Viral meningitis

Viral meningitis is a relatively common self-limiting type of CNS infection clinically diagnosed based on the cerebrospinal fluid analysis and proportionally more frequent in young children than adults. Enteroviruses represent nowadays the most common cause of viral meningitis for all ages in the developed world. 

For viral infection of the brain parenchyma, please refer to the general article on viral encephalitis. 

Terminology

Aseptic meningitis is a term frequently used as a synonym for viral meningitis (as distinct from bacterial meningitis). To avoid misleading interpretations, we prefer to save this term only for those noninfective inflammatory processes involving the meninges (e.g. leptomeningeal carcinomatosis, sarcoidosis, vasculitis, and connective tissue diseases).

Epidemiology

Viral meningitis is more common in young children, particularly in those with less than one year or between 5 to 10 years. 

After a long term period since the implementation of highly effective conjugate vaccines, there has been a significant change in the etiology and the incidence and hospital admissions for viral meningitis have dropped ^1,3. 

Clinical presentation

Classically, patients present with fevers and clinical signs of meningeal irritation (e.g. neck stiffness, Brudzinski sign). Other nonspecific symptoms such as vomiting, headache, anorexia, exanthems, and myalgia may be also associated. As most of the viral infections start affecting the mucosa of the respiratory or gastrointestinal tracts, symptoms involving these organs may be present ^4,5. 

Neurologic dysfunction (e.g. altered state of consciousness, cranial nerve palsies, abnormal reflexes, and paralysis) is not a feature of meningitis and its presence should alert to a mixed picture with also the involvement of the brain parenchyma, meningoencephalitis ⁴. 

Pathology 

Most viral infections will primarily affect the respiratory and gastrointestinal tracts, as the are either inhaled (e.g. measles, mumps) or ingested (e.g. enteroviruses), and thus will have their replication in the regional lymph nodes or small bowel lymphoid tissues. From the secondary lymphoid tissues, the viruses get into the blood stream resulting in systemic infection and subsequent seeding of other organs ⁵. 

Etiology

• enteroviruses ^1-5
  • represent ~90% of cases of viral meningitis 
  • genus Enterovirus (family Picornaviridae) includes ⁵
    • 3 polioviruses
    • 28 echoviruses
    • 23 group A coxsackieviruses
    • 6 group B coxsackieviruses
    • 4 numbered enteroviruses
  • respiratory and fecal-oral transmission 
• human parechoviruses
• herpesviruses (family Herpesviridae)
  • herpes simplex viruses (HSV)
    • HSV 1
    • HSV 2
  • varicella-zoster virus (VZV)
  • human herpesvirus (HHV) 6 and 7
  • Epstein-Barr virus (EBV)
  • cytomegalovirus (CMV)
• polyolmaviruses
  • JC virus
• arboviruses

Radiographic features

Viral meningitis is usually not directly visible on CT and can be subtle on MRI. The most common finding is of leptomeningeal enhancement with high signal in the sulcal CSF on FLAIR. 

Not infrequently, however, inflammation will also involve the brain parenchyma (meningoencephalitis) with areas of high T2 seen in a variety of locations, depending on the individual patient and on the specific virus ⁶. 

Treatment and prognosis 

Most cases require only supportive management ^1,3.