what do you mean, RMD purposes?
An RMD is the minimum amount you must withdraw after age 72-1/2, based on your life expectancy and the account balance. There are no exceptions to the requirement to withdraw at least the minimum amount. If you miss the December 31 deadline, you can take the RMD as soon as you remember and ask the IRS to waive the penalty. If you are asking, "Can I ask for a waiver because I forgot my RMD because I was distracted by illness", then yes, you can use your diagnosis to request a waiver. Most waivers are granted as long as you have any reasonable sounding explanation.
Do you mean the 10% penalty for early withdrawal before age 59-1/2? Disability is a reason to be exempt from the penalty, but for the IRS, disabled means unable to perform gainful work. Many people with medically recognized disabilities are able to work (blind, amputee, etc.). So the question really should be, are you unable to work because of your diagnosis, and that inability to work is expected to last at least one year.
