Fetal Arachnoid Cyst / Midline Cyst

Step-by-step decision

Choose step by step as prompted; reaching an endpoint gives the management recommendation. You can go back a step or restart anytime.

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Full pathway

1. [Decision] Site + mass effect/hydrocephalus + differentiation
   Site + mass effect/hydrocephalus + differentiation? (Arachnoid cyst = a benign extra-axial CSF cyst, isointense to CSF on all sequences, no enhancement, not communicating with the ventricles/subarachnoid space; mass effect (displaces, does not destroy, adjacent structures). Mostly supratentorial (middle cranial fossa most common). Usually found after the second trimester.)
   • Extra-axial CSF-signal cyst, isolated, no mass effect/hydrocephalus (middle fossa/convexity) → Isolated · follow-up
   • Mass effect causing ventricular dilation/obstructive hydrocephalus (quadrigeminal/suprasellar/posterior fossa) → Mass effect/hydrocephalus · neurosurgery
   • Midline cyst needing differentiation (vs cavum septi pellucidi enlargement/Blake pouch/DWM/vein of Galen/callosal-related) → Midline cystic lesion differentiation
2. [End] Isolated · follow-up
   Isolated extra-axial arachnoid cyst, no mass effect/hydrocephalus: mostly good prognosis; serial MRI/ultrasound monitoring of size and ventricles + check associated CNS (especially ACC) and extracranial anomalies; isolated cases followed postnatally, about 30–60% have postnatal surgery (fenestration/shunt) for symptoms/enlargement/hydrocephalus.
3. [End] Mass effect/hydrocephalus · neurosurgery
   Mass effect causing ventricular dilation/obstructive hydrocephalus (quadrigeminal cistern/suprasellar compressing the aqueduct, posterior fossa): MRI to assess the mass and hydrocephalus; monitor progressive ventricular enlargement and cortical mantle; multidisciplinary + postnatal neurosurgery (fenestration/shunt); for suprasellar, watch the visual pathway/endocrine.
4. [End] Midline cystic lesion differentiation
   Midline cystic lesion differentiation: an arachnoid cyst does not elevate the pontine-vermian angle, does not communicate with the fourth ventricle (vs Blake pouch/DWM); differentiate from cavum septi pellucidi/fornix position (vs midline cavum velum interpositum), vein of Galen (has flow, not CSF), callosal-related interhemispheric cyst (with ACC); midsagittal localization + check ACC; manage by characterization and associated findings.

Source guidelines & references

• Fetal arachnoid cyst MRI and midline cystic lesion differentiation (extra-axial CSF cyst; does not elevate the pontine-vermian angle)

This pathway is our own synthesis of the decision logic in the guidelines above (not the guideline verbatim); thresholds and workflows change as guidelines update — in practice follow the latest guideline, your institution's protocol and the individual patient.

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