BrainTF13-Herpes Simplex Encephalitis

Clinical Profile:

H/O fever and memory impairment.

Findings:

There are diffuse areas of hypointensity on the T1W images that turn hyperintense on the T2W images involving the cortical gray and underlying white matter in the fronto-temporal lobes bilaterally. The insular cortex is also involved. 

Discussion: 

Herpes viruses are a group of double stranded DNA viruses and include viruses like HSV-1, HSV-2, CMV, EBV, VZV, etc.. After the initial viremia, seeding of the CNS may occur. CMV, HSV-2, neonatal cases of HSV-1 and EBV gain access by diffusion through the blood-brain barrier or by infecting the endothelial cells within intracranial blood vessels. In non-neonatal HSV-1 and VZV the infection starts in a peripheral neuron and is transported centrally.

HSV-1 is the causative organism in 95% of the herpetic infections. There is a mortality rate of 50% to 70% in HSV encephalitis. In adults it is usually a result of reactivation of a latent HSV-1 infection in the trigeminal ganglion with spread along the branches of the trigeminal nerve that innervate the meninges of the anterior and middle cranial fossae. Pontine infection may occur due to retrograde transmission along the cisternal component of the trigeminal nerve. In children, HSV-1 is usually post-natal whereas HSV-2 is seen in neonates and congenital afflictions.

Patients may present with headaches, fever, altered mental status and focal or diffuse neurologic deficit.

On MRI:

• HSV-1 may result in a necrotizing encephalitis, involving the temporal lobes and orbital surfaces of the frontal lobes. The insular cortex, cerebral convexity and posterior occipital cortex may be involved. Affliction is usually bilateral and the basal ganglia are spared. Lesions are hypointense on the T1W sequences and hyperintense on the T2W, FLAIR and PD images. There is involvement of the cortex and white matter.
  

• The areas enlarge and coalesce with associated mass effect. Enhancement is absent in the early stages. Gyriform enhancement may be seen later. Focal hemorrhage may be seen.
  

• Later on atrophy and areas of encephalomalacia are seen.
  

• MRI is used to monitor response to acyclovir therapy.
  

• MRI is usually positive within 48 hours (CT - 3-5 days, SPECT - 4-11 days).
  

• In AIDS, HSV may be diffuse rather than the classical distribution.
  

1. Intracranial Infection. In: Atlas SW: Magnetic Resonance Imaging of the Brain and Spine, Lippincot-Raven, pp:708-711, Second Edition.