病例报道：CT误诊为SAH的原因---低颅压

 头颅CT对于急性SAH敏感性和特异性很高，但是有些情况下头CT会误诊为SAH。比如：encephalopathy with brain swelling (hyponatremia,anoxic–ischemic and metabolic encephalopathy), pyogenic leptomeningitis, intrathecally administered contrast material, and leakage of high-dose intravenous contrast medium into the subarachnoid spaces.

下面的病例报道为一心肺复苏后患者的头颅CT：

在上述的病例中，产生假SAH的机制：
It has been speculated that the causal mechanism of such spurious findings is a combined effect of decreased CT attenuation of the brain parenchyma and distention of the superficial vessels as a consequence of elevated intracranial pressure associated with severe brain edema. However, no systematic investigation has examined pseudo-SAH in terms of its incidence,the difference from true SAH on CT, and its clinical significance.

The relative hypoattenuation of the parenchyma and displacement of the CSF simulates the appearance of subarachnoid blood.

下面的一个误诊为SAH的病例报道病因为低颅压：

作者分析，低颅压所致假SAH的机制为：In our patients, MRI showed dense and pachymeningeal enhancement corresponding to the area of increased attenuation of the tentorium cerebelli on CT, confirming that venous vascular dilatation, according also to Monro-Kellie hypothesis, can be the substrate for this CT finding.