Arachnoid cyst

A left parietal Arachnoid cyst with typical imaging findings though location is uncommon.
An extra axial cyst.
Uni locular, no septations.
Clear cyst isodense to Csf.
An associated hypoplastic adjacent left parietal lobe.
Scalloping of inner table of bony calvarium.
Not communicating with ventricular system.
Adjacent brain parenchyma show normal attenuation. No Gliosis in surrounding brain parenchyma.
No mass effect or volume loss.
Enhancement not expected on post contrast.

Typical locations of Arachnoid cyst includes Middle cranial fossa and Cp angles.
Case 1: A right middle cranial fossa Arachnoid cyst with an associated hypoplastic right temporal lobe.

Case 2: A left side Cp angle Arachnoid cyst with an associated hypoplastic left cerebellar hemisphere.

On MRI;
Arachnoid cysts have typical imaging findings as in these cases seen as a sharply demarcated cyst.
Extra axial, does not communicate with ventricular system.
Isointense to Csf on all pulse sequences.
Complete suppression of fluid signal on FLAIR.
No restricted diffusion on Dw images.
No septations or loculations.
No calcification on T2*GRE.
No Gliosis in adjacent brain parenchyma. No volume loss.
An associated hypoplastic adjacent temporal lobe or cerebellar hemisphere.
May show an associated mild scalloping of inner table of adjacent bony calvarium.
Usually no mass if any is mild, decently displaces adjacent vessels and nerves.
Non enhancing on post contrast.

Needs to be differentiated from its closest DDs;
1. Epidermoid cyst which typically shows restricted diffusion MRI Diffusion. Mass effect on adjacent parenchyma instead of hypoplasia, insinuates in cisternal spaces, encases nerves and vessels implies to its plasticity. Signal suppression on MRI Flair may not be as complete as arachnoid cyst.
2. Porencephalic cyst show Gliosis in adjacent brain parenchyma and volume loss like ex vacuo dilatation of adjacent ventricle. May communicate with ventricular system. It’s a cortical area of Csf signals involves cortical grey white mater where as arachnoid is an extra axial lesion.
3. Neuroglial cyst is intra axial, rare, commonly seen in frontal lobes.

Arachnoid cyst MRI

A 32 y o male with left hemi facial pain for last 1 year. 

Here is his MRI Brain Axial FLAIR and Diffusion.

This MRI study of Brain shows a sharply demarcated left Cp angle cystic lesion. 

Extra axial, not communicating with ventricular system.
Isointense to csf, complete suppression of fluid signal on FLAIR. No septations or loculations.
No restricted diffusion on Dw images.
An associated hypoplastic left cerebellar hemisphere. No gliosis in adjacent brain parenchyma. No volume loss.
Mass effect is mild, decently displaces adjacent vessels and nerves without encasement. Left trigeminal nerve is stretched may be the cause of left hemi facial pain. Left 7-8th CN complex mildly displaced. No significant mass effect fourth ventricle to cause obstructive hydrocephalus.
Non enhancing on post contrast.

Imaging diagnosis : Left Cp angle Arachnoid Cyst.

Needs to be differentiated from its closest DDs;
1. Epidermoid cyst which typically shows restricted diffusion. Mass effect on adjacent parenchyma instead of hypoplasia, insinuates in cisternal spaces, encases nerves and vessels implies to its plasticity. Signal suppression on FLAIR may not be as complete as arachnoid cyst.
2. Porencephalic cyst show Gliosis in adjacent brain parenchyma and volume loss like ex vacuo dilatation of adjacent ventricle. May communicate with ventricular system. It’s a cortical area of Csf signals involves cortical grey white mater where as arachnoid is an extra axial lesion.
3. Neuroglial cyst is intra axial, rare, commonly seen in frontal lobes.

Related post :
Arachnoid-cyst parietal region - uncommon for site.
Epidermoid-vs-arachnoid-cyst

ARACHNOID CYST

AC belongs to a category of primary non neoplastic intracranial Cysts.
An intra arachnoid CSF-filled sac that does not communicate with ventricular system.
ACs contributes to ~ 1% of all intracranial masses and as incidental finding on imaging for seizure in ~ 2 % of cases.

Imaging wise diagnostic clue is sharply demarcated round/ovoid extra-axial cyst that follows CSF attenuation on CT or signal on all MRI pulse sequences. No restricted diffusion on DWI (vs Epidermoid cyst)

Most common location is middle cranial fossa (~50-60%), Cerebellopontine angle (~ 10%), Suprasellar arachnoid cyst (~10%), Cerebral convexity, quadrigeminal cistern (~10%)

Imaging findings
NECT: Hypodense with density same as that of Csf.
Calcification is not a feature.
Non enhancing on post contrast.
Hyperdense if intracyst hemorrhage present and is rare.
MRI
Iso intense to Csf on T1, T2w and FLAIR images. Complete signal suppression of FLAIR.
T2* GRE: No blooming unless hemorrhage present and is rare.
DWI, no restriction
T1 C+, doesn't enhance
MRA, cortical vessels displaced away from calvarium.

Associated abnormalities are scalloping of adjacent bony calvarium.
Hypoplastic adjacent part of brain for example temporal pole in case of middle cranial fossa ACs and Cerebellar hemisphere in cases of Posterior fossa ACs.

Pathogenesis 
AC is usually sporadic, non-syndromic, rarely familial.
Frontal, temporal embryonic meninges (endomeninx) fail to merge as sylvian fissure forms, remain separate, forming duplicated arachnoid. Proposed mechanism behind development of cyst are active fluid secretion by cyst wall, Slow distention by CSF pulsations, CSF accumulates by ball-valve mechanism.
Microscopic Features are wall consists of flattened but normal arachnoid cells. No inflammation, neoplastic cells.

Clinical presentation
Often asymptomatic and found incidentally
If symptomatic, symptoms vary with size, location of cyst. Headache, dizziness, sensorineural hearing loss, hemifacial spasm/tic.

ACs can be found at any age, 3-5 times more common in male.
Usually don't enlarge. If at all grows very slowly.

Treatment
Often requires no treatment.
May need endoscopic resection, Fenestration, Cystoperitoneal shunt.

Reference : Diagnostic imaging Osborn.

Epidermoid cyst MRI

A 35 yo female with left side Trigeminal Neuralgia clinically.
Here is her MRI Brain Axial T1 T2 FLAIR and Diffusion. 

This MRI study of Brain shows:
A lobulated extra axial left Cp angle cystic space occupying lesion.
Signals hypo intense on T1, hyper intense T2w images, hypo intense on Flair but signal suppression of fluid is not complete, not as clear as csf.
Restricted diffusion on Dw images. 

Cyst is insinuating in cisternal spaces. Significant mass effect, adjacent left cerebellar hemisphere, brain stem and fourth ventricle are compressed, left side trigeminal nerve stretched.
No obvious parenchymal invasion.
Contrast enhanced T1w images not available.

Imaging diagnosis : left Cp angle Epidermoid cyst.

Closest differential is Arachnoid cyst which shows complete signal suppression of fluid on FLAIR, as clear as csf, no restricted diffusion on Dw images, an associated hypoplasia of adjacent brain parenchyma, mass effect if any is mild and not significant as in this case.

Left retro mastoid craniotomy perfomed. Epidermoid cyst confirmed by intraoperative and histopathological findings.


EPIDERMOID CYST

Belongs to a category of primary non neoplastic intracranial Cysts.

Intracranial epidermoids are congenital inclusion cysts, arise from ectodermal inclusions during neural tube closure. Acquired develops as a result of trauma and is very very rare.

EC contributes to 0.2-1.8% of all primary intracranial tumors.
4-9 times more common than dermoid.
EC is a most common congenital intracranial tumor and is third most common CPA/lAC mass, after vestibular schwannoma, meningioma.
Associated abnormalities may be occipital / naso frontal dermal sinus tract.

EC is a lobulated, irregular, "cauliflower-like" cystic mass with "fronds", insinuates cisterns and encases nerves/vessels. Restricted diffusion on MRI DWI is a diagnostic clue.

Location
A. Intradural, in ~ 90% of cases, primarily in basal cisterns.
M/c is Cerebellopontine angle (40-50%), Fourth ventricle (~17%), Para sellar/middle cranial fossa (~10-15%), Rarely in cerebral hemispheres (~1.5%), Brain stem (rare), Intraventricular within tela choroidea of temporal horn, 3rd, or 4th ventricles.
B. Extradural, in ~ 10% , intradiploic to bony calvarium, spine.

Imaging findings
NECT
Often a round / lobulated mass, hypodense in more than 90% of cases, resembling a focal CSF density. In 10-25% an associated calcification noted.
"Dense" epidermoid – a rare variant secondary to hemorrhage, high protein, saponification of cyst debris to calcium soaps or iron-containing pigment.
Contrast enhancement is none. Very minimal enhancement may be present at its surface.
Intradiplioc epidermoid shows bony erosion with sharply corticated margins.

MRI
Signal on TlWI often slightly hyperintense to CSF.
Uncommonly hyperintense to brain ("white epidermoid") due to high triglycerides & unsaturated fatty acids
Uncommonly hypointense to CSF ("black epidermoid") due to presence of solid crystal cholesterol & keratin and lack of triglycerides & unsaturated fatty acids.
On T2WI, often isointense to slightly hyperintense to CSF.
Very rarely hypointense due to calcification, hydration, viscous secretions, iron pigments.
On FLAIR, hypo intense but slightly hyper intense than Csf due to incomplete suppression of fluid signals.
DWI, restricted diffusion is pathognomonic.
Post contrast Tl, usually none. Margin of cyst may show minimal enhancement.
MRS: lactate peaks.

Histopathology 
EC grows by progressive desquamation with conversion to keratin/cholesterol crystals.
Gross features are outer surface often has shiny, glistening "mother of pearl" appearance ("beautiful tumor"), Cyst filled with soft, waxy, or flaky material. Lobulated excrescences. Soft and pliable, conforms to shape of adjacent local structures/spaces. Insinuating growth pattern (extends through cisterns, surrounds and encases vessels/nerves.
Microscopic Features are Cyst wall shows internal layer of simple stratified cuboidal squamous epithelium covered by fibrous capsule and contents is solid crystalline cholesterol, keratinaceous debris. No dermal appendages.

Clinical Presentation
Age of presentation is usually between 20-60 y with peak at 40; no gender preponderance.
Symptoms depend on location, growth pattern.
Headache is most common.
Cp angle lesion present with 5, 7,8 CN neuropathy.
Cerebellar signs.
Increased intracranial pressure rare due to 4th ventricular compression.
Less commonly hypopituitarism, diabetes insipidus.
Seizures if in Sylvian fissure/temporal lobe.
May remain clinically silent. Grows slowly.
Chemical meningitis possible from content leakage.
Rare malignant degeneration into squamous cell carcinoma (SCCa) reported.

Treatment: 
Often surgical.
Complete resection often complicated by investment of local structures
Recurrence common if incompletely removed.
Subarachnoid dissemination of contents may occur during operative/postoperative course may cause chemical meningitis.
Rare malignant degeneration of resection bed into Sq Cell Ca reported, may occur years after surgical resection.

Reference: Diagnostic imaging Osborn.