In addition to what follows in this post, please see my post The Imaging of Idiopathic Intracranial Hypertension And Of Spontaneous Intracranial Hypotension Posted on March 2, 2017.
This post contains excerpts from Headache Narrative & Rating Table – American College of Radiology [Full Text PDF] 1996 Last Review Date: 2013.
And each table variant (there are 16 variants) covers the differential diagnosis of a different variant (scenario) for a headache that can be serious.
Be sure to read carefully the literature review and summary on pp 10 to 22 of the pdf above. It is also outstanding.