Updated: Aug 24, 2009
Despite advances in antiviral therapy over the past 2 decades, herpes simplex encephalitis (HSE) remains a serious illness with significant risks of morbidity and death.1,2,3 HSE occurs as 2 distinct entities.
Unless noted otherwise, this article describes HSE as it occurs in older children and adults. (See Special Concerns for a discussion of neonatal HSE.) HSE must be distinguished from herpes simplex meningitis, which is more commonly caused by HSV-2 than by HSV-1, and which often occurs in association with a concurrent herpetic genital infection. Like other forms of viral meningitis, herpes simplex meningitis usually has a benign course and is not discussed in this article.
The pathogenesis of HSE is poorly understood. Neurons are quickly overwhelmed by a lytic and hemorrhagic process distributed in an asymmetric fashion throughout the medial temporal and inferior frontal lobes. Involvement of the basal ganglia, cerebellum, and brainstem is uncommon. The exact mechanism of cellular damage is unclear, but it may involve both direct virus-mediated and indirect immune-mediated processes. The ability of HSV-1 to induce apoptosis (programmed cell death, or "cellular suicide") in neuronal cells, a property not shared by HSV-2, might explain why the former causes virtually all cases of HSE in immunocompetent older children and adults.4,5
A vivid description of the temporal course of tissue destruction is given in an immunohistological autopsy study of patients succumbing to HSE over periods of days to weeks in the era prior to acyclovir:
The impression gained is of a rapidly spreading wave of viral infection within limbic structures, probably starting on one side of the brain and spreading within it and to the other side, lasting about 3 weeks and leaving in its wake a trail of devastatingly severe necrosis and inflammation in infected parts of the brain.6
Brain infection is thought to occur by means of direct neuronal transmission of the virus from a peripheral site to the brain via the trigeminal or olfactory nerve. Factors that precipitate HSE are unknown. The prevalence of HSE is not increased in immunocompromised hosts, but the presentation may be subacute or atypical in these patients. HSV-2 may cause HSE in patients with HIV-AIDS.7,8,9
HSE represents a primary herpes simplex virus (HSV) infection in about one third of cases. The remaining cases occur in patients with serologic evidence of preexisting HSV infection and are due to reactivation of a latent peripheral infection in the olfactory bulb or trigeminal ganglion, or reactivation of a latent infection in the brain itself. A substantial number of neurologically asymptomatic individuals may have latent HSV present in the brain. In a postmortem study, HSV was present in the brains of 35% of patients with no evidence of neurological disease at the time of death.10
HSE is the most common cause of sporadic lethal encephalitis, occurring in about 1 person per 250,000-500,000 population per year.
The mortality rate in untreated patients is 70%. Among treated patients, the mortality rate is 19%, and more than 50% of survivors are left with moderate or severe neurological deficits.
No predilection for race exists.
The male-to-female ratio is 1:1.
HSE has a bimodal distribution by age, with the first peak occurring in those younger than 20 years and a second occurring in those older than 50 years. HSE in younger patients usually represents primary infection, whereas reactivation of latent infection occurs in older persons.
Herpes simplex encephalitis (HSE) is an acute or subacute illness, causing both general and focal signs of cerebral dysfunction. It is sporadic and occurs without a seasonal pattern. Although the presence of fever, headache, behavioral changes, confusion, focal neurological findings, and abnormal CSF findings are suggestive of HSE, no pathognomonic clinical findings reliably distinguish HSE from other neurological disorders with similar presentations (eg, non-HSV encephalitis, brain abscess, tumor).11 Confirmation of the diagnosis depends on the identification of HSV in the CSF by means of a polymerase chain reaction (PCR) or on the identification of HSV in brain tissue by means of brain biopsy (see Workup).
Typical symptoms include the following:12
The most frequent findings on physical examination are fever and mental status abnormalities. Meningismus is uncommon.
Typical findings on presentation include the following:12
A causal or temporal relationship between peripheral lesions (eg, herpes labialis) and HSE does not exist. Also, many febrile diseases may precipitate herpes labialis. Therefore, the presence or absence of such lesions neither confirms nor excludes the diagnosis.
| Brain Abscess | Pediatrics, Febrile Seizures |
| Encephalitis | Pediatrics, Meningitis and Encephalitis |
| Epidural and Subdural Infections | Stroke, Hemorrhagic |
| Neoplasms, Brain | Stroke, Ischemic |
Prehospital care consists of supportive management of the patient's airway, breathing, and circulation (ABCs).
Goals of therapy are to shorten the clinical course of the disease, to prevent complications, and to prevent recurrences.
The treatment of choice for herpes simplex encephalitis (HSE) is acyclovir.1,3,19 Through a series of in vivo reactions catalyzed by viral and host cellular enzymes, acyclovir is converted to acyclovir triphosphate, a potent inhibitor of HSV DNA polymerase, without which viral replication cannot occur. Human cells are not affected.
Acyclovir has relatively few serious adverse effects. The drug is excreted by the kidney, and the dose should be reduced in patients with renal dysfunction. Crystal-induced nephropathy may occur if the maximum solubility of free drug is exceeded. Risk factors for this are intravenous administration, rapid infusion, dehydration, concurrent use of nephrotoxic drugs, underlying renal disease, and high doses. The risk of renal toxicity is reduced by adequately hydrating the patient (eg, 1 mL fluid per day for each 1 mg/d of acyclovir).
Because of its high pH, intravenous acyclovir may cause phlebitis and local inflammation if extravasation occurs. Gastrointestinal disturbances, headache, and rash are among the more frequent adverse reactions. Acyclovir is considered appropriate for serious infections during pregnancy. The manufacturer cautions that it should be used in pregnancy only when the potential benefits outweigh the potential risks. However, a prospective registry of acyclovir use in pregnancy between 1984 and 1999, including 756 first trimester exposures, demonstrated a 3.2% rate of birth defects, similar to that expected in the general population.20
Since most relapses occur within 3 months of completing an initial course of intravenous acyclovir, a prolonged course of an oral antiviral agent (eg, valacyclovir) has been suggested following initial treatment. An ongoing clinical trial is currently evaluating a 90-day course of valacyclovir versus placebo post treatment with acyclovir in patients with HSE.21
The role of steroids in the treatment of HSE remains uncertain. To the extent that cellular damage in HSE is the result of immune-mediated inflammatory processes triggered by the viral infection, the anti-inflammatory effects of steroids may be beneficial. However, there is also concern that steroids might suppress immune responses of the host that are necessary to limit viral replication. Animal studies have demonstrated a beneficial effect of steroids on outcome, without evidence of increased viral replication or dissemination.22,23 Steroids have been used to reduce cerebral edema in patients with severe HSE.
One nonrandomized, retrospective human study compared the outcomes of patients with HSE who received steroids in addition to acyclovir versus those receiving acyclovir alone.24 The steroid group had improved outcomes at 3 months. While these results suggest a possible role for steroids in HSE, definitive recommendations must await the results of larger prospective studies.
DOC for HSE; selectively taken up by infected cells; has inhibitory activity against HSV-1 and HSV-2.
10 mg/kg (or 500 mg/m2) IV q8h for 14-21 d; dose infused over 1 h
Administer as in adults
Concomitant probenecid or zidovudine prolongs half-life and increases CNS toxicity
Documented hypersensitivity
B - Fetal risk not confirmed in studies in humans but has been shown in some studies in animals
Caution in renal failure or when using nephrotoxic drugs
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National Institute of Allergy and Infectious Diseases. A Phase III Double-Blind, Placebo-Controlled Trial of Long Term Therapy of Herpes Simplex Encephalitis (HSE): An Evaluation of Valacyclovir (CASG-204). ClinicalTrials.gov. Available at http://clinicaltrials.gov/ct/show/NCT00031486?order=1. Accessed August 8, 2007.
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herpes simplex encephalitis, HSE, herpes encephalitis, herpes simplex virus, HSV, herpes simplex virus type 1, HSV-1, herpes simplex virus type 2, HSV-2, sporadic fatal encephalitis, sporadic lethal encephalitis, viral encephalitis
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