Paradoxical Hyperreactivity in Bilateral Carotid Stenosis as a Potential Mechanism for Convexity Subarachnoid Hemorrhage: A Case Report.

While internal carotid artery (ICA) stenosis is typically linked to ischemic stroke, some patients may present with convexity subarachnoid haemorrhage (cSAH). Usually, carotid stenosis leads to decreased vasoreactivity due to chronic hypoperfusion; an exhausted reactivity has been proposed as a causative mechanism for cSAH in these patients. We present a case of cSAH with paradoxically increased vasoreactivity. A 65-year-old male presented with a headache and was diagnosed with bilateral cSAH and severe bilateral ICA stenosis (right near-occlusion, 85% left). MRI showed recent small asymptomatic ischemic lesions in the left hemisphere. Pre-stenting transcranial Doppler (TCD) revealed marked asymmetry in cerebral flow velocities (left > right) and a blunted waveform on the right MCA. Vasoreactivity testing demonstrated bilateral hyperreactivity (breath-holding index (BHI) >2.5). The patient underwent left ICA stenting, resulting in restored symmetry of flow velocities and normalisation of vasoreactivity indices. This case highlights the complex interplay between cerebral autoregulation, vasoreactivity, and stroke and cSAH risk in carotid disease. The normalisation of vasoreactivity post-stenting suggests that pre-existing hyperreactivity was a compensatory response to chronic hypoperfusion. Hyperreactivity may have favoured the cSAH. Vasoreactivity testing provided valuable insights into cerebrovascular compensation and treatment response.