Radiography
Findings
Plain radiographs are not useful in assessing HSV encephalitis.
Computed Tomography
Findings
In adults, CT classically reveals hypodensity in the temporal lobes either unilaterally or bilaterally, with or without frontal lobe involvement. Hemorrhage is usually not observed. A gyral or patchy parenchymal pattern of enhancement is observed. Contrast enhancement generally occurs later in the disease process.24,26
The herpes virus preferentially involves the temporal lobe and orbital surfaces of the frontal lobes. This involvement may extend to the insular cortex, posterior occipital cortex, and cerebral convexity; however, the basal ganglia are spared. Bilateral involvement is frequent. Involvement of the cingulate gyrus occurs later in the disease. The classic involvement of the medial temporal and frontal lobes is consistent with intracranial spread along the small meningeal branches of the fifth cranial nerve. Cingulate gyrus involvement may arise from efferent hippocampal connections. A rhomboencephalitis resulting from pontine involvement may occur and likely arises from retrograde viral transmission along the cisternal portion of the trigeminal nerve to the brainstem.
In neonates, involvement is in the periventricular white matter, sparing the medial temporal and inferior frontal lobes. In addition, meningeal enhancement may be observed following contrast.27,28
Taira et al analyzed specific variables as predictors of a need for a prolonged course of acyclovir therapy in 23 patients with HSV encephalitis. A lower Glasgow Coma Scale score and a greater number of lesions detected on CT were predictors of prolonged therapy. The investigators therefore concluded that standard initial ACV treatment may not be sufficient for patients with such predictors and that initial treatment modifications may be necessary in such patients.16
Degree of Confidence
Late in the disease process, when temporal and frontal lobe involvement is seen, CT findings may be characteristic. Reports exist of patients with normal CT results and CSF studies in the presence of abnormal MRI and EEG findings, indicating that MRI is more sensitive. Early in the disease process, limited sensitivity of approximately 50% was noted. When typical findings of HSV encephalitis are observed on CT, they often are associated with severe brain damage and a poor prognosis.24
False Positives/Negatives
Because of bone artifact, the temporal lobes can be difficult to assess on CT. Early in the disease process, CT may be normal. Other causes of encephalitis, tumor, or lymphoma may appear similar.
Magnetic Resonance Imaging
Axial proton density–weighted image in a 62-year-old woman with confusion and herpes encephalitis shows T2 hyperintensity involving the right temporal lobe.
Axial nonenhanced T1-weighted image shows cortical hyperintensity (arrows) consistent with petechial hemorrhage. In general, this is a common pathologic finding but less commonly depicted in herpes encephalitis.
Axial gadolinium-enhanced T1-weighted image reveals enhancement of the right anterior temporal lobe and parahippocampal gyrus. At the right anterior temporal tip is a hypointense, crescentic region surrounded by enhancement consistent with a small epidural abscess.
Findings
In adults, T2-weighted MRI reveals hyperintensity corresponding to edematous changes in the temporal lobes (see Image 1), inferior frontal lobes, and insula, with a predilection for the medial temporal lobes. Foci of hemorrhage occasionally can be observed on MRI (see Image 2). MRI is preferred for imaging and follow-up studies of herpes encephalitis. Typically, temporal lobe T2 hyperintensity spares the basal ganglia. Although this appearance was previously believed pathognomonic for herpes involvement, similar findings can be observed in progressive multifocal leukoencephalopathy and primary CNS lymphoma. Patchy parenchymal or gyral enhancement can be observed (see Image 3).29,30,31,32,33,34
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy.
NSF/NFD has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.
Reports of restricted diffusion in herpes encephalitis exist (see Image 4), with corresponding T2 hyperintensity reflecting edema (see Image 5). Reports suggest diffusion-weighted imaging may be more sensitive in the detection of HSV involvement than conventional MRI sequences and may mimic an infarct with involvement of the cortical regions of the temporal lobe.
Axial diffusion-weighted image reveals restricted diffusion in the left medial temporal lobe consistent with herpes encephalitis. This patient also had a positive result on polymerase chain reaction assay for herpes simplex virus, which is both sensitive and specific. In addition, the patient had periodic lateralized epileptiform discharges on EEG, which supports the diagnosis of herpes encephalitis.
Coronal T2-weighted image reveals hyperintensity in the left temporal lobe (arrows) in a distribution similar to the restricted diffusion abnormality seen in Image 4 in Multimedia. This finding is typical for herpes encephalitis. In patients with HHV6 infection, one series has noted that in addition to mesial temporal lobe abnormality, abnormal T2 hyperintensity has been seen in the insular and inferior frontal region, which may suggest the diagnosis. There are felt to be 2 typical imaging appearances: one seen in older adults involves T2 hyperintensity confined to the medial temporal lobe; in young adults, a more varied pattern has been described that includes foci of restricted diffusion with an otherwise normal MR, diffuse cortical necrosis, or small focal regions of abnormal T2 hyperintensity.
In neonates, T2-weighted MRI shows hyperintensity of the periventricular white matter, with the medial temporal and inferior frontal lobes spared. Meningeal enhancement also may be observed.35
MR spectroscopy using proton spectroscopic MRI has demonstrated a reduction of the N -acetylaspartate (NAA)-to-choline ratio. A decreased NAA peak has been reported 7-14 weeks after onset, and in some cases, an elevated choline peak is seen. Occasionally, the lactate peak may be elevated. The NAA decrease is believed to reflect neuronal injury. NAA recovery has been noted to parallel clinical improvement.36,37
Degree of Confidence
The diagnosis of herpes encephalitis can be strongly suggested by the typical appearance of medial temporal abnormalities that do not respect hippocampal borders.
Ultrasonography
Findings
The role for sonography in herpes encephalitis is limited. For in utero or neonatal evaluation, ultrasonography may have a limited role in identifying the periventricular destructive process.
Nuclear Imaging
Findings
The use of technetium-99m hexamethylpropyleneamine oxime (HMPAO) single-photon emission CT in evaluating herpes encephalitis is limited. Results demonstrate that the encephalitis matches the distribution of hyperintensity on T2-weighted MRIs, with increased HMPAO uptake in the acute stage. Late sequelae are characterized by decreased HMPAO uptake and postnecrotic widening of the temporal horns.
Degree of Confidence
When abnormal uptake involves the temporal lobes, with characteristic involvement of the limbic system, the diagnosis of herpes encephalitis is likely.
Angiography
Findings
Angiography has no significant role in the diagnosis of herpes encephalitis.
More on Herpes Encephalitis |
| Overview: Herpes Encephalitis |
 Imaging: Herpes Encephalitis |
| Follow-up: Herpes Encephalitis |
| Multimedia: Herpes Encephalitis |
| References |
| Further Reading |
| « Previous Page | Next Page » |
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Further Reading
Related eMedicine topics
Herpes Simplex Encephalitis (Emergency Medicine)
Herpes Simplex Encephalitis (Neurology)
HIV-1 Associated Opportunistic Infections: Cytomegalovirus Encephalitis
Viral Encephalitis
Clinical guidelines
Neurologic complications in HIV-infected children and adolescents.
New York State Department of Health - State/Local Government Agency [U.S.]. 2003 Mar. 19 pages. NGC:003047
Treating opportunistic infections among HIV-infected adults and adolescents. Recommendations from CDC, the National Institutes of Health, and the HIV Medicine Association/Infectious Diseases Society of America. Centers for Disease Control and Prevention - Federal Government Agency [U.S.]. 2004 Dec 17. 112 pages. NGC:003994
2007 national guideline for the management of genital herpes.
British Association for Sexual Health and HIV - Medical Specialty Society. 1999 Aug (revised 2007). 26 pages. NGC:006299
Screening for genital herpes: recommendation statement.
United States Preventive Services Task Force - Independent Expert Panel. 2005 Mar 18. 11 pages. NGC:004067
Viral encephalitis: a review of diagnostic methods and guidelines for management.
European Federation of Neurological Societies - Medical Specialty Society. 2005 May. 13 pages. NGC:005163
Clinical trials
Long Term Treatment of Herpes Simplex Encephalitis (HSE) With Valtrex
Structure of the Herpes Simplex Virus Receptor
Keywords
herpes encephalitis, viral encephalitis, herpesvirus infection, HSV encephalitis, HSV-1, HSV-2, herpes-induced encephalitis, herpes infection, brain herpes, herpes simplex virus type 1, herpes simplex virus type 2, herpes simplex virus encephalitis
