【Reading notes】Arachnoid cyst

Arachnoid cyst definition: an arachnoid cyst filled with cerebrospinal fluid that does not communicate
with the ventricular system.

Etiology (middle cranial fossa arachnoid cyst as an example): 1.
When the lateral fissure is formed during the embryonic stage, the inner mater of the frontotemporal lobe fails to confluence; 2.
Keep separated from each other to form a "repeating" arachnoid.

Possible mechanisms: 1.
Active fluid secretion of the cyst wall; 2.
Slow expansion caused by cerebrospinal fluid pulsation; 3.
Cerebrospinal fluid flows and converges
in one direction (ball valve).
Complications of cerebrospinal fluid shunting with the formation of arachnoid cysts are rare
.

Usually asymptomatic, mostly found by chance, symptoms vary with the size and location of the cyst, headache dizziness, neurological deafness, hemilateral muscle spasm, suprasellar arachnoid cyst can cause obstructive hydrocephalus
.

General imaging features: well-defined, round or oval intracranial extraparenchymal cystic lesions consistent with CSF density
/signal.
The sites of occurrence are in order of probability: middle cranial fossa, pontocerebellar angle, occipital pond, suprasella, and other parts
.
The lesions range from several millimeters to large, with clear and sharp boundaries, and more intracranial extracranial mass features
.

CT findings: non-contrast scan usually shows cerebrospinal fluid density, rare cases of hemorrhage in the capsule, which can be manifested as different degrees of high density, adjacent skull elongation, thinning, remodeling, adjacent brain parenchyma compression and shifting; The lesion is not strengthened after enhancement, and ciciliography can show communication
between the lesion and the subarachnoid space.

MRI findings: T1, T2, FLAIR are manifested as intracranial extracerebral fluid accumulation, consistent with cerebrospinal fluid signal, clear boundaries, T2* no "flowering sign" (except when combined with hemorrhage), DWI is not diffuse limited, no strengthening after enhancement, MRA can show cortical vascular displacement away from the skull, MRV can show abnormal venous return (in the case of large cysts).

Middle fossa arachnoid cyst Galassi classification: with an increase in size and mass effect, communication with the basal cistern decreases
.
Type I: small, spindle-shaped, confined to the anterior part of the middle cranial fossa; Type II: extends upward along the lateral fissure, temporal lobe shifted; Type III: huge, filling the entire middle fossa, frontal/temporal/parietal lobe shifted
.

Differential diagnosis:

1.
Epidermoid cyst: the edge is fan-shaped, hidden growth, spreading in the cerebrospinal fluid pool, wrapping blood vessels and nerves (arachnoid cyst displaces blood vessels and nerves, but does not wrap), FLAIR imaging is not inhibited, and DWI diffusion is limited as a highlight signal
.

2.
Chronic subdural hematoma: the signal is different from cerebrospinal fluid, often in the shape of bilateral lenses, some cases may have meningeal strengthening, gradient echo imaging can see "flowering signs"
in most cases.

3.
Subdural hydroma: usually crescent-shaped or flat, bilateral onset is more common, signal or density is similar
to cerebrospinal fluid.

4.
Brain penetrating cyst: surrounding brain tissue with glial hyperplasia can be seen around, no cortical compression, common in patients with a history of brain trauma and stroke, and a small number of cases are congenital abnormalities
.

5.
Neural tube gastral cyst: relatively rare, the spine and posterior cranial fossa are the most common sites, and the fluid in the capsule is usually protein-based, so CT non-contrast scan is higher than cerebrospinal fluid, and the T1 sequence of MRI examination can show high-signal changes
.