Arachnoid Cyst

FACTS

congenital aberrant splitting of arachnoid layers → accumulation of CSF

walls: flattened arachnoid cells creating a translucent thin membrane

HPI

universal ROS

ask about seizures - one of the rare reasons to operate

PHYSICAL EXAM

universal exam

extra focus on cranial neuropathies

IMAGING

Locations

40% middle fossa-Sylvian fissure

25% posterior fossa

other locations: suprasellar, quadrigeminal cistern, inhermispheric fissure, convexity

evaluate for cyst hemorrhage or associated subdural hemorrhage

DDx: epidermoid vs. arachnoid cysts

ㅤ	Arachnoid	Epidermoid
DWI	dark (like CSF)	bright
Displace vessels?	yes	No

A/P

Counseling parents who are often shocked that a large mass like this is just watched, especially when radiology reads talk about shift, mass effect, etc.

In general, candidates for surgical intervention: patients with clear symptoms of increased ICP that correlate with cyst expansion, such as progressive macrocephaly, papilledema, and progressive cranial neuropathy,

People (both parents and referring doctors) will be surprised the management is nothing for these, even when they're massive (> 5cm). You need to reassure them this is standard management.

"What caused this cyst?"

https://ispn.guide/hydrocephalus-and-other-anomalies-of-csf-circulation-in-children/arachnoid-cysts-of-the-head-and-spine-in-children-homepage/complications-of-therapies-for-arachnoid-cysts-of-the-head-and-spine-in-children

No unifying theory for origin: Many theories exist about the origin of arachnoid cysts, including infection, trauma, hydrocephalus, and metabolic derangements, but no unique explanation has been accepted (25).

"Will this cyst keep growing?" Rarely grow after 4 years of age

What are neurosurgical management paradigms for arachnoid cyst?

Useful to know this so you can explain that treatment is not straightforward and complication-laden. Conceptually it's basically like treating "hydro," you need to shunt the fluid whether intracranially (via fenestration into an open basal cistern) or extracranially (i.e. shunt).

Paradigm of treatment	Complications	Outcomes
Fenestration - You need to find a way to communicate the cyst with normal cerebrospinal fluid (CSF) pathways of the brain. You basically need to find an open cistern to drain the cyst permanently	• Hemiparesis • CN3 palsies • Hypothalamic injury • ↑ seizure frequency • Surgical Hematomas	• 60% success (lower in infants, up to 90% if no hydro/ventriculomegaly/macrocephaly) • 40-80% will eventually require a shunt
Shunting if fenestration fails or there is no open cistern to communicate the cyst to.	• infection • malfunction • overdrainage • (the usual)	• 55-100% of middle fossa cysts with hydro/ventriculomegaly will require permanent shunt • small cysts may become obliterated and not require functioning shunt anymore