My issues started with sicca syndrome.

I now have lip biopsy proven, seronegative sjogrens.

Depending on what material you read, sjogren's is blown off as dry eyes and mouth, or a systemic disease.

I would not rule out spondyloarthropathies coexisting with neuropathy. Sjogren's also comes with neuropathy. There is growing evidence that seronegative, biopsy +, Sjogrens is associated with neuropathy. Make sure they draw your 'complements.'

You may be seronegative, and depending on the doc, you will be told either you do have or do not have Sjogren's.

You can get a minor salivary gland biopsy. Mine is full of lymphocytes....It is such a mess, I don't think they have a focal score for it. I was shocked when my doc showed me the slide.

Sicca can come just from neuropathy which has affected the nerves that innervate those organs, however, you won't have the lymphocytic infiltration, which is inflammatory.

I would pursue this.

Also, it is probably possible that the CPAP is drying your mouth and nose....some mask leakage would dry the eyes too, or it can be pushing up air thru your nasolacrimal duct which terminates in the inner canthus of the eye in a little hole. Can they humidify the CPAP?