Herpes Simplex Encephalitis (HSE)

Clinical presentation of HSE in children over 3 months and adults is typically is HSV-1, acute, or subacute, and generally has non-specific signs such as:

  • Alteration of conscious (97%)
  • CSF Pleocytosis (97%)
  • Fever (90%),
  • Personality Change (85%)
  • Headache (81%)
  • Seizures(67%)
  • Vomiting (46%)

In newborns it typically presents 6-12 days after birth with general lethargy, poor feeding, and/or seizures and is HSV-2.


CSF should be taken immediately when HSE is suspected and sent for a PCR study. This is the gold standard and is sensitive 94-98% and specific 98-100%.

Lab studies are non-specific. Imaging (CT and MRI) and Electroencephalography abnormalities can take days to a week to appear on scans.

  • CSF in patient with HSE will have elevated WBC, RBC, elevated protein, and normal glucose level with lymphocytic pleocytosis
  • MRI findings if present would demonstrate temporal lobe lesions
  • Electroencephalography (EEG) has characteristic periodic high-voltage spike wave activity emanating from the temporal lobes and slow wave complexes are highly suggestive of HSE.


IV acyclovir should be started immediately , before PCR results confirm, because its toxicity is rather low and HSE prognosis is poor untreated. In adults 10-15mg/kg q8h x14-21days, 3 months-12years 20mg/kg x10 days, neonates 30mg/kg/day.

In keeping with the theme of turning what basic science we are learning in class into clinical scenarios, Sakib suggested I write up a piece on HSE relating to our Infectious Disease class. Struggling to stay focused through learning tons of boring viral info, bacteria, fungi, etc. I wondered what we’ll ever need to know again, especially in emergency medicine. There are actually quite a lot of emergencies that can arise, and HSE is one.

HSE is the most common cause of sporadic fatal encephalitis in the US (Meyer). HSV is a dsDNA virus in the Herpesviridae family, of which there are 8 types that affect humans. HSV-1 and HSV-2 are spread when an infected person is shedding virus during the primary infection or a secondary infection. The viron fuses with a cell’s plasma membrane where it uncoats and a Viral Host Shutoff protein is expressed. Herpesvirus then replicates using sequential control of protein synthesis expressing mRNA for first alpha, then beta, then gamma proteins. Herpes is well known for its characteristic of viral latency, where it remains dormant in cell bodies of neurons, safe from immune cell destruction.

Primary illness with HSV1&2 is typically the most severe illness, latent ones not as severe. 90% are symptomless, 9% minor illness, 1% have severe illness with gingivostomatitis, the most common symptom. Infections “above the belt” are usually (but not always) HSV-1 and “below” HSV-2.

PCR is used diagnostically to differentiate 1&2 and is the gold standard, because the viruses differ in 50% of their DNA. Immunocytochemistry and Western Blot can be used but are less accurate.


In regards to diagnosis, the presentation is described above. It has a somewhat non-specific presentation, but needs to be treated immediately. Untreated, death can occur in 7 days, which can vary with severity. The pathogenicity is not known, neither is the mechanism of cell damage leading to encephalopathy.

A short list of differential should include:

  • Meningitis
  • Infective encephalitis causes
  • Abscesses
  • Intracranial tumors

CSF analysis is used to diagnose. CT and MRI don’t show abnormalities for several days. On CT changes in temporal or frontal lobe are seen as low density lesions after 3-4 days. In T2 MRI, early involvement of white matter is seen. The inferomedial portion of the temporal lobe is most commonly affected on MRI (Anderson).

62-year-old woman with confusion and herpes encephalitis shows T2 hyperintensity involving right temporal lobe:

source = eMedicine

Acyclovir is used to treat. When HSE is suspected it should be started immediately, however in one study at an academic emergency medicine institution showed only 29% of patients were given acyclovir with presentation of HSE1 (Benson).

What better way to end this post than to get a quick review on serious CNS infections from Scott Weingart at EMCrit podcast


Anderson, DO, Wayne. “Herpes Simplex Encephalitis Treatment & Management.” Medscape Reference. 07 2011

Benson PC, Swadron SP. Empiric acyclovir is infrequently initiated in the emergency department to patients ultimately diagnosed with encephalitis. Ann Emerg Med. Jan 2006;47(1):100-5. [Medline].

Meyer Jr., M.H. et al., 1960. CNS Syndroms of “Viral” Etiology. Am. J. Med. 29, 334-247.

Compiled by: Nick Mancuso, M1

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One thought on “Herpes Simplex Encephalitis (HSE)

  1. Adolfo says:

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    I believe that you could do with some p.c. to force the message home a bit,
    however other than that, that is fantastic blog.
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