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Hydranencephaly

Hydranencephaly is a rare encephalopathy that occurs in-utero. It is characterised by destruction of the cerebral hemispheres with transformation into a membranous sac containing cerebrospinal fluid and the remnants of cortex and white matter 1.

Epidemiology

This is a rare disorder with an incidence of 0.2% in infant autopsies 5.

Clinical presentation

The condition may be diagnosed prenatally using ultrasound or foetal MRI. However, it may present in neonates with seizures, respiratory failure, flaccidity or decerebrate posturing with a vegetative existence 5.  

Associations

Pathology

There is complete absence of the cerebral hemispheres and often, the falx. They are replaced by a sac-like structure containing CSF surrounding the brainstem and basal ganglia 1

The outer layer is composed of leptomeninges while the inner layer of cortex and white matter filled with CSF and necrotic material. It may be considered a gross polporencephaly.

Aetiology

Five aetiologies have been described.

  1. infarction - bilateral occlusion of the supra-clinoid segment of the internal carotid arteries or of the middle cerebral arteries 3
  2. leukomalacia - an extreme form of leukomalacia formed by confluence of multiple cystic cavities 4
  3. diffuse hypoxic-ischemic brain necrosis fetal hypoxia due to maternal exposure to carbon monoxide or butane gas may result in massive tissue necrosis with cavitation and resorption of necrotized tissue 1
  4. infection - necrotizing vasculitis or local destruction of the brain tissue secondary to intrauterine infection, e.g. congenital toxoplasmosis, cytomegalovirus, and herpes simplex infections (HSV) 1,8
  5. thromboplastic material from a deceased co-twin -  
    • monochorionic twins have presented with a variety of cerebral lesions. 
    • lesions in the recipient twin result from emboli or thromboplastic material originating from the macerated co-twin 4

Radiographic features

All modalities able to image brain parenchyma can be used to identify the features of this hydranencephaly including ultrasound (ante and post natal), MRI (ante and post natal) and CT. In all cases, the anatomical features are the same, although they are demonstrated to a variable degree according to the abilities of each modality. MRI is, as is usually the case, the gold standard. 

Typically there is essentially no remaining cortical tissue, with preserved thalami and posterior fossa. Often islands of residual tissue is seen typically at the occipital poles and orbitofrontal regions. The falx is usually present, and the hemicranium filled with fluid, in which choroid can often be identified. 

Antenatal ultrasound, or vascular imaging will demonstrate absence of the middle cerebral arteries.

Treatment and prognosis

Hydranencephaly is not compatible with a prolonged life after birth, with the vast majority of live births dying prior to one year of age. Termination of pregnancy is usually considered justifiable due to this reason.

Differential diagnosis

General imaging differential considerations include:

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