HNPP is a hereditary demyelinating neuropathy. Most cases come from a 1.5 Mb deletion on chromosome 17p11.2 that removes one copy of PMP22. PMP22 is a Schwann cell protein that helps compact and stabilize peripheral myelin. With only half the normal amount, myelin becomes fragile and forms tomacula (sausage-like thickenings) with disorganized paranodes and shorter internodes. Potassium leak lowers conduction safety, so nerves block under pressures most people tolerate. That biology explains the