That will likely be your next step, then. What they've got right now are some rule-outs they need to make. From here, they will want to confirm a diagnosis through further testing. X-rays or MRI will give you an idea if there is a visible mass on the brain. Alternatively, you could look for SARDS instead by going to a veterinary opthamologist to have an electroretinogram (which is like an EEG for the eye).
If they are able to confirm the diagnosis of SARDS, they may still wish to do the x-rays and/or MRI, since a positive diagnosis for SARDS does not rule out the possibility of an abnormality to the brain. The diagnosis is more one to describe the condition, and no one is quite sure yet what causes it. So, they will likely want to see if a brain lesion or some other process is connected to the blindness.
If it does turn out to be a mass or tumor on the brain, they will want to do a biopsy to determine the type of tumor. Once they have diagnosed what type of lesion it is, they will take steps to reduce or remove it, based on location. They can possibly remove it during surgery, and if it is malignant, they may discuss chemotherapy and radiation as an option.
It is absolutely possible that SARDS can exist without a mass or brain lesion, and they have had several cases that were simply spontaneous for no known cause, and other cases that were linked to diseases such as Cushing's disease (which is an adrenal gland disorder). So, while the behavior may appear to be neurological in nature (such as the stumbling, and circling), they will need to do further tests to determine if it's actually related to the brain or if it has some other root cause.
Does that help to answer your question?