= CSF-containing intraarachnoid cyst without ventricular communication / brain maldevelopment

Incidence: 1% of all intracranial space-occupying lesions

Origin:

1. congenital: arising from clefts / duplication / “splitting” of arachnoid membrane with expansion by CSF due to secretory activity of arachnoid cells = true arachnoid cyst
2. acquired: following surgery / trauma / subarachnoid hemorrhage / infection in neonatal period / associated with extraaxial neoplasm = loculation of CSF surrounded by arachnoidal scarring with expansion by osmotic filtration / ball-valve mechanism = leptomeningeal cyst = secondary arachnoid cyst = acquired arachnoid cyst

Histo: cyst filled with clear fluid; thin wall composed of cleaved arachnoid membrane; lined by ependymal / meningothelial cells

Age: presentation at any time during life

• often asymptomatic
• symptomatic ← mass effect, hydrocephalus, seizures, headaches, hemiparesis, intracranial hypertension, craniomegaly, developmental delay, visual loss, precocious puberty, bobble-head doll syndrome

Location: in CSF cisterns between brain + dura

1. floor of middle fossa near tip of temporal lobe (50%): common in sylvian fissure
   • usually asymptomatic
2. suprasellar / chiasmatic cistern (10%)
   • propensity to become symptomatic in early childhood:
     • hydrocephalus (most common manifestation)
     • visual impairment
     • endocrinopathy: eg, precocious puberty
3. posterior fossa (⅓–¼): cerebellopontine angle (11%), quadrigeminal plate cistern (10%), in relationship to vermis (9%), prepontine / interpeduncular cistern (3%)
   Associated with: Aicardi syndrome, glutaric aciduria type I, unbalanced X,9 translocation
4. interhemispheric fissure, cerebral convexity, anterior infratentorial midline

• forward bowing of anterior wall of cranial fossa + elevation of sphenoid ridge
• extraaxial unilocular thin-walled CSF-density cyst with well-defined smooth angular margins
• compression of subarachnoid space + subjacent brain (minimal mass effect)
• may erode inner table of calvarium
• NO enhancement (intrathecal contrast penetrates into cyst on delayed scans)
• NO calcifications

MR (best modality):

• well-circumscribed extraaxial fluid collection / cyst isointense to CSF
• lack of complete signal suppression (on fluid-attenuated inversion-recovery sequence) ← proteinaceous content
• free water motion / facilitated diffusion similar to CSF on DWI
• thin cyst wall not visualized on MR

Cx:

1. hydrocephalus (30–60%) + remodeling / thinning of overlying occipital bone ← secondary obstruction of ventricular system ←mass effect on cerebellum + vermis ← enlargement during infancy
2. concurrent subdural / intracystic hemorrhage

Prognosis: favorable if removed before onset of irreversible brain damage

Rx: resection, fenestration, endoscopic ventriculocystostomy, cystoperitoneal shunting

CT-DDx:

• epidermoid cyst, dermoid, subdural hygroma, infarction, porencephaly

US-DDx:

• choroid plexus cyst, porencephalic cyst (communicates with ventricle), cystic tumor (solid components), midline cyst associated with agenesis of corpus callosum, dorsal cyst associated with holoprosencephaly, Dandy-Walker cyst (extension of 4^th ventricle, developmental delay), vein of Galen aneurysm