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Findings: High attenuation is seen diffusely within the sulci on a noncontrasted head CT. High attenuation collections are also present within the occipital horns of the lateral ventricles. Moderate hydrocephalus is present with dilatation of the temporal horns of the lateral ventricles as well as the third ventricle.
Diagnosis: Subarachnoid hemorrhage with a small amount of blood present in the occipital horns of the lateral ventricles. Moderate hydrocephalus is also present.
Discussion: The most common cause of subarachnoid hemorrhage is head trauma. With trauma, small subarachnoid vessels can be disrupted or direct extension of a contusion or hematoma into the subarachnoid space can occur resulting in a subarachnoid hemorrhage. Another common cause of subarachnoid hemorrhage is rupture of intracranial aneurysms. Other causes include secondary leakage of blood from a primary intraparenchymal hemorrhage, intracranial AVM, hemorrhagic tumor, dural AVM, spinal cord AVM, blood dyscrasias or bleeding diatheses, and complications of pregnancy.
On a CT examination of the head, subarachnoid hemorrhage appears as linear areas of high attenuation within the cisterns and sulci which differentiates it from a subdural hematoma which does not extend into the sulci. The dependent portions of the subarachnoid space, the interpeduncular fossa and the posterior occipital horns of the lateral ventricles, are the most sensitive places to look for subarachnoid hemorrhage because this is where gravity causes the blood to settle. With small amounts of blood, it is possible that the only finding on CT may be apparent effacement of the sulci. Other situations in which it is difficult to detect subarachnoid hemorrhage on CT include a low hematocrit and a delay in scanning. In these instances, a lumbar puncture revealing rbc’s or xanthochromia may be the only way to confirm a suspected subarachnoid hemorrhage. In most cases, acute subarachnoid hemorrhage cannot be seen on MR.
Patients with a subarachnoid hemorrhage may develop hydrocephalus as in this case due to impaired CSF resorption at the arachnoid villi. Intraventricular blood which is also present in this case results from retrograde flow of subarachnoid hemorrhage into the ventricular system through the fourth ventricle outflow foramina.
References:
Brant and Helms. Fundamentals of Diagnostic Radiology. 1994:46.
Grossman and Yousem. Neuroradiology. 1994:136-140.
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