Original sentence: In American English, the phrase "I could care less" (without the "n't") is synonymous with this in casual usage.Intonation no longer reflects the originally sarcastic nature of this variant, which is not idiomatic in British English and might be interpreted as anything from nonsense to an indication that the speaker does care
Edited sentence: In American English, the phrase "I could care less" (without the "n't") is synonymous with this in casual usage (although this stems from misusage of the phrase and is not an intrinsic linguistic difference), while in British English, "I could care less" is not, and might be interpreted as anything from nonsense to an indication that the speaker does care