While unilateral chronic isodense subdural hematomas as a result of indirect signs of a space-occupying lesion are easily recognizable on computed tomography (CT), bilateral isodense chronic subdural hematomas may cause considerable difficulty.
In two cases with CT false negative findings we observed, retrospectively, significant small cellae mediae and also the main part of the anterior horns sharply pointed and approaching one another. Three further cases showed the same ventricular configuration, which we called “hare's ears sign”. This sign together with clinical data is always suspicious of chronic bilateral isodense subdural hematomas and carotid angiography is indicated. Other possible signs are: subtle midline shift if the size of the hematoma varies, changed formation of density of brain tissue, non-appearance of cerebral sulci especially in elderly patients, and eventually the visualization of a membrane after intrevenous injection of contrast material 1).
A case was a 43-year-old woman who suffered from a sudden onset of headache and nausea. She had no past history of head injury. CT scans on admission did not clearly reveal the Sylvian fissures and the mesencephalic cistern, without any mass effects. A lumbar puncture demonstrated xanthochromic cerebrospinal fluid (CSF), which was considered to be responsible for her headache. Cerebral angiography performed on day 4 failed to demonstrate any cerebral vascular disorders. Follow-up CT scans on day 7 demonstrated a high density lesion in the left subdural space. Magnetic resonance images (MRIs) confirmed a diagnosis of bilateral chronic subdural hematomas. Removal of the hematomas cleared all signs and symptoms smoothly.
The second case was a 44-year-old man who was referred from another hospital because of xanthochromic CSF found by lumbar puncture. He began to suffer headache and be subject to vomiting 6 weeks earlier and these symptoms were still present on the day of admission. CT scans did not clearly show the cerebral cisterns without mass effects. Because the second lumbar puncture showed xanthochromic CSF again, SAH from aneurysm was suspected. However, emergency cerebral angiography failed to demonstrate cerebral aneurysms. MRI performed two days later demonstrated bilateral chronic subdural hematomas. Following surgery, the patient improved immediately and was discharged from hospital without any complications. In both cases, a retrospective study of the angiograms revealed the evidence of bilateral avascular areas over the convexities in the venous phase. The reason why these subdural hematomas were missed at the time of angiography was due to too much attention being paid to the arterial phase in an effort aimed only at identifying cerebral aneurysms. There are no reports of chronic subdural hematoma which demonstrated sudden onset of headache associated with xanthochromic CSF 3)