The common abnormalities of intracranial hypotension or CSF volume depletion, such as subdural fluid collections, meningeal enhancement, engorgement of venous structures, pituitary hyperemia and sagging of the brain [2], are based on the Monro–Kellie hypothesis, which indicates that the sum of the volumes of the brain, CSF, and intracranial blood is constant [3]. However, the clinical features of SIH are more variable, and the relationship between SAH and SIH is increasingly being investigated.
SIH may mimic SAH in clinical symptoms, radiological features and CSF findings. Approximately 14% of SIH patients suffered from thunderclap headache [4], CSF xanthochromia could also be observed in SIH [5, 6], and increased attenuation on CT was interpreted as a pseudo-SAH [7]. These abovementioned manifestations are easy to misdiagnose as SAH.
We presented two cases of clearly diagnosed non-aneurysmal SAH with clinical features of nuchal rigidity and meningeal irritation signs, the CT scans showed diffuse SAH, and lumber puncture revealed bloody CSF with low intracranial pressure, which is an uncommon complication secondary to SIH. In our cases, the duration between SAH and SIH was short, and bloody CSF was obtained immediately after SAH, implicating a closer relationship between SAH and SIH.
In the literature, there are 5 other cases in four studies of SAH secondary to SIH (Tab. 1) [8-11]. Tomokiyo et al reported the first case of SAH associated with SIH in Japan [8]. The patient complained of an orthostatic headache in January 2001 and presented with a sudden onset of severe nuchal pain 4 months later (on May 19th). On May 28th, a lumber puncture was performed, showing xanthochromic CSF with hypotension. Although the SAH appeared in the setting of SIH, the slightly prolonged duration and xanthochromic CSF are not sufficient evidence. Two other patients were accompanied by cortical venous thrombosis (CVT) [9, 10]. Schievink et al reported two cases of unresponsive patients with a long history of CSF leak diagnosed with SAH [11].
We speculate that the main reason for SAH secondary to SIH stems from an intravenous source for the following reasons: the patients’ clinical performance did not have serious consequences, the CSF pressure was still below the normal range for SAH, the CT scans demonstrated a relatively low bleeding volume and no aneurysms were observed in the cerebral vascular angiographies, which is not very similar to the signs of artery-related SAH. Some articles speculated that as the CSF volume decreases, the intracranial venous structures dilate, and the venous blood flow velocity slows down, resulting in venous thrombosis being limited to a single cortical vein. Thus, SAH was induced by damage to the vessel wall due to blood and pressure accumulating backwards in the subarachnoid segment of the cortical vein [9, 10]. Even though venous thrombosis was not found in our two cases, we speculate that SIH might mechanically lead to congestive intracranial veins, which in turn rupture the cortical veins, basilar plexus or bridging veins, finally resulting in SAH.
In conclusion, special attention should be paid to orthostatic headache in patients with SAH-like CT scans. Moreover, we should be more vigilant about noting SIH patients’ clinical features, such as a new persistent severe headache or a suddenly changed state of consciousness during a therapeutic procedure.