Prepared by John F. Healy, MD, of VA Medical Center San Diego
and University of California, San Diego; and Steven Eilenberg, MD
of Tri-City Radiology, Oceanside, CA.
An elderly woman was noted to have a declining mental status,
mimicking dementia, over a period of several weeks. The patient was
physically feeble, but no focal neurologic signs or symptoms were
noted. Noncontrast computed tomography (CT) of the brain was
obtained (figure 1). Three findings on the CT were noted. An MR
scan was requested to clarify these findings (figure 2).
Subdural hematoma involving congenital arachnoid cyst
Noncontrast CT exam (figure 1A) revealed slight asymmetry of
cerebral sulci over the left convexity suggesting the possibility
of an intra-axial lesion such as a subacute infarct or an
intracranial mass, or an extra-axial isodense subdural hematoma.
There was no vasogenic edema or midline shift, making an
intracranial mass lesion less likely. A more caudal image (figure
1B) revealed bony asymmetry of the sphenoid wings with the left
sphenoid wing smoothly expanded, similar to that usually seen with
arachnoid cysts of the middle cranial fossa. However, there was no
cerebrospinal fluid density.There was also no anterior left
temporal horn, suggesting that it is either compressed by mass
effect or does not exist because of congenital absence of the
anterior temporal lobe.
MR examination clearly showed a left subacute subdural hematoma
and hemorrhagic arachnoid cyst (figure 2). The classic geometric
configuration of an arachnoid cyst was appreciated best on the
axial T2-weighted images (figure 2B), but the expected
cerebrospinal fluid intensity of an arachnoid cyst was absent.
Blood products averaged with cerebrospinal fluid within the
arachnoid cyst yielded an intermediate signal intensity.
Methemoglobin producing T1 shortening was seen within the cyst
and subdural space on the T2-weighted images (figure 2A).
CT and MR findings in chronic subdural hematomas have been well
The elderly or individuals with brain atrophy are more prone to
develop subdural hematomas because the stretching of cortical veins
between atrophic brain and the dural sinuses make these veins more
prone to tear with trivial head trauma. Because of the atrophy,
however, a significant quantity of blood may accumulate in the
enlarged subdural space without becoming symptomatic. Chronic
subdural hematomas in the elderly also may be asymptomatic because
the patients have "extra space" inside the cranium resulting from
cerebral atrophy. The patient was found at surgery to have a
chronic subdural hematoma which may have decompressed itself into a
pre-existing arachnoid cyst. Conversely, the cyst itself may have
bled, decompressing into the subdural space. Rebleeding from
subdural membranes may increase the size of the hematoma and
eventually cause symptoms of a space-occupying lesion.
Bank et al
have described subarachnoid hemorrhage into arachnoid cysts as a
potential pitfall in both CT and MR interpretation. Subdural
hematoma and intracystic hemorrhage are both known complications of
arachnoid cyst. Cases of subdural hematoma decompression into an
arachnoid cyst are less common, but have also been documented.
This elderly individual had symptoms of dementia, but the added
space available for the subdural hematoma to decompress into the
arachnoid cyst may have delayed the onset of more definitive
neurologic symptoms. The diagnosis of an isodense subdural hematoma
is difficult on this noncontrast CT scan, but should be suspected
because of the sulcal asymmetry. The bony expansion of the sphenoid
wing is classic for arachnoid cyst. Because no CSF density lesion
is noted, however, this finding could easily be overlooked. A clue
to this finding is the non-visualizaton of the left temporal horn.
The absence of the left temporal horn should also be noted by the
astute observer. However, it is often more difficult to notice
missing anatomy than to notice altered anatomy. The missing
temporal horn in this case may be due to congenital absence of the
anterior temporal lobe or, less likely, may result from compression
effect of the arachnoid cyst by the mass.
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