ok, i've made a few references to this in general threads.. but posting it here is where you put up or shut up so to speak..

Sasuke, since 'gaining' his MS, has used it a few times and is apparently already starting to feel the effects of the 'losing your sight' aspect of the MS. before I get into my theory, lets review the known facts about MS/EMS and how Sasuke's different path to each

FACT (at least up until Sasuke): the upgrade from 3 tomoe sharingan to MS ends up with a MS pattern with 3 distinctive sides.. Sasuke's MS - 6 sided..

FACT (up until Sasuke.. and to an extent kakashi. but we've never learned how kakashi was able to activate his MS): to activate the MS, you had to kill your best friend. Sasuke has, if we really want to be overly technical, only killed Itachi (and to an extent, even that is questionable). he didnt kill Orochimaru, he allowed him to start the soul transfer ritual, then simply overpowered him in his own dimension with his willpower. Oro was effectively trapped in Sasuke's body with no say in what was going on (at least until Sasuke totally ran out of chakra fighting Itachi). and the MS techniques he does have (the ones we've seen him use at least) were transferred to him by Itachi right before he died. its possible that Itachi's were just the base dojutsu that all uchiha MS get upon activation, but thats a theory for another day

FACT (again, up until Sasuke): the six sided EMS was gained by removing your brothers MS activated eyes and implanting them in your own head. Sasuke's MS is already 6 sided..

FACT (up until sasuke again, though we havent seen Madara activate his beyond normal 3 tomoed sharingan yet as a comaprison): when a person activates the MS (at least when thier eye is open to see it) the tomoes from the sharingan 'evolve' into thier MS pattern. Sasuke has never has his eyes open during the evolution process on the MS that we know of

ok, heres where my theory comes in..

1)unless Itachi was Sasuke's true best friend and not Naruto, he shouldn't be able to activate MS at all. let alone have a 6 sided EMS style pattern

2) Then there's the MS pattern itself. Kishi has gone to at least some effort to make every other MS at least look like the next step in sharingan evolution, yet makes Sasuke's MS 6 sided. there has to be plot hole no jutsu explanation somewhere down the line

3) the EMS pattern we have seen (along with being 6 sided) is the melding of the 2 MS patterns used to create it. Sasuke's natural 6 sided MS seems to have no compatibility with Itachi's 'normal' 3 sided MS pattern. this is the part where I am the most confused about, and trying to figure it out is making my head hurt.. so i'll leave this be for now

4) if Sasuke's MS is really an EMS, he hasn't met the requirement for THAT either (though he has itachi's eyes handy should that prove needed). and as a result, his eyes are burning out at least twice as fast as they would if it were a 'normal' MS. Assuming Itachi didnt totally burn out his eyes during thier fight (which is entirely possible), I suppose he can at least fill that requirement

I've probably left holes in this big enough to put the hokage's mansion in, but im doing this in total off the top of my head mode using as many actual facts as possible, and its 1 in the morning as im writing it

I hope its an enjoyable read, and lok forward to the more in the know types here blowing it to shreds or actually supporting it