Acquired cases start with an event: a stroke, a head injury, progressive blindness. Afterward, sounds may carry flashes or touch may carry color, in a person who had typical perception before.
The pattern differs from developmental synesthesia in revealing ways. Acquired pairings are often less specific, less stable, and more intrusive; developmental ones are precise and lifelong. Sensory loss cases especially suggest that deprived cortex gets recruited by neighboring senses, the same plasticity that sensory substitution devices exploit deliberately.
For researchers, acquired cases are natural experiments about how much crosstalk adult brains permit once inhibition changes.