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Polyneuropathy is an anatomic pattern of nerve damage most commonly seen in Diabetes mellitus.

To quote Robbin's Pathology,

In most polyneuropathies, axons are affected in a length dependent fashions leading to deficits that start in the feet and ascend with disease progression. The hands often start to show involvement by the time deficits extend to the level of the knee, resulting in a characteristic 'stocking and glove' distribution of sensory deficits

What is the reason behind longer neurons being preferentially affected?

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Polyneuropathy is actually not a very specific term, so the reasons will no doubt vary depending on which specific class you are talking about.

That said, here are two reasons that longer neurons are preferentially affected:

  1. Some polyneuropathies involve conduction problems that include "failures" where action potentials aren't propagated down the full length. It doesn't matter where along an axon the failure occurs, and longer axons therefore have more "chances" to fail.

  2. Axons are somewhat isolated from the soma where most proteins are created, and it is common in certain neuropathies for degradation to begin furthest from the soma, presumably because the neuron has less capability to deal with problems or repair damage in those areas far from the soma. Neurons with longer axons by definition will have some part of their axons that are further from the soma than neurons with shorter axons. This is the answer that is most relevant to your question, see for example this source that reviews the causes of diabetic neuropathy.

References:

Yagihashi, S., Mizukami, H., & Sugimoto, K. (2011). Mechanism of diabetic neuropathy: where are we now and where to go?. Journal of diabetes investigation, 2(1), 18-32.

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  • $\begingroup$ Thanks. Id like to add some more. Further reading the same chapter I came across another line. It says neuronopathies (which attack the soma of the neuron eg zoster and platinum) unlike axonopathies (which polyneuropathies mostly are) are equally likely to affect both proximal and distal parts of the body since here it's the cell body and not the axon to be damaged. So, this helps as a differentiating point bet the two $\endgroup$ – Polisetty Mar 2 '17 at 20:26
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    $\begingroup$ @Polisetty I don't think it is accurate to think of neuronopathies and polyneuropathies as independent groups; there is definitely a Venn diagram-like association: it is just as possible for a neuronopathy to be a polyneuropathy as not (zoster would be a good example of a neuronopathy that is not a polyneuropathy). $\endgroup$ – Bryan Krause♦ Mar 2 '17 at 21:25
  • $\begingroup$ Yup one is an anatomical classification while the other is a functional one... $\endgroup$ – Polisetty Mar 2 '17 at 21:26

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