Most people who have HNPP have heard of the tomacula, sausage shaped swellings, which are found along the nerve fibers of those who have HNPP (and a few other neuropathies). Indeed at one time HNPP was often called Tomaculous neuropathy.
But just how big are these sausage shaped swelling. Typical sausage size? No, not at all, they can only be seen by microscopic inspection of individual nerve fibers, and are typically approx. 100 microns long (1/10 of a millimetre) and between 10-20 microns wide (about 1/100 – 1/50 of a millimetre).
Why they cause so much trouble is that the nodes of ranvier, which are gaps in the myelin sheath which allow nerve impulses to jump from one node to another, are usually around 1 millimetre apart. A typical tomacula is somewhere between 1/10 and 1/8 of this distance, and thus causes disruption of the nerve impulse, by elongation of the node of ranvier and /or shortening of the compact myelin between the nodes. (This might not be 100% correct, but it’s the best I can do at this time in the morning)