How does the Mangekyo Sharingan blind its wielder, and why are some wielders not blinded? A disadvantage of gaining the Mangekyo Sharingan is that it leads to blindness over time. 


Source: Chapter 385, Mangashare/Binktopia scanlation
We have seen Madara, Itachi and Sasuke, all wielders of the Mangekyo Sharingan, use Susanoo. All three have gone blind or have worsened eyesight at some point*. Itachi and Sasuke have also been shown using Amaterasu which makes the eyes bleed afterwards. 

Danzo Shimura, Obito Uchiha and Kakashi Hatake, also wielders of the Mangekyo Sharingan, on the other hand, have never used any of the above techniques and have never shown any sign of blindness*. Why didn't they go blind or have worsened eyesight? 

What causes the blindness effect of the Mangekyo Sharingan? How does the Mangekyo Sharingan eventually blind its wielder?



*Please correct me (or suggest an edit) if I'm wrong.
Also, please feel free to suggest edits adding what chapters the events mentioned above happened, so question answerers can review it.
An original Mangekyo Sharingan puts a strain on the eye. The ocular muscles are placed under a constant strain due to the unique chakra the Mangekyo consumes. That strain causes the user to gradually go blind.

Another person's eye doesn't (for some reason) produce the same strain. So the user's vision is intact.

Let's review the list:


Itachi - Awakened his Mangekyo on his own, going blind.
Sasuke - Awakened his Mangekyo on his own, gone almost completely blind, then took Itachi's eyes, and got his vision back.
Madara - Same as Sasuke.
Kakashi - Remember that the eye didn't originally belong to him. His Mangekyo is more similar to an Eternal than a normal one.
Obito - Presumably, it's because Obito has a high concentration of Hashirama Senju's cells. These constantly heal the eye, preventing it from going blind.
Danzo - Same as Kakashi, the eye didn't belong to him. Although using Sishui's eye did put a huge stress on his chakra and body.