal fluid; viruses infect meningeal and ependymal cells.
Immune System Levels of IgG and IgA in the CSF are approximately 0.2 to 0.4 percent of the serum levels. IgM is present at even lower levels. There is also no lymphatic system in the usual sense, and few phagocytic cells. Complement is also largely excluded.
When trauma or inflammation disrupts the blood-brain barrier, antibody molecules passively leak into the CNS along with other serum proteins. When an inflammatory reaction has been mounted against an infection, B cells from the peripheral circulation can move into the perivascular spaces of the CNS and generate immunoglobulins intrathecally. Types of CNS infections include; meningitis, encephalitis, myelitis, and meningoencephalitis or meningoencephalomyelitis
Meningitis • Meningitis is an infection which causes inflammation of the membranes covering the brain and spinal cord. • Bacterial meningitis may be referred to as ‘purulent meningitis’.Non-bacterial meningitis is often referred to as ‘aseptic meningitis’ – eg. viral meningitis • Causes and risks - The most common causes of meningitis are viral infections that usually resolve without treatment. - Bacterial infections of the meninges are extremely serious illnesses, and may result in death or brain damage even if treated.
Aseptic Meningitis • Definition: A syndrome characterized by acute onset of meningeal symptoms, fever, and cerebrospinal fluid pleocytosis, with bacteriologically sterile cultures. • Laboratory criteria for diagnosis: - CSF showing ≥ 5 WBC/cu mm and elevated protein - No evidence of bacterial or fungal meningitis. • Confirmedcase: a clinically compatible illness diagnosed by a physician as aseptic meningitis, with no laboratory evidence of bacterial or fungal meningitis
Viral meningitis is more common than bacterial meningitis and it is often less severe than bacterial meningitis. Recovery is almost always complete, since only meningeal and ependymal cells are involved. The disease is benign and tends to be seasonal.
Aseptic meningitis syndrome Acute onset of fever, headache, neck pain/stiffness, vomiting, and meningeal irritation signs No confusion/stupor CSF: WBC protein normal glucose Negative bacterial culture of CSF
Clinical Manifestations Manifestations vary depending on age; below 2 years and above 2 years. In older children and adults: main manifestations of meningitis are headache, fever, and nuchal rigidity. Flexion of the neck may also cause reflex flexion of the legs (Brudzinski sign), and meningeal irritation may limit extension of the leg when flexed at the knee (Kerning sign). Meningeal inflammation may also cause some degree of obtundation (reduced consciousness), and seizures are common in children.
Symptoms of meningitis Adults and children Babies, neonates and the elderly often present atypically .
Systemic clinical signs sometimes suggest the agent (e.g., the rash or herpangina of enterovirus infections and the parotitis of mumps). Examination of the CSF provides the most important diagnostic information. In general, viruses produce a modest mononuclear cell response, and although the CSF protein may be elevated, CSF sugar is normal or only mildly depressed.
Causative Agents of Aseptic Meningitis Viruses Enteroviruses Herpesviruses (HSV- 1, HSV- 2, EBV, HHV- 6, VZV) HIV Lymphocytic choriomeningitis Mumps Arboviruses West Nile virus Numerous other causes e.g., Lyme disease, leptospirosis, 2º syphilis, partially-treated bacterial meningitis, TB, Cryptococcus, autoimmune disease, and medications
Enteroviruses (echoviruses and Coxsackie's viruses) are the most common cause, and they cause disease primarily in the late summer and early fall. Mumps virus is the second most common cause in unvaccinated communities and it spreads predominantly in the spring. Lymphocytic choriomeningitis virus is more common in winter, since this virus is acquired from mice, which move indoors during cold weather and increase human exposure.
Most viruses invade the CNS from the blood, and the risk of CNS invasion has been shown to be related to the magnitude and duration of viremia. The virus may cross the BBB directly at the capillary endothelial level or through natural defects such as the area postrema and other sites that lack a BBB. Particles in the blood, including bacteria or viruses, are normally cleared by the reticuloendothelial system, and speed of removal is proportional to size. A variety of viruses elude clearance by replicating within blood cells.
Enteroviruses and some arboviruses are cleared less effectively from blood because of their small size. Some viruses enter the CNS by infecting endothelial cells or choroid plexus epithelium. Indeed, in mumps virus meningitis, choroid plexus cells containing viral nucleocapsids are frequently found within the CSF.
Diagnostic test for meningitis : lumbar puncture • A lumbar puncture collects cerebrospinal fluid to check for the presence of disease or injury. • A spinal needle is inserted, usually between the 3rd and 4th lumbar vertebrae in the lower spine.
Typical CSF findings in Meningitis • Viral meningitis • Presence of lymphocytes is associated with infection by viruses or mycobacteria. Neutrophils increase in the first 24-48 hours, then replaced by lymphocytes. • CSF protein level reflects the degree of meningeal inflammation:- • 2-3 X in viral CNS infection • CSF glucose levels: • normal with viral infections • Bacterial meningitis • Presence of neutrophils in the CSF is associated with infection by N. meningitidis, S. pneumoniae etc. • CSF protein level reflects the degree of meningeal inflammation:- • 10 X in bacterial infections • CSF glucose levels:- • very low in bacterial infections
Epidemiology Incidence varies depending on age and geography, ranging from 11-219 cases per 100,000 population. Incidence is highest in children aged 1-4 years and it decreases with age Males are more commonly affected than females. No specific racial predilection has been identified. Mortality is low being less than 1%
Management Most cases of viral meningitis require only symptomatic treatment. Since the disease is self-limited; the prime management problem is to rule out nonviral, treatable illnesses that can mimic acute viral meningitis. Examples are; partially treated bacterial meningitis, tuberculous or fungal meningitis, syphilis, Lyme disease, etc.)
Enteroviral Meningitis • Enteroviruses are thought to be the most common cause of viral meningitis • Account for >50% of cases and approximately 90% of cases in which a specific etiologic agent is identified. • Majority of cases are in children or adolescents, but patients of any age can be affected. • Transmitted primarily by fecal-oral route, but can also be spread by contact with infected respiratory secretions. • The incidence is increased in the summer months, but cases occur throughout the year. • Sporadic outbreaks are generally associated with specific serotypes (eg, ECV-30), typically related to introduction of new virus strain to a region.
Enterovirus Lab Findings • CSF- findings typical of viral meningitis - Lymphocytic pleocytosis of generally <250 cells/mm3, with modest protein elevation generally <150 mg/dl, and normal glucose - Viral cultures positive in 40-80% of cases but it usually takes 4-12 days to become positive - PCR is the most specific (close to 100%) and sensitive (97-100%) test and is positive in more than 2/3 of culture negative CSF in patients with aseptic meningitis
Herpes Simplex Meningitis • Generally caused by HSV-2 (as opposed to encephalitis which is caused by HSV-1) • Increasingly recognized as a cause of aseptic meningitis, with improving diagnostic techniques and a continued increase in the transmission of HSV-2 • Can be due to primary or recurrent HSV infection • Between 13 and 36% of patients presenting with primary genital herpes have clinical findings consistent with meningeal involvement including headache, photophobia, and meningismus. • Occasionally patients present with more severe signs including urinary retention, paresthesias, weakness of upper or lower extremities, or ascending myelitis.
The genital lesions are typically present (85% of the time), and usually precede the CNS symptoms by seven days. • HSV meningitis can be recurrent, these patients may not have clinically evident genital lesions. • For patients with benign recurrent lymphocytic meningitis, careful analysis has revealed that over 80% are due to HSV meningitis. • It is also likely the cause of a large percentage of patients with Mollaret’s meningitis, which is a form of recurrent meningitis characterized by large monocytic/ macrophage lineage cells in the CSF.
HSV Diagnosis • CSF- typical of a viral meningitis, with lymphocytic pleocytosis, modest elevation in protein, and normal glucose. • Viral cultures are positive in about 80% of patients with primary HSV meningitis, but less frequently positive in patients with recurrent HSV meningitis. • HSV PCR of the CSF is the single most useful test for the evaluation of a patient with suspected HSV meningitis.
HIV meningitis • A subset of patients with primary HIV infection will present with meningitis or meningoencephalitis, manifested by headache, confusion, seizures or cranial nerve abnormalities. • HIV atypical meningitis is characterized by chronicity and recurrence. • Reports have suggested that as many as 5-10% of HIV infections can be heralded by meningitis. • Aside from the usual meningeal signs, HIV infections may also cause global encephalopathy, seizures, and focal neurologic deficits.
Diagnosis of HIV Meningitis • Serum might reveal atypical lymphocytosis, leukopenia, and elevated serum aminotransferases. • Documentation of seroconversion or detection of HIV plasma viremia by nucleic acid techniques can be used for diagnosis. • CSF- might show a lymphocytic pleocytosis, elevated protein, and normal glucose. • CSF cultures are often positive, but are not available in most centers.
Lymphocytic Choriomeningitis Virus LCM is thought to be an underdiagnosed cause of viral meningitis, in one review it was noted to be responsible for 10-15% of cases. LCM is excreted in the urine and feces of rodents, including mice, rats, and hamsters. It is transmitted to humans by either direct contact with infected animals or environmental surfaces Transmission can occur by inhaling aerosolized particles of rodent excrement, by ingesting food contaminated with the virus or by direct contact of mucus membranes with infected fluids (lab and pets). Infection occurs more commonly in the winter months.
Incubation Period = 8 –13 days Symptoms: Biphasic febrile illness. Initial phase: fever, malaise, headache, muscle aches, anorexia, nausea and vomiting. Second phase: After a few days of remission, symptoms of meningitis and rarely encephalitis. The mortality rate is<1%. Most patients recover completely Treatment: Supportive care
LCM Diagnosis • CSF- typical of other viral meningitis causes except that 20-30% of the time low glucose levels are present, and cell counts of > 1000/mm3 are not unusual • Diagnosis is made by documentation of seroconversion to the virus in paired serum samples.
Mumps Meningitis • Prior to the introduction of the mumps vaccine in 1967, it accounted for 10-20% of all cases of viral meningitis. • Even now this virus causes a significant minority of cases in unvaccinated adolescents and adults. • In patients who do acquire mumps, CNS infection occurs rather frequently, with CSF pleocytosis detected in 40-60%.
Males aged 16-21 years are at highest risk of developing this infection, with a 3:1 male/female ratio. Clusters of cases occur in schools and colleges in the winter months. Concomitant parotitis is a helpful clinical tool but may be absent in as many as half of cases with CNS involvement.
Mumps Diagnosis • CSF- similar to other viral causes, but like LCM it can induce a lymphocytic pleocytosis with cell counts >1000/mm3 or a decreased glucose <50mg/dl, can isolate the virus from the CSF • Can document seroconversion • Clinical correlation is very helpful, especially if the patient has parotitis or orchitis.
West Nile virus • Most infected individuals develop WNV fever • Time from mosquito bite to illness ranges from 3-14 days • Fever, chills, headache, fatigue can be severe • Nausea and vomiting can develop • Rash, usually not itchy, lasting a few days, mainly on chest, back, abdomen, and/or arms • Persistent fatigue is common and may continue for months, even among otherwise healthy persons • In one study, “~96% of patients with WNF described post-illness fatigue that lasted a median of 36 days” (Sejvar, 2007)
West Nile Virus • Season: summer • Mosquito transmission (currently infects 43/ 174 different types of North American mosquitoes) • Other routes • Placenta • Lactation • Transfusion • Organ transplant
WNV • Presenting symptoms • Headache, fever, mental status changes • CN findings, optic neuritis • Myoclonus • Flaccid Paralysis • With or without encephalitis • Asymmetric weakness/paralysis, no sensory loss • Anterior horn cells (polio like)
WNV • Movement Disorders • Parkinsonian • Tremors • Bradykinesia • Cogwheel rigidity • Postural instability • Masked facies • 80-100% will have rest or intention tremor • 30% will have myoclonus
WNV: Predictors • Admission diagnoses: • 30%: aseptic meningitis • 15%: fever • 18%: viral infection • 14%: UTI • 10% pneumonia • 7% : encephalitis • 5%: probable WNV (year 2001) • Mortality rates highest with: • Initial diagnosis of encephalitis (35% of those who died), • No headache (50% had headache, 7% those that died had headache), and • Initial mental status changes