Synesthesia and Mitempfindung
Mitempfindung has been thought to be related to synesthesia in some individuals, specifically digit-color synesthesia. Digit-color synesthesia is the phenomenon in which affected individuals associate color with number counting; individuals are said to "count in colors."
Both synesthesia and mitempfindung develop in early childhood and are highly variably among individuals. Furthermore, synesthesia and mitempfindung are both unidirectional. A scratch on the trigger zone creates a referred sensation in a distant body part but not vice versa. Likewise, in synesthesia, the ability to "hear" a color does not mean that a color produces sound.
Read more about this topic: Referred Itch