Acute necrotizing encephalopathy

Acute necrotising encephalitis of childhood (ANEC) is a rare type of encephalopathy characterised by multiple bilateral brain lesions, mainly involving the thalami, but also the putamina, internal and external capsules, cerebellar white matter, and the brainstem tegmentum.

Epidemiology

ANEC is a rare condition with less than 150 cases reported in literature. From the first case described in 1995, many cases have been reported both in Asia as well as western countries. Most cases are sporadic; however, few cases of recurrent and/or familial episodes have been reported suggesting an inherited pattern. 

Clinical features

ANEC is seen in children around the age of 4 years. Clinically, it is characterized by acute encephalopathy, with dramatical neurological deficits/symptoms. A viral etiology has been proposed in some cases; viral prodrome hence may precede the neurological deficits.

Pathology

The aetiology and the pathogenesis of ANEC remain partially clear. Usually, it develops secondary to viral infections, including influenza A and influenza B, parainfluenza, varicella and enterovirus.

Pathologically, the lesions show oedema, haemorrhage, and necrosis. ANEC is characterised by the lack of inflammatory cells in affected brain parenchyma in comparison to the more common entities of acute disseminated encephalomyelitis and acute hemorrhagic encephalitis ¹.

Radiographic features

CT

On a CT-scan, the corresponding thalamic, putamina, cerebral, cerebellar and brainstem density abnormalities are hypodense.

Intracranial hemorrhage and cavitation may also be seen, both of which are associated with a worse prognosis.

MRI

In most cases of ANEC, there is bilateral symmetrical thalamic involvement. Abnormal signals on MRI are hypointense on T1 and hyperintense on T2. TheseRestricted diffusion of the involved regions can be seen. ⁷ These findings can be quite extensive. Haemorrhage, cavitation, and post contrast enhancement are also seen.

Treatment and prognosis

ANEC carries a very poor prognosis; the mortality rate is near 70%. Treatment is mainly supportive, as the precise etiology remains unknown.

Differential diagnosis

If characteristic symmetric imaging findings are present, the differential diagnosis is limited. Lactic acidosis is not a classic finding of ANEC. Otherwise, the differential diagnoses include:

• acute disseminated encephalomyelitis (ADEM)
• Leigh syndrome
  • associated with hypoglycemia, hyperammonemia and lactic acidosis
• Reye syndrome
  • associated with hypoglycemia, hyperammonemia and lactic acidosis

Clinically, ANEC may be differented from ADEM by early onset of encephalitic features just after the prodromal illness while in ADEM, they may take 1 to 2 weeks to develop.