The goal of this observational study is to understand if angiographic cerebral vasospasm (CV) can be predicted by levels of nitric oxide (NO) consumption in patients with aneurysmal subarachnoid hemorrhage (SAH). To reach this goal, the investigators will compare NO consumption levels in SAH and non-SAH patients. NO consumption levels will be analyzed from samples of participants' cerebral spinal fluid (CSF).

After enrollment, the investigators will collect patients' demographics and most relevant clinical characteristics of the patients.

For patients with SAH, the investigators will collect an-1 ml aliquot of CSF from the drainage circuit daily, (using a sterile procedure that involves a valve of the closed system circuit located distally to the patient, before the collection container), for a period of 15 days following SAH. The samples will be stored at -80 degrees.

The investigators will perform daily Transcranial Doppler (TCD) monitoring to assess whether the patients have cerebral vasospasm. In case of suspected vasospasm, and in all comatose patients after 4 days from the bleeding, a CT angiography will be performed to define the angiographic CV.

For the control group, discarded 1 ml-aliquot of CSF will be collected at the time of needle insertion for spinal anesthesia.

At the end of the protocol, the study population will be divided in 3 groups:

• patients with SAH and vasospasm
• patients with SAH and no vasospasm
• control group

Inclusion Criteria:

• Adult
• patient with SAH and with an external ventricular drainage (EVD) placed within 24 hours after the onset of the hemorrhage

Exclusion Criteria:

• pregnancy
• pre-existing neurological comorbidities
• coagulation disorders
• anemia
• absence of a good cranial window to perform Transcranial Doppler (TDC)
• presence of >50% stenosis of carotid arteries confirmed by doppler ultrasound
• Technical problems on EVD that prevent CSF withdrawals

For the control group, we will invite to participate patients without neurological comorbidities undergoing spinal anesthesia in the operating room or with a spinal drain for major vascular surgery.

The cohort of patients with SAH will be selected from patients in the Intensive Care Unit at Gemelli Hospital in Rome, Italy, within 24 hours after the onset of the hemorrhage and with an external ventricular drain. The control group will be patients at Massachusetts General Hospital who are undergoing spinal anesthesia in the operating room, or those with a spinal drain for major vascular surgery.