If and when you are diagnosed with an important illness such as MS or FND your first reaction will probably be to assume that you will need to stop almost all physical activity immediately, but this is rarely the case and very often you will be able to continue cycling - or in some cases start cycling! - after your diagnosis.
For those unfamiliar wth the terms, MS is Multiple Sclerosis and FND is Functional Neurological Disorder. Both of these illnesses can have a very wide rnge of symptoms, and a very wide rate range in the speed at which they progress, but will often include weakness in the limbs, balance problems, fatigue and pain among their most important symptoms and which can affect your ability to ride a bike.
Without going into medical details here, MS can sometimes stop progressing and typically (in the most common type of MS) has times when the symptoms are much less severe, and FND can sometimes be cured altogether, so please don't abandon hope as soon as you receive a diagnosis.
Perhaps the most important thing to remember is that although you are likely to fatigue more quickly - often a great deal more quickly than before - if you have one of these illnesses, the general consensus is that exercise and sport will never make these illnesses worse and will typically bring significant health benefits, so if at all possible you should try to continue. How easy this is will depend on your current health...
The most limiting medical symptom for sport is often fatigue, which with both MS and FND can be a severe fatigue that leaves you feeling unable to get out of a chair after even a very modest effort. Fatigue also has a secondary effect: it is very difficult to motivate yourself to get off the chair and go and do some sport if you already feel exhausted, even if you know it will do you good.
My own level of fatigue is less severe than for many people but still provides many challenges - usually after I have cycled about five kilometres I am fighting to keep my eyes open and to stop myself falling into a ditch for a welcome nap! Happily for me my own fatigue then passes for a while, before returning every few kilometres during my ride.*
* My own cycle rides are now typically 20 - 30 kilometres, before my illness they were 60 - 80 kilometres average. I have cycled for many years and I think it is probably easier to continue cycling after a diagnosis than to start if you have never cycled before.
Balance and stability problems are also symptoms that do not necessarily stop you from cycling. Personally I stumble for no reason all the time when I am at home, I limp due to left leg weakness and I have constant dizziness and poor balance so I often almost fall over, and I have difficulty walking very far because of these problems, but none of these has any effect at all on my cycling.
Quite remarkable really, but I have no balance or dizziness problems at all when I am on a bike ride. I have seen one of our neghbours cyling into the town centre with his walking stick clipped to the bike frame for when he arrives...
Among the other symptoms I metioned, limb weakness varies from person to person but at least in early cases of these illnesses will not usually be so severe as to stop you cycling, and pain levels and thresholds vary so much between individuals that it is not possible for any one person to comment on another persons level of suffering but will typically be treated with medication to keep it under control.
This raises another problem. Many medicines that are used to treat pain, sleep problems, fatigue and other symptoms of MS and FND have side effects that include making you sleepy or lowering your energy levels as well as causing weight gain.
None of these side effects are likely to fill you wth enthusiasm for cycling or any ther sporting activities, but I all can suggest is that you try to take the minimum amount of medicines possible for your own degree of ilness, and talk to your doctor or neurologist about alternatives if any medicine you are taking has a particular side effect that stops you remaining active.
The type of bike you should use depends on what you are used to. I still like to use a road bike because that is what I have always used but I think if I had never cycled before I would probably buy a more upright or cross-country / off-road bike to use sice the goal is no lonfer to go as far and fast as possible, but to have a good time, keep active and fit and get out in the fresh air.
I should also mention electric bikes. Like most 'enthusiastic' cyclists I have always been opposed to electric assistance when cycling, and I have still never used an e-bike myself, but I can now see that they are a good option if the alternative is not to go cycling at all because of fatigue, or if other health problems prevent you cycling, or if you live somewhere where the hills are just too long or steep for you to cycle up!
Certainly depending how my own illness progresses I can imagine the time when an e-bike will be the only option if I want to keep cycling. One wrd of caution though, e-bikes can have quite a small electric motor or a very powerful electrc motor. If your own electric bike has a 700 watt motor, I think you will probably be more or less riding a motorbike and getting very little exercise!
If you have any doubts at all about your own cycling abilty, or abilty to do any exercise at all, or you have other health conditions eg with your heart, you should talk to your doctor before heading off to cycle up the local mountains! I am happy to share my own thoughts and experiences but some symptoms of MS and FND or side effects from medicines you are taking might make cycling hazardous for both yourself and for other road users so please be careful!