The Ottawa Subarachnoid Hemorrhage Rule is a clinical calculator used in the field of emergency medicine to determine the likelihood of a subarachnoid hemorrhage in patients presenting with acute headache. This rule applies specifically to patients who are alert and over 15 years old with a new severe non-traumatic headache reaching maximum intensity within 1 hour. 

The clinical utility of this calculator is to help clinicians decide whether a lumbar puncture or computed tomography scan is necessary to rule out a subarachnoid hemorrhage. It is designed to be 100% sensitive for identifying subarachnoid hemorrhage, meaning that if a patient meets none of the criteria, the likelihood of them having a subarachnoid hemorrhage is extremely low.

Exclusion criteria for the use of the Ottawa Subarachnoid Hemorrhage Rule include patients with new neurological deficits, patients with a history of three or more headaches of the same character and intensity as their current headache over a period of six months, and patients with previous diagnoses of brain tumors or subarachnoid hemorrhage.

Ottawa Subarachnoid Hemorrhage Rule

# Criteria
 A. Age >= 40 (+1)
B. Neck pain or stiffness (+1)
C. Witnessed loss of consciousness (+1)
D. Onset during exertion (+1)
E. Thunderclap headache (instantly peaking pain) (+1)
F. Limited neck flexion on examination (+1)

# Conditions
- score == 0
- score >= 1