[Neuronal Intranuclear Inclusion Disease]

Neuronal intranuclear inclusion disease (NIID) is a gradually progressing neurodegenerative disease characterized by eosinophilic hyaline intranuclear inclusions in the central and peripheral nervous systems and in certain visceral organs. The number of reported cases of NIID has recently increased after the introduction of skin biopsy for the diagnosis of NIID. Patients with sporadic NIID usually present with dementia, elevation of overall protein levels in the cerebrospinal fluid, leukoencephalopathy on head magnetic resonance imaging (MRI), and high signal intensity in the corticomedurally junction on a diffusion-weighted (DW) imaging. Among patients with familial NIID with age at onset <40 years, muscle weakness is observed most frequently; these cases are classified in the "limb weakness group." Among patients with familial NIID with age at onset >40 years, dementia is the most prominent symptom; these cases are classified in the "dementia dominant group." We proposed a flowchart for diagnosis adult-onset NIID. The prevalence rate of adult-onset NIID may be higher than previously thought, and NIID may be underdiagnosed in the population. NIID must be considered during the differential diagnosis of leukoencephalopathy and neuropathy, for which this flowchart for adult-onset NIID diagnosis may prove beneficial.