Hydranencephaly

Introduction

• Hydranencephaly (HE) is a congenital central nervous system anomaly characterized by the absence or near-total absence of the cerebral hemispheres, which are replaced by a fluid-filled sac. 

  
  

• It is an encephaloclastic process - meaning it involves the destruction of already formed brain tissue, differentiating it from primary developmental malformations.

Etiology

Hydranencephaly is an acquired, destructive lesion occurring after the first trimester, following the formation of the skull and meninges.

The most widely accepted pathophysiological theory involves bilateral vascular compromise within the fetal brain:

1. Bilateral ICA Occlusion: Most commonly accepted hypothesis.

   
   

Occlusion of the supraclinoid ICAs

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Widespread infarction

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Necrotic tissue is gradually reabsorbed

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CSF-filled cavity.

2. Preserved Structures: Structures supplied by the posterior circulation (vertebrobasilar system) remain intact.

   
   

3. Other Causes: Severe intrauterine infections (e.g., congenital cytomegalovirus, toxoplasmosis, herpes simplex),

   
   

Imaging features

A. Ultrasound (US)

• Typically identified at or after the mid-second trimester.

• Cranial vault appears almost completely filled with a large, anechoic (fluid-filled) space.

• "Floating" Structures: The preserved thalami and brainstem appear as solid structures floating within the fluid.

  
  

• Color Doppler: Absence of flow in the anterior and middle cerebral arteries - a highly specific sign.

  

B. CT/MRI

• No remaining cortical tissue.

  • Few islands of residual tissue may be preserved.

  • medial temporal tissue may be present, as the medial temporal lobes receive blood supply from the basilar circulation.

    
    

• Thalami and posterior fossa are preserved

  
  

• Falx is typically present.

C. Angiography

• Abrupt cut-off or severe hypoplasia of the distal ICAs.

Differential Diagnoses

Hydranencephaly must be differentiated from two other conditions with similar imaging appearances:

Condition	Radiological Feature	Hydranencephaly (HE)
Severe Hydrocephalus	Cortical Mantle: Cortex is always present, even if severely stretched. Middle cerebral arteries are preserved.	Absent or replaced by fluid.
Alobar Holoprosencephaly (HPE)	Falx Cerebri is absent. Fused thalami.	Usually present and intact.

Severe hydrocephalus

Clinical Implications:

• Severe Neurological Deficit: Infants may be in a vegetative state, without cognitive functions.

  
  

• Presentation: Signs develop within weeks to months, including:

  • Abnormal muscle tone

  • Increasing head size

  • Seizures

    
    

• Prognosis: Typically poor, with death usually within the first year.

  
  

Tips About Reporting Hydranencephaly

1. Be Definitive about the Cortex: Explicitly state the absence of a cortical mantle or the presence of only minor, non-contiguous remnants

   1. e.g. "Near-total absence of supratentorial parenchyma with no identifiable cortical mantle."

      
      

2. Confirm the Falx: Always confirm the status of the falx cerebri.

   1. e.g. "Falx cerebri is present and intact/incomplete," to help rule out alobar holoprosencephaly."

      
      

3. Validate Posterior Circulation: Confirm that the posterior fossa structures and thalami are preserved and appear normal.