The world lost one of it's great creative talents with the death of David Bowie this month. A musical genius, his appearance was striking due to his asymmetric eyes.
Bowie was punched in his left eye by a friend when he was 15 years old, in a fight over a girl. The trauma apparently damaged the iris sphincter muscle, which controls constriction of the pupil. This left him with a permanently dilated left pupil, giving the illusion of differently coloured eyes. Read more about Bowie's eyes here.
Unequal pupils, medically described as anisocoria, can be due to a number of different causes. Most commonly it is physiological, ie normal for a particular person, but it can signify a more serious cause.
When we assess anisocoria it is helpful to know how long the condition has existed for, and whether the size difference is greater in bright light or dim light. Anisocoria that is greater in bright light signifies failure of one pupil to constrict fully, whereas a greater difference in pupil sizes in dim light indicates a failure of an eye to dilate properly. If the level of anisocoria is similar in all light conditions, then the cause is probably physiological. Old photos can be useful in determining how longstanding the condition is.
As always with eye conditions, any sudden changes in pupil size should be assessed by a good optometrist or your GP fairly smartly.