A 43-year-old male physician with a history of mild hypertension
and a 20-year history of migraine headaches presented with a
headache that seemed to him to be different in character and much
more intense than usual. Medications included atenolol 50 mg QD and
ibuprofen PRN. Laboratory data and physical examination were
unremarkable. An MRI of the brain was performed (figures 1 and
The sagittal TI-weighted MR image (figure 1) demonstrates a
low-signal intensity, homo-geneous, extra-axial collection in the
left frontal region with mild mass effect on the adjacent brain
(arrows). The axial T2-weighted MR image (figure 2) shows the
lesion as homogeneous high-signal intensity. The signal intensity
of the lesion exactly follows that of CSF on all pulse sequences.
Also seen on both images is characteristic bony remodeling of the
inner table of the overlying calvarium, indicating the extremely
slow growth of the lesion. No other abnormalities were noted. After
consulting a neurologist, the patient started propranolol 80 mg QD
and naproxen sodium PRN. Six months later, the patient reports no
True arachnoid cysts are the most common congenital cystic
lesions in the brain, accounting for approximately 1% of
intra-cranial mass lesions.
Secondary arachnoid cysts have been reported after trauma and
infection, forming when an inflammatory process causes
arachnoiditis and subsequent encystment of the subarachnoid space.
In most cases, however, the pathogenesis remains obscure.
About 50% of such lesions occur in the middle cranial fossa, the
most common location overall. There is an association with temporal
lobe hypoplasia when arachnoid cysts occur in this region, although
the exact relationship is controversial.
About one third occur in the posterior fossa, particularly the
retrocerebellar, cerebellopontine, and quad-rigeminal plate
cisterns. Posterior fossa cysts should be differentiated from a
mega cisterna magna and Dandy-Walker malformation. Approximately
10% of these lesions are found in the suprasellar region, where
they can mimic an enlarged third ventricle. An ependymal cyst
within the third ventricle could produce an identical image.
Occasionally, cysts are found in the perisellar region and over the
convexities. A large convexity arachnoid cyst can resemble a
chronic subdural hematoma. Rare intraventricular cysts have been
CT imaging features include a CSF density mass that effaces
adjacent sulci and remodels bone. Small cysts may not be seen due
to partial volume averaging, especially when located in the middle
cranial fossa, and expansion of the sylvian fissure or bony
scalloping may indicate the presence of a cyst.
On MRI, the arachnoid cyst appears as an extra-axial mass that
follows CSF intensity on all pulse sequences. Occasionally, the
signal intensity of an arachnoid cyst can be similar or identical
to that of an epidermoid tumor.
Smooth erosion of the inner table of the skull is seen on both CT
and MRI, which is thought to be the result of extremely slow growth
and transmitted CSF pulsations.
There is no enhancement. It is unclear if the arachnoid cyst in
this case contributed to the patient's symptoms, even though the
headaches improved with a change in medication. Arachnoid cysts are
usually regarded as incidental findings of no clinical
significance. Symptoms can occur when cysts enlarge and exert
sufficient mass effect to result in CSF obstruction and subsequent
hydrocephalus, a situation commonly seen in children with large
posterior fossa cysts.
Symptoms associated with arachnoid cysts include headache,
seizures, mental retardation, and cognitive impairment.
However, since these nonspecific symptoms represent the most common
indications for neurologic imaging, a causal relationship may not
exist. Therefore, it is understandable that therapy of headaches
and seizures in a patient with a concomitant arachnoid cyst is
Cysts in a suprasellar location are associated with symptoms
specific to their location, including endocrine abnormalities and
damage to the optic chiasm. Sudden expansion of cysts has been
reported and can result in hydrocephalus and symptoms of elevated
Our patient had no prior imaging studies that would have been
helpful to confirm cyst chronicity without a significant recent
change in size. In addition to sudden expansion, other reported
complications include hemorrhage and secondary infection of an
arachnoid cyst in the setting of meningitis.
In the presence of acute hemorrhage, the original arachnoid cyst
may be difficult to detect, resulting in the appearance of an
unusual looking hemorrhage. Therefore, awareness of this
complication is important. Middle cranial fossa cysts have a much
higher association with hemorrhage than cysts in other
Indications for surgical intervention include hydrocephalus and
elevated intracranial pressure.
Surgery for asymptomatic cysts is controversial. Surgical options
include marsupialization, fenestration, and cystoperitoneal
Spontaneous rupture of arachnoid cysts with development of subdural
hygromas has been reported.
In addition, total disappearance of arachnoid cysts has been
reported after minor head trauma, presumably due to a tear in the
cyst membrane, leakage, and subsequent resorption of cyst fluid.
In conclusion, arachnoid cysts typically represent benign
incidental lesions with characteristic imaging features.
Occasionally, they can become symptomatic and require surgical
intervention. Arachnoid cysts should be included in the
differential diagnosis of cystic extra-axial lesions of the
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Prepared by Glenn G. Gray, MD and Lewis Rothman, MD,
Department of Radiology, Lenox Hill Hospital, New York, NY.