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Objective: Neuronal Intranuclear Inclusion Disease (NIID) is a neurodegenerative disease affecting the central and peripheral nerves. We aimed to assess the pathophysiological features of peripheral nerve dysfunction in NIID., Methods: We observed six unrelated NIID patients through clinical records, nerve conduction studies, and multiple measures of motor nerve excitability. Additionally, we reviewed one NIID patientt who underwent a nerve biopsy. Control measures were obtained from 22 age-matched normal subjects., Results: The NIID patients exhibited mild conduction slowing and distinct nerve excitability abnormalities, including a significant decrease in excitability through hyperpolarizing threshold electrotonus (TE) and increased overshoots in both depolarizing and hyperpolarizing conditions. Histopathology revealed thinly myelinated fibers and axonal degeneration. Mathematical modeling suggested that reduced leak conductance was the key factor contributing to the observed excitability changes., Conclusions: The findings indicate that NIID involves a complex interplay of axonal degeneration and myelin dysfunction, leading to unique peripheral nerve excitability changes. These results provide new insights into the pathophysiology of NIID., Significance: Nerve excitability testing offers insight into particular axonal excitability abnormalities especially combined with histopathologic studies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.)