Neuronal intranuclear inclusions: Case report of an unusual cause of rapidly progressive dementia

Fragile X-associated tremor/ataxia syndrome (FXTAS) and neuronal intranuclear inclusion disease (NIID) are caused by CGG repeat expansions in the FMR1 and NOTCH2NLC genes, respectively. They have similar clinical, radiological, and pathological features. Neither typically presents as rapidly progressive dementia (RPD). Here we report a case that highlights that FXTAS and NIID should be considered in the differential for RPD. A 62-year-old Caucasian man with diabetes and obstructive sleep apnea presented to our movement disorders clinic with a chief complaint of cognitive impairment and falls. His family confirmed he worked full time until late 2022 as a closing agent without any concern for his work performance. In late 2022, he developed decreased stamina, tremors, and urinary incontinence. In early 2023, he started falling and was referred to physical therapy. By summer of 2023, he had clear cognitive impairment with an incident of getting lost while driving in his own neighborhood. His family reported fluctuating cognition but unchanged personality. He was now having trouble keeping up at work. Before he could be seen in neurology clinic, he was hospitalized in October 2023 with COVID-19, at which time he was also started on levodopa, which improved his tremor. In November 2023, he went on medical leave from work due to his rapid cognitive decline. He required round-the-clock assistance from his family to complete activities of daily living. There was no history of REM behavior disorder or hallucinations.