A postscript, hopefully helpful. The text in question is the note to line 149 (on page 137 ff in my old Dutton edition), which details first the geography of the Zembla peninsula, bifurcated by the Bera Range but connected by "two asphalted highways." The peninsula is also said to be "cut off basally by an impassable canal from the mainland of madness", suggesting that the geography is actually a metaphor for what is described by neuroscientists as "brain architecture."
It would be interesting to have a neurologist read this note and get his/her reaction!
The Karlists' escape route follows one of the highways, but is blocked "when a confused blaze in the darkness before them, at the intersection of the old and new highways, revealed a road-block that at least had the merit of canceling both routes at one stroke."
The word stroke of course is the give-away. When Shade has his last one in his bath, he is shaving, stroke after stroke. Aunt Maud died of a stroke, but I can't locate it in the book just now.
Another clue is also found on page 137, the note to "line 143: a clockwork toy. By a stroke of luck, I have seen it!" The toy turns up again in the poem's last two lines: "Some neighbor's gardener, I guess --goes by/Trundling an empty barrow up the lane."
Carolyn