Definition and Developmental Origin

  • Blake’s pouch is a normal embryonic structure:
    • Dorsal evagination of the 4th ventricle ependyma.
    • Occurs through the midline foramen of Magendie.
  • Normally perforates at 9–10 weeks post-conception (p/c):
    • Enables cerebrospinal fluid (CSF) communication between the 4th ventricle and subarachnoid space.

Pathogenesis

  • Failure of Blake’s pouch to perforate:
    • Leads to formation of a cyst.
    • Results in enlargement of the 4th ventricle.
    • Causes mild-moderate upward rotation of the vermis.
    • Increases tegmento-vermian angle.

Diagnostic Criteria

  • Enlarged fourth ventricle.
  • Normally formed vermis (no hypo/dysplasia):
    • Mild to moderate rotation of vermis.
    • Widened tegmento-vermian angle.
  • Normal-sized posterior fossa.
  • Normal-sized cisterna magna (distinguishes from mega cisterna magna and DWM).

Distinction from Dandy-Walker Malformation (DWM)

  • Both involve abnormalities in foraminal development but differ significantly:
    • Blake’s pouch cyst: originates from posterior membranous area (PMA).
    • Dandy-Walker malformation: originates from anterior membranous area (AMA).
  • Key imaging distinction:
    • Blake’s pouch cyst features a normal vermis and normal posterior fossa.
    • DWM features vermian hypoplasia/dysplasia, posterior fossa enlargement, and elevation of torcular/tentorium.
  • Blake’s pouch cyst often resolves spontaneously by late gestation (24–26 weeks), not seen in DWM.

Imaging Signs

  • Identification of 4th ventricular choroid plexus along:
    • Inferior surface of the vermis.
    • Roof of the cyst.
  • Although challenging to detect on conventional imaging, this is a key diagnostic indicator.

Natural History and Prognosis

  • Spontaneous resolution is common:
    • Approximately 50% resolve around 24–26 weeks gestation.
    • Nearly two-thirds resolve by term gestation.
    • Resolution likely due to delayed fenestration of 4th ventricular roof.
  • Hydrocephalus is rare but possible:
    • Typically mild if present; severe (tetraventricular) hydrocephalus uncommon.
  • Long-term clinical prognosis generally excellent and benign:
    • Often incidental diagnosis.
    • Normal neurodevelopmental outcome in most cases.

Relationship with Other Posterior Fossa Cysts (Controversies)

  • Blake’s pouch cyst sometimes grouped with Dandy-Walker variant and mega cisterna magna:
    • Proposed as a spectrum disorder.
    • Differences attributed to timing and extent of Blake’s pouch fenestration (controversial viewpoint; Robinson & Goldstein, 2007).
  • Clinical significance:
    • Distinction between entities crucial due to differing prognoses.

Clinical Management

  • Usually no treatment required.
  • Monitoring for resolution via serial imaging:
    • Typically, prenatal and postnatal follow-up recommended.
  • Interventions (CSF diversion) rarely necessary, reserved for unusual cases with hydrocephalus or symptomatic mass effect.