--paraneoplastic syndromes are one cause of non-length dependent neuropathies/neuronopathies (though not the most common).

There are specific antibodies that savvy doctors test for to see if such syndromes are possibilities, as the neuropathy is thought to be caused by a cross-reaction of the nerves to the body's attempt to attack protein produced by the tumor--the most common is the anti-Hu test, but there's also the anti-Yo and anti-Ri. Take a look at:

http://neuromuscular.wustl.edu/nother/paraneo.htm

http://neuromuscular.wustl.edu/antibody/sneuron.html#hu

The tests for these specific antibodies are in addition to the tests for tumor markers.