I think it is primarily due to meds and not the neurological system in our bodies.
It may be that because of discomfort, all the aches and pains become so distracting that it makes it difficult to concentrate, as well.
But I can not see any correlation between neuropathy of the limbs and mental concentration. The neurons mis-firing to our brains from feet or hands, are not related to mental acuity. Drugs may affect it. Distraction, maybe at a subconscious level - constantly ignoring pain, may take over and make it difficult to read.
I find that I constantly fidget, unable to get comfortable and read the same paragraph a zillion times..