The title, "In extremis," means at the point of death or in utter crisis, and the poem reads like a few seconds caught at that edge. Pain arrives, and the speaker pictures it as "a drowned light in the sand": something bright but submerged, glowing faintly under a surface that won't let it out. Then pain becomes "white" and "lace," an odd, delicate image for something that hurts. Lace suggests fine threads and holes, as if the body is being worked into a pattern, or worn thin.
The most unsettling moment is "I am itself." The grammar slips, and the speaker no longer says "I am in pain" but becomes the pain. What remains is "the cast of a person / I never saw before," like a mold or shell, or a stranger's outline. The mood is hushed, strange and detached. The speaker seems to watch their own self dissolve, with more bewilderment than panic.