Imaging description

Pseudo-subarachnoid hemorrhage (pseudo-SAH) refers to increased attenuation within the basal cisterns and subarachnoid spaces that mimics subarachnoid hemorrhage (SAH), but has a different etiology. The causes of pseudo-SAH include diffuse cerebral edema, meningitis, and intrathecal contrast [1].

Diffuse cerebral edema is the most common cause of pseudo-SAH. Cerebral edema leads to decreased attenuation of the brain parenchyma. There is also compression of the dural venous sinuses, which may lead to venous congestion and engorgement of the superficial veins. The combination of decreased brain attenuation and venous engorgement is postulated to be the etiology of pseudo-SAH in the setting of cerebral edema (Figure 4.1) [2].

The measured attenuation of the subarachnoid spaces will be lower than that seen with true SAH. Venous engorgement will demonstrate attenuation coefficients of 30–42HU. SAH will demonstrate higher attenuation. Therefore, if accurate measurements can be made, the distinction of pseudo-SAH from true SAH can be made in the setting of cerebral edema [3]. When cerebral edema is caused by a hypoxic event, there may be loss of the gray–white matter differentiation, especially involving the basal ganglia (Figure 4.2).

Exudative meningitis leads to increased protein content within the subarachnoid space. This may rarely produce a pattern of pseudo-SAH [4]. Similar findings may be seen along the pachymeninges (Figure 4.3).