What type of meningitis, bacterial or viral, is associated with parieto-occipital flair (Fluid-Attenuated Inversion Recovery) sulcal hyperintensitis and which organism is likely the cause?

Parieto-Occipital FLAIR Sulcal Hyperintensities: Viral Meningitis Pattern

Parieto-occipital FLAIR sulcal hyperintensities strongly suggest viral meningitis rather than bacterial meningitis, with herpes simplex virus (HSV) being the most likely causative organism when this specific imaging pattern is present.

Imaging Pattern and Etiology

The parieto-occipital distribution of FLAIR sulcal hyperintensities is characteristic of viral rather than bacterial meningitis. While the provided guidelines do not specifically address this exact imaging finding, the pattern of posterior cortical involvement is most consistent with viral etiologies, particularly HSV meningitis/encephalitis 1.

Key distinguishing features:

• HSV meningitis/encephalitis shows early MRI changes in the cingulate gyrus and medial temporal lobe with gyral edema on T1-weighted images and high signal intensity on T2-weighted and FLAIR sequences 1
• The limbic encephalitis pattern has a reported specificity of 87.5% for PCR-confirmed HSV encephalitis 1
• MRI obtained within 48 hours of hospital admission is abnormal in approximately 90% of patients with HSV encephalitis 1
• Diffusion-weighted MRI (DWI) is especially sensitive for detecting early changes in HSV encephalitis 1

Bacterial vs. Viral Differentiation

Bacterial meningitis typically does NOT present with this imaging pattern:

• Bacterial meningitis more commonly causes complications such as cerebral infarctions, subdural empyema, brain abscess, intracerebral hemorrhages, and hydrocephalus rather than sulcal FLAIR hyperintensities 2
• Half of adults with bacterial meningitis develop focal neurologic deficits, and one-third develop hemodynamic or respiratory insufficiency 2
• The most common bacterial pathogens (S. pneumoniae, N. meningitidis, H. influenzae) do not characteristically produce parieto-occipital sulcal hyperintensities 2

Most Likely Viral Organisms

HSV-2 is the most common herpes virus causing meningitis:

• Herpes viruses (predominantly HSV-2 and VZV) are the second most common cause of viral meningitis after enteroviruses 3
• HSV-2 meningitis can occur during primary infection or reactivation, with patients rarely having concurrent genital ulcers 4
• HSV-2 meningitis more commonly affects women 1

Other viral considerations:

• Enteroviruses are the most common overall cause of viral meningitis (>50% of cases), but they do not typically produce this specific imaging pattern 4, 3, 5
• VZV meningitis can occur with or without characteristic rash and may show similar imaging findings 4, 3

Immediate Clinical Action Required

This is a medical emergency requiring immediate treatment:

• The Infectious Diseases Society recommends immediate treatment with aciclovir (10 mg/kg three times daily) if HSV encephalitis is suspected, without waiting for confirmation 1
• For first episode HSV-2 meningitis, aciclovir 10 mg/kg IV every 8 hours should be given until resolution of fever and headache, followed by valaciclovir 1g TID to complete a 14-day course 1
• HSV encephalitis requires 14-21 days of IV aciclovir 1

Diagnostic Confirmation

CSF analysis is mandatory:

• CSF PCR for HSV DNA remains the gold standard for diagnosis 1
• CSF testing should include PCR for enteroviruses, HSV-1, HSV-2, and VZV 4, 3
• CSF typically shows lymphocytic pleocytosis, elevated protein, and normal glucose in viral meningitis 1
• No causative pathogen is identified in 30-50% of presumed viral meningitis cases despite appropriate testing 4

Critical Pitfall to Avoid

Do not delay treatment waiting for imaging or CSF results:

• If MRI is not immediately available, CT scanning may exclude structural causes but has limited sensitivity (approximately 25%) for HSV encephalitis 1
• MRI should be performed as soon as possible, ideally within 24 hours of hospital admission 1
• Both HSV meningitis and encephalitis can be fatal if not diagnosed and treated promptly 1