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Neuroradiology Case of the Week

Case 159

Rajashree Vyas, MD and PL Westesson MD, DDS, PhD

Clinical Presentation:  Patient is a 7-year-old with severe global developmental delay, spastic quadriplegia secondary to infantile herpes encephalitis.

Radiological Findings: FLAIR, T1 and T2 weighted images demonstrate CSF density, cystic areas within the supratentorial compartment with multiple septations. There is near complete absence of supratentorial brain parenchyma. Bilateral thalami (white arrows), brainstem and the cerebellum are preserved.

Differential Diagnosis:

1. Multicystic encephalomalacia
2. Hydranencephaly
3. Porencephaly

Diagnosis: Multicystic encephalomalacia secondary to herpes simplex encephalitis

Clinical Discussion: Herpes virus is included in the congential TORCH group of infections (toxoplasmosis, other agents, rubella,cytomegalovirus and herpes simplex). Herpes is a DNA virus with two known serotypes, types 1 and 2. Approximately 75% or more of neonatal infections are caused by type 2. The most common mode of transmission is parturitional, with infection caused by direct contact of the infant’s eyes, skin or oral cavity in the cervix or vagina.
     The neuropathological and radiological findings depend upon the age of infection. Herpes virus has a predilection for endothelial cells, which explains the resulting thrombosis and hemorrhagic infarctions.
     Regardless of the specific radiologic findings, neonatal meningoencepalitis is usually diffuse and overwhelming, resulting in widespread brain destruction. Late imaging in the disease process demonstrates diffuse volume loss throughout the cerebral hemispheres, hydrocephalus and multicystic encephalomalacia.
     Pathologically, astroglial proliferation and multifocal gray and white matter involvement with cystic infarction and demyelination leads to cystic encephalomalacia.
     Septations form within areas of damaged brain parenchyma. This distinguishes it from porencephaly and hydranenecephaly.

The location of the lesions varies with the nature of the insult.
Causes include:

1. Thrombotic/embolic infartion. The lesion lies within the distribution of a branch of a major cerebral artery.
2. Mild-moderate hypotension. Lesions are primarily located at the intervascular boundary zones (“watershed areas”) in the cortex and peripheral white matter.  In severe cases, only the immediate periventricular white matter may be spared.
3. Post-infectious eg. HSV.  Lesion site is usually non-specific representing the area of brain affected by the infectious etiology. (Refer to Case #50.)

     Severe cerebral necrosis and resultant cystic encephalomalacia are characteristic of infections with Proteus, Enterococcus, Citrobacter and Serratia species.

Radiological Discussion: Ultrasound (US) is the most sensitive modality to detect glial septae but offers poor evaluation of the entire brain. After injury, US shows increased echogenicity of affected regions within 2-5 days after the injury. Although US has shown the appearance of cystic degeneration between 7-30 days after injury in term infants, the time to cavitation is presumably related to the severity of injury.
     CT has inherent limitations on contrast resolution making it difficult to reliably distinguish porencephaly from encephalomalacia. CT initially shows diffuse hypoattenuation within affected regions and progresses to hypodense tissue with cysts of varying sizes within.
     MR demonstrates areas of  reactive astrogliosis and tissue injury as areas of prolonged T1 and T2 relaxation. In infants, subacute phase of injury will show T1 and T2 shortening of injured cortex and white matter. The combination of glial septae and CSF creates a heterogeneous appearance.  This is most apparent on FLAIR sequences where the glial strands appear hyperintense in contrast to the hypointense CSF.
     Overall, MR is much better in detection of glial septae than CT or US.

Prognosis: Long term sequelae of survivors can be markedly debilitating with major neurological problems, including mental retardation, blindness and spastic quadriparesis as is seen in our case.

References:

1. Barkovich AJ. Pediatric Neuroimaging. Lippincott Willims and Wilkins 4th ed.; Chapter 4, pgs. 191-195.
2. West A. Postinfectious encephalomalacia secondary to herpes simplex type 2 virus. Applied Radiology Online 30(9), September 2001. [PDF]
3. Leonard JR, Moran CJ, Cross DT 3rd, Wippold FJ 2nd, Schlesinger Y, Storch GA. MR imaging of herpes simplex type 1 encephalitis in infants and young children: a separate pattern of findings. AJR Am J Roentgenol. 2000 Jun;174(6):1651-5. [Medline]