Hi and welcome to our MS community,
Q: "Based on the MRI, could this be MS or do I need to keep looking for answers?"
A: the type of information from your brain MRI is not specifically suggestive or consistent towards anything, which basically means it's 'non specific' which wouldn't put a neurological condition like MS at the top of your causation list BUT nor does your 'brain' MRI completely rule out MS either!
Unilateral muscle weakness is a neurological abnormal clinical sign that indicates a nervous system malfunction but not which specific causation and the cause might be with the spine and not brain so IF you haven't as yet had a spinal MRI, as well as blood, peripheral nerve tests etc etc it is far to early in your diagnostic work up to point towards or away from anything yet.
In regards to symptoms worsening in the heat and remitting in the cold, which IF it is similar as the heat intolerance associated with MS, keep in mind MS is not the only cause and you still need to factor in the possibility of inadvertently overlooking the simple things eg like your hydration levels, sleep quality, how physically active you are/were etc if it's only during exposure to hot sunny weather or any type of heat ie, cooking, baths, showers, exercising, menstrual cycle, intercourse, drying your hair, electric blankets, partners body heat, infections and fevers etc etc etc
Sometimes when symptoms seem to get worse in heat and remit in cold, heat gets focused on as what makes it worse but we totally miss all the other behavioural factors which could also be causing symptoms to get worse and then remit with cold temperatures....
eg it's a very hot summer, too many hot days and nights in a row and being out in the heat is taking it's toll on everyone but regardless of the weather you still have to work a full day getting the new stock sorted, pick up the kids from school and daycare, pick up the groceries and get everyone home in time for dinner before you can finally get off your feet and out of the heat. You've been sweating and running on empty all day focused on everyone else's needs but not your own and your body is playing up because of the heat and it won't stop until you cool down in the air-conditioning.....the muscle fatigue symptom's getting worse in this senario is less likely due to heat intolerance and more likely because of the amount of physical activity is too much, and the symptoms are remitting in the cold due to resting those fatigued muscles so in this case thinking the worsening in the heat and remitting in the cold would be overlooking all the other possible factors. (hope that wasn't too wishy washy an example and it made some sense)
You've only got a few weeks before you'll see the neurologist and you'll likely need further tests before you get any idea what could be causing this, try to keep an open mind it just might not be a neurological condition at all and it will likely take a bit of time to work it out.
Hope that helps..........JJ
Unfortunately it's not uncommon to be diagnosed with more than one autoimmune condition, symptoms often overlap which makes it harder to diagnose and treat accurately, there are a few MS mimics that i am aware of that are connected with Small Fiber Neuropathy Dysfunction (with autonomic) eg Celiac, Lupus, Sjögren and Sarcoidosis but i don't know much about them sorry and there are most likely more than i'm aware of.
Lupus is said to be the greatest mimic and it can be very hard to dx too because the blood work doesn't always pick it up so off the top of my head symptom wise that could be another direction to get looked into with a specialist.
I don't want to give you too much and send you down the garden path so to speak but structural and spinal cord related conditions would have to be included in your potential causes. I would suspect that if you've only been dx with Small Fiber Neuropathy Dysfunction (with autonomic) there is definitely more to it and it's only one component of what it is you're dealing with.
I know it's frustrating not knowing and not treating but if you can try not to let it get too ingrained, the answers typical do get discovered but sometime we just don't have a label that matches everything, so stay strong, stay positive and don't be a stranger!
Let us know how you get on.......JJ