SCA is a heritable disease. The certainty of a diagnosis would need to look at your family history and DNA testing to figure this out. Are there any other members of your family that have or had SCA? Have they done DNA testing? If the answer to both of those is yes, then it is more likely than not that you have SCA if they found positive results in the DNA test.
ALS diagnosis is not as cut and dried. 1 in 10 (+/-) of us are familial and inherited, and 90% are sporadic cases. With ALS diagnosis we have a 40% misdiagnosis rate, or 4 out of 10 are told that we have something else before we are given a correct diagnosis. The likelihood that you would be given a diagnosis of SCA, and then ALS, would be rare I think because SCA should be confirmed by DNA. Genetics may just crossover too, but that would be such a rare event.
On the other hand, if you have no relatives with SCA, it could be a misdiagnosis. The key to this may be your vision, ability to learn and remember would be affected in SCA. Our eyes and minds are usually not affected this way in ALS (ALS-FTD will affect some mental abilities but not so much vision issues). Not being an MD, I consider all info that I can find when talking about this kind of thing because it is a consideration that needs to be made by patients looking at serious conditions like these. ALWAYS talk to the specialists that you are working with because they have a duty to keep you informed.
If they say what it is NOT, the next question you ask about should be what it IS.
