Key Differences Between Hydranencephaly and Anencephaly
Hydranencephaly and anencephaly are fundamentally different conditions: anencephaly results from failure of anterior neural tube closure with exposed, undifferentiated neural tissue above the orbital rims and is virtually always fatal at birth, while hydranencephaly occurs after neurogenesis during embryogenesis with near-complete absence of cerebral hemispheres but intact skull and meninges, allowing for potential survival beyond infancy. 1
Embryological Timing and Pathophysiology
Anencephaly:
- Represents failure of anterior neural tube closure during early embryonic development 1
- The cranial analog of myelomeningocele (MMC) 1
- Results in exposed, undifferentiated neural tissue present above the orbital rims and nuchal line 1
Hydranencephaly:
- Occurs during embryogenesis after neurogenesis has already occurred 2
- Characterized by near-complete absence of cerebral hemispheres replaced by cerebrospinal fluid and glial tissue 2, 3
- Meninges and skull are well-formed, consistent with earlier normal embryogenesis of the telencephalon 3
- Bilateral occlusion of internal carotid arteries in utero is a potential mechanism 3
Clinical Presentation at Birth
Anencephaly:
- Malformation is obvious at birth 1
- Open, exposed neural tissue visible externally 1
- Virtually always fatal 1
Hydranencephaly:
- Head size and spontaneous reflexes (sucking, swallowing, crying, moving arms and legs) may all appear normal at birth 3
- Intact brainstem reflexes without evidence of higher cortical activity 3
- Diagnosis may be delayed for months because early behavior appears relatively normal 3
- After a few weeks, infants typically become irritable with increased muscle tone 3
Prognosis and Survival
Anencephaly:
- Virtually always fatal at or shortly after birth 1
- No treatment available 1
- Neurologic outcome in the few identified survivors is poor 1
Hydranencephaly:
- Prolonged survival is not unusual in the modern treatment era 2
- Survival can extend from several weeks to months, with documented cases surviving up to 19 years 4
- Survival possible even without rostral brain regions, with isoelectric electroencephalograms, and absent-evoked potentials 4
- Four patients in one series survived over 5 years with appropriate management 2
Management Considerations
Anencephaly:
- No treatment available 1
Hydranencephaly:
- Ventriculoperitoneal shunt (VPS) placement is standard management 2
- Major complications include recurring shunt malfunction and shunt infection (occurring in 3 of 4 cases in one series) 2
- Choroid plexus cauterization (CPC) with shunt removal may be considered after shunt infection to avoid ongoing CSF shunt problems 2
- After a few months, seizures and hydrocephalus typically develop requiring intervention 3
Important Diagnostic Distinction
Critical differentiation from maximal hydrocephalus:
- True hydranencephaly shows minimal occipital brain parenchyma connected by thin bridge of tissue to intact basal ganglia, with absent EEG activity 5
- Maximal hydrocephalus shows minimal frontal cerebral mantle with presence of electrical activity on EEG 5
- This distinction is crucial because maximal hydrocephalus patients show remarkable progress with bilateral shunts and appearance of considerable brain substance on serial CT scans, whereas true hydranencephaly patients show no improvement neurologically or radiologically 5