Sprue is celiac, and/or gluten intolerance.

Sjorgrens is an autoimmune disease more common in women, but may rarely occur in males. It can be tested for, but there are seronegative people out there with it.

21 patients is not a very large cohort.

Notice the mean age was 50...most of the diseases listed occur in that age group. PNs typically start in middle age.

Be careful reading online studies, if you do not have the background to interpret them.

Diabetics, or those poisoned by toxins may show PN signs earlier. You can get PN from food poisoning. Toxic well water.
Inhalants like paint strippers, or pesticides in the garden or on the farm.

You can receive contaminated recreational drugs, including MJ. (pesticides)

In fact there are so many causes, some doctors don't even look for them. You can have thyroid problems. You can have drug reactions...statins, fluroquinolone antibiotics, etc.

In fact now that this is vaccine season, you can get PN from vaccines.