At best, depending on your machine, the CPAP allows for a 3cm difference ("pressure support") between IPAP and EPAP. In MND, the recommended minimum difference is 4 and more often higher -- essentially, as part of the goal to keep a minimum EPAP so as to reduce the work of breathing muscles that get weak in MND. The absolute minimum of 3 or 4 may not work for your apnea at this point but as the MND progresses, it may. So you will always want to monitor that, and I'm happy to help. The point is, CPAP isn't set up to minimize EPAP. That's most often the reason that PALS move from CPAP to BiPAP (as my husband did as well).
However, with UMND, respiratory impairment is typically more mild than ALS and often comes later in the game. So I don't know what influences, if any, MND has had on your respiratory function as yet.
If it turns out that you are dealing with both MND and the pre-existing sleep disorder, then, depending on the type of apnea(s) you have -- central, mixed, obstructive, and in what ratio-- there is a BiPAP model that will allow for more tailored pressure support while supporting the EPAP or range that will address your apnea. A Trilogy or Astral in AVAPS-AE or the equivalent mode would as well, but depending on your FVC/MIP, those may not be reimbursable at this point.
Have you had recent PFTs? That would tell you more clearly where you are and what reimbursement you might be eligible for in re the BiPAP.
Best,
Laurie