Viral infections are produced by both conventional viruses which contain DNA or RNA and unconventional (slow / prions ) viruses which contain neither DNA  nor RNA. 

They gain access from the skin, the gastrointestinal tract or the respiratory system to the CNS via neural (axoplasmic and perineural lymphatics ) and more commonly, hematogenous routes.

They may cause only relatively mild systemic illnesses; however if they invade the CNS, they may be fatal or cause severe morbidity.

The most common acute viral encephalitis that requires neurosurgical intervention is herpes simplex.

The most common post or para infectious viral encephalopathy encountered in neurosurgery is Reye's syndrome. 

The most common unconventional (slow ) viral disorder of interest to the neurosurgeons is Creutzfeldt-Jakob disease.

A virus etiology of CNS tumors is an area of interest to the  neurosurgeons. 

HERPES SIMPLEX :

It is the commonest virus encephalitis and caused by Herpes Simplex Virus- type I (oral)  with spread to CNS. 

Latent infection of trigeminal ganglia is common (oral-facial herpes). 

Congenital infection caused by type 2 (genital) virus produces a neonatal meningoencephalitis. All mothers with genital herpes should have a cesarean section before their amniotic membranes have ruptured or within 4 hrs of rupture.

Pathology: 

There is acute necrotizing (often hemorrhagic) encephalitis and mild meningitis with mild meningeal and perivascular lymphocytic infiltration. Microglial activation and nodule formation are also seen. There may be neuronal necrosis and loss (Neuronophagia ) and brain edema can be severe. Cowdry type A intranuclear inclusions may be seen.

The temporal lobes, orbital regions, insular cortices, cingulate gyri and brain stem are more commonly involved.

Diagnosis:

Clinically, there may be fever, personality changes, fluctuating level of consciousness and seizures, mimicking a bacterial meningitis. 

CSF shows mononuclear leucocytosis, reduced glucose and elevated protein levels. Recently developed PCR test for Herpes Simplex Virus in CSF is highly sensitive.

EEG may show periodic high-voltage discharges localized to the temporal lobes.

CT and MRI scans demonstrate edema, hemorrhage in the temporal lobes and in the orbital-frontal regions.

Biopsy can confirm diagnosis by culture, histology or immunohistochemistry, but rarely required these days.

Treatment:

Acyclovir (10mg/kg intravenously every 8 hrs for three days ) is relatively effective.

Sub-temporal decompression may be life saving at times.

The mortality is high and death may occur in 1-3 weeks. The survivors often have severe neurological deficits due to extensive tissue necrosis and  involvement of temporal lobes often results in memory disorders and/or seizures.

HERPES ZOSTER (VARICELLA):

It is often a reactivation of latent chickenpox virus infection and may occur in all age groups but predominantly the elderly. It may also involve the dorsal root ganglia, the trigeminal and the  geniculate ganglion. The virus spreads through the nerve to the nerve endings localized in the dermatome corresponding to the root ganglion in which the infection is reactivated. More than one dermatome may be involved at a time. Motor involvement such as facial weakness or localized wasting of muscles may occur occasionally.

Trigeminal nerve involvement is the commonest of ' cephalic zoster '.

Facial nerve involvement is called 'Ramsay-Hunt Syndrome '.

' Otitc zoster ' involves the vestibulocochlear nerve. 

Very painful skin eruptions are called ' Shingles '.

Rarely the virus will spread centripetally to cause an encephalomyelitis (usually in severely immunocompromised patients).

Transverse myelitis, Guillain-Barre syndrome, Reye's syndrome and acute cerebellar ataxia may be associated, as  in any other virus infection.

Acyclovir ( 5mg / kg intravenously every 8 hrs) is effective and the recovery is usual with few residual effects. Post herpetic neuralgia may be distressing.

Reye's syndrome:

It is a para infectious process and usually affects children on recovery from a viral illness. Clinically, they develop headache, vomiting and confusional states. There is severe brain edema and liver failure. If untreated , it is fatal. Investigations reveal hypoglycemia and elevated serum ammonia. 

Use of aspirin in children  and also in adults with viral infections is blamed lately.

Creutzfeldt-Jakob Disease:

It is a  presenile dementia caused by an unconventional virus which induces a subacute spongiform encephalopathy. It usually affects the middle aged with a mean survival time of 5 months. 

Clinically, there is progressive dementia with myoclonus which differentiates this from Alzheimer's.

CT and MRI scans reveal bilateral cortical atrophy.

Brain biopsy may be required for diagnosis. The method of transmission is iatrogenic through contaminated instruments.

OTHERS:

Epilemic acute encephalitis is caused by arbovirus (RNA viruses) infections. It is seasonal (summer). There is widespread meningoencephalitis with no viral inclusions. The diagnosis often is made only by convalescent immunoglobulin.

There is no viral-specific therapy is available. The mortality and morbidity varies. 

Rabies is caused by bite of a rabid animal (dog, skunk, fox, etc.) There is a long but variable incubation period - 2 months (average). Rhabdovirus ( RNA virus ) migrates to CNS via peripheral nerves. There is acute encephalomyelitis (gray matter infection) and the lesions are distributed in the  spinal cord, brain stem,  cerebellum and the temporal lobes.

Viral inclusions (Negri bodies) in neuronal cytoplasm are diagnostic.

Poliomyelitis ( Polioencephalomyelitis ) is by poliovirus infection, enteric virus (seasonal - summer months). The primary  enteric infection is followed by hematogenous spread to the CNS, certain strains are neuronotropic. there is mild diffuse meningitis with microglial nodules, neuronophagia and gliosis.

It is extremely rare since the advent of polio vaccines.

AIDS-RELATED (OPPORTUNISTIC ) INFECTIONS:

A clinically important group of CNS infections occur in the immune-suppressed patient. The advent of the AIDS epidemic has increased the frequency of many of these diseases, but some are more commonly seen in patients who are immunosuppressed for other reasons (transplantation, chemotherapy).

In addition to HIV infection, AIDS patients(are prone for infections due to cytomegalovirus, herpes, typical and atypical mycobacteria, the fungi, such as cryptococcus and  toxoplasmosis due to T.gondii .

Primary HIV infection of the CNS ( HIV-Associated dementia complex ):

HIV infects macrophages and microglia but does not produce significant infection of neurons or neuroglia. The principal pathology is aggregates of macrophages and multinucleated cells in the white matter. Atrophy of the brain with neuronal loss may occur in later stages.30 to 40 % of the AIDS patients have neurological involvement.

The neurologic effects of virus may be mediated by cytokines. The clinical picture varies from a chronic asymptomatic state to full blown AIDS. 

Rarely, acute reversible encephalitis or meningitis, or acute myelopathy or neuropathy occurs. 

Subacute encephalitis causes dementia. Chronic meningitis and peripheral neuropathy have also been reported. 

Vacuolar degeneration of the spinal cord result in paresis, ataxia and incontinence.

Most patients develop CSF evidence of HIV infection at the time of seroconversion.

The early stages of the disease are pathophysiological and may be self limiting in a few weeks and most patients die within six months.

Cytomegalovirus infection:

Most adults have been infected by the cytomegalic virus by the age of 35 years. The infection is usually  asymptomatic, chorioretinitis often a more serious problem. Acquired cytomegalic virus disease is very rare except in those who are immunosuppresed and is the most common opportunistic infection in AIDS; also common in organ transplant.

Congenital infection is  by transplacental route. It is characterized by low birth weight, jaundice, hepatosplenomegaly, thrombocytopenia, hemolytic anemia, chorioretinitis, microcephaly and periventricular calcifications. 

There is nodular encephalomyelitis with predilection for the subependymal germinal matrix surrounding the lateral ventricles.Calcification within the walls of these ventricles occurs frequently. 

At the present time, there is no effective treatment or prophylaxis.       

Progressive Multifocal Leukoencephalopathy (PML):

It is a demyelinating disorder caused by papovavirus (DNA) infection (JC virus) usually occurring in patients with immunosuppression or lymphoid malignancies (relatively common in AIDS). It is either a primary infection or reactivation of latent infection (systemic JC virus infection is common in the general population, but CNS disease is rare)

The virus attacks oligodendroglia leading to demyelination; particularly involves the cerebral hemispheres, but can involve posterior fossa also. There typically is little inflammatory reaction other than for debris ( laden macrophages ). Oligodendrocytes have glassy intranuclear viral inclusions. Reactive astrocytes often have enlarged, pleomorphic nuclei.

There is no effective therapy and the clinical course typically is 3-9 months until death.