Subdural hematomas, whose absorption values approximate those of adjacent brain, are not visualized in routine computed tomography.

While unilateral chronic isodense subdural hematomas as a result of indirect signs of a space-occupying lesion are easily recognizable on computed tomography (CT) and clearly diagnosed on the angiogram, bilateral chronic isodense subdural hematomas may cause considerable difficulty.

see Bilateral isodense chronic subdural hematoma

Two clues indicating the presence of such “isodense” subdural hematomas are:

(1) unilateral effacement of cerebral sulci on the convexities, and (2) midline shift or mass effect on the ventricles in the absence of abnormal areas of diminished or increased density in the brain. Nine cases were detected on pre- and postcontrast studies in 2,500 CT scans of the brain over a 10 month period. Delayed CT scanning 4-6 hr after intravenous contrast injection showed enhancement of the subdural hematoma in three of seven cases 1).

The time that a chronic subdural hematoma may become isodense on computed tomography (CT) varies from 1 week to 90 days after trauma.

In most instances, it appears isodense in 2-6 weeks.

There has been little mention of isodense subdural hematomas in the acute clinical setting. In patients with low hemoglobin levels, an acute subdural hematoma may be isodense from the outset.

Ten patients were studied in whom precontrast computed tomography (CT) revealed an isodense mass which obliterated cerebral sulci and deformed the ventricle(s). Upon rapid-high-dose (RHD) contrast enhancement, the cerebral hemispheric surface was so well demonstrated, that in 4 patients the possibility of an isodense subdural hematoma was exluded, while in the other 6 it was correctly diagnosed. Preoperative angiography was not necessary 2).

Amendola MA, Ostrum BJ. Diagnosis of isodense subdural hematomas by computed tomography. AJR Am J Roentgenol. 1977 Oct;129(4):693-7. PubMed PMID: 409245.
Hayman LA, Evans RA, Hinck VC. Rapid-high-dose contrast computed tomography of isodense subdural hematoma and cerebral swelling. Radiology. 1979 May;131(2):381-3. PubMed PMID: 441324.
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