Neonatal herpes simplex encephalitis

Neonatal herpes simplex encephalitis is caused by vertical transmission of infection during passage from birth canal with diffuse cerebral involvement within the first month after birth; in contrast to adult herpes simplex encephalitis, it is commonly related to HSV-2. 

Epidemiology

The incidence of neonatal HSV infection at all is usually low and it varies by country. About 80% of cases are due to HSV type II-2. 

Clinical presentation

There are three types of clinical manifestations related to this infection ²:

• skin lesions without any visceral or central nervous system (CNS) involvement, also known as skin, eye and mouth disease
• CNS disease (with or without skin lesions, but without involvement of visceral organs); usually this presentation has non-specific signs including decreased level of consciousness, seizures, lethargy and fever
• disseminated form characterised as a sepsis with multi-organ failure 

Newborn babies are initially asymptomatic for one or two weeks.

The diagnosis is confirmed by detection of HSV DNA in the cerebrospinal fluid.

Radiographic features

It is important to appreciate that the radiographic appearance of neonatal HSV encephalitis is different from its more common adult counterpart. Changes are typically diffuse which can be difficult to identify due to normal immature myelin (see normal myelination) and more commonly involving the cerebral cortex, the deep white matter, and thalamus ⁴. The medial temporal and inferior frontal lobes may be spared and haemorrhagic change is uncommon but can develop later and is best seen on gradient echo/susceptibility sequences ¹.

As a sequel, areas of leukomalacia, diffuse multicystic encephalomalacia, and parenchymal punctate or gyral calcifications may be seen ³. 

CT

• may be negative in the early course of the disease
• in advanced stages may present with extensive areas of parenchymal hypoattenuation, predominantly in the white matter, as a result of oedema or necrosis ³
• enhancement usually presents in a gyriform pattern

MRI

Affected areas, however, have a similar appearance regarding signal characteristics:

• T1
  • may show general oedema in the affected region
  • if complicated by subacute haemorrhage there may be areas of hyperintense signal
• T2
  • hyperintensity of affected white matter and cortex, predominantly due to oedema
  • as previously discussed, the incomplete white matter myelination in the neonate's brain and its high water content may mask subtle signal changes
• DWI/ADC
  • restriction diffusion is demonstrated in approximately half of all patients, which tends to be diffuse and bilateral
  • restricted diffusion is common due to cytotoxic oedema
  • restricted diffusion is less intense compared to infarction
  • beware of T2 shine through due to vasogenic oedema
• GE/SWI: may demonstrate blooming if haemorrhagic 
• T1 C+ (Gd): enhancement usually presents in a gyriform pattern

Treatment and prognosis

Neonatal herpes simplex encephalitis is highly lethal (in about 50% of cases) and can cause permanent disability if left untreated ². 

Treatment is with intravenous antivirals (aciclovir is usually the drug of choice).  

Sequelae are mostly seen in neurodevelopment, including deafness, vision loss, cerebral palsy, and seizure.