Thanks Marc.  According to the French version of Wikipedia (look up l'algie vasculaire de la face), the cost of oxygen therapy is only reimbursable by the state health insurance organisation (l'assurance maladie) if it is prescribed by a neurologist, or a pain control specialist.  Instead, my neurologist prescribed Imiject (Sumatripan), which seems to be the preferred alternative here.  I didn't learn about oxygen therapy until I discovered this website, following the consultation with my neurologist, however I will ask about it if/when I am referred back to her.

Thanks BD.  I hope you enjoyed living in Verdun and weren't too troubled  by the ghosts.  It's interesting that you too experience memory loss.  I'm currently recovering from several days when the pain ramped up from it's usual 5/10 level to about 8/10.  Not as bad as the 9/10ers that I get from time-to-time, but I find being able to remember anything through the fog of pain virtually impossible.  In my case, I don't have any shadows to chase away, as I'm in constant pain, however I have got into the habit of drinking a can as soon as I get up, because I find it definitely helps reduce the congestion in my left nostril and also seems to reduce the pain in my head somewhat.

The good news is that when I saw my cardiologist on 20 October, following ECG and ultrasound scans, he agreed to allow me to take Verapamil and use Imiject.  Apparently his brother is a fellow sufferer (episodic, 4-year cycle), so unusually he was familiar with the condition.  Also, I've now an appointment for an MRI scan on 28 Ocotber.  So, today's my 3rd day on Verapamil (I'm still taking Gabapentin).  I haven't detected any change in the level of pain in my head (yet, ever hopeful). How long do I need to allow for it to take effect?  Thanks.

Thanks lorac.  So, on that basis I should expect a reduction in my pain by the weekend.  Roll on next week! 

The Verapamil certainly has slowed me down, but my cardiologist warned me it would take several days for my body to readjust.  He recommended that I take the Verapamil in the morning, rather than evening.

I hope you have told your doctor about doubling the dosage ...

The way I think of CH is that my cycles seem to be a bit like a sine wave, with peaks and troughs. 

Imagine a piece of paper with the horizontal X and vertical Y axes of a graph drawn on it.  With Y-axis having a scale 0-10, where zero represents being pain free and 10 represents suicidal pain.

Now, imagine a sheet of transparent plastic overlaying the graph with a sine wave drawn on it, with the wave moving left to right horizontally and the peaks and troughs swinging up and down, with an amplitude greater than the 0-10 scale on the Y-axis.

Sliding the film down the Y-axis so the peaks are level with 9 on the scale would cause the sine wave to straddle the X-axis.

That is how I imagine my CH when it was episodic. I didn't consider the periods between attacks to be troughs. They were below zero on the Y-axis, consequently I wasn't aware of them.

Now imagine a different sheet of transparent film with another sine wave drawn on it.  This time with the overall swing amplitude less than 9 on the scale.  Once again, overlay the film and slide it so the peaks of the wave line up with 9 on the scale, keeping the wave parallel with the X-axis.

I think this is what happened when my CH became chronic. The up and down swing (ie amplitude) reduced, so the troughs never drop to  zero.  Hence my pain never drops to zero on the pain threshold scale.  In my case the amplitude swings up and down between 2 and 9.  Consequently, it's the amplitude that changed, not necessarily the frequency.

Since my CH toggled from episodic to chronic, I've been keeping a daily record of my pain, on a 0-10 scale, which is easy enough to do. However, working out what the frequency over time is would be a bit more difficult and not something that I feel inclined to do while I'm in this level of pain.

Apologies for the use of some jargon here, but hopefully it all makes sense.

Thanks Linda.  Wow, that's a first!  Nobody has ever called me sir before.  As regards the link, thanks, but of course I was already aware of that.  You do realise, don't you, that I wasn't attempting to define, even redefine, Bob Kipple's 0-10 scale?  Rather, I was trying to explain what I think happened, in my case, when my CH changed from being episodic to chronic.  Sorry, I should have explained myself more clearly.

Update:  Am taking 480mg Verapamil & 2400mg Gabapentin per day, plus now have oxygen (2 large cylinders at home, plus small one in car).

I would like to thank everyone who has helped me, particularly those who made me aware of oxygen therapy.  This, above all else, has transformed my life.  It really works for me and I am most grateful.  Thank you.