In researching something else, I learned that Imitrex may actually treat the headache associated with subarachnoid hemorrhage-- which is a bad thing, because you're still going to die.
Subarachnoid hemorrhage, the "worst headache of your life"-- comes on suddenly, lasts for hours (even days, yes, days), worse in light or with sounds, but not affected by movement-- is the result of an aneurysm (usually middle cerebral artery) popping. CT is positive in 95% of cases if taken early-- the longer time passes, the less sensitive CT becomes.
A case report of a woman whose headache improved with Imitrex (6mg SQ) but still ultimately died. The authors said this was the only case report they found, but in the same issue is another such case report (improved after 6mg SQ and died later that day) , and a year later some British guys reported three other migraine patients who came in with undiagnosed SAH and their headache got better after getting Imitrex. (Two got 6mg, the other got 3x100mg). They were correctly diagnosed only after they came back with headache and meningeal signs, and got CTs.
The editor of the first journal notes that sumatriptan is not "migraine-specific" and is effective in treating other head pains (such as viral meningitis, and, I discovered, orgasm headaches *.) The authors of the earlier SAH report hypothesize that since triptans block transmission at the trigeminal nucleus caudalis, any pain from the meninges should be blocked. (In bacterial meningitis, the pain relief may also be augmented by the 5HT1D and B agonism, which (in mice) reduces inflammation, decreases intracranial pressure, and reduces white blood cells in the CSF(!)) This may be only true in acute meningitis, as failure in two meningitis patients may have been the result of sensitization of the caudalis neurons (where triptans are supopsed to block input) and spontanueous activity. (So get your triptans early.)
The obvious message here, given the efficacy iin SAH with such low doses of Imitrex, is that one should not assume efficacy is diagnostic of a migraine. Triptans seem to be efficacious across a variety of trigeminal neuropathies; which, like everything else in medicine, is good and bad.
* Orgasm headache: apparently triptans can treat or prevent "orgasmic headaches." The funniest line is in the abstract of that paper: "In patients who chose to predict their sexual activity, short-term prophylaxis with oral triptans 30 min before sexual activity might be a therapeutic option.."