| Summary: | Late-onset progressive cerebellar ataxia is a diagnostic challenge because of a poor correlation between genotype and phenotype, and a broad range of secondary causes that extend beyond the neurological field. We report the case of a 45-year-old woman admitted after 2 years of slowly progressing cerebellar ataxia, dysarthria, and emotional instability. Notably, she was diagnosed with diabetes insipidus at the age of 35. As ‘idiopathic cerebellar ataxia’ was suspected, diagnostic tests, including genetic testing as well as serum and cerebrospinal fluid analyses, and brain magnetic resonance imaging (MRI) were performed. All results were normal except those of MRI, performed 9 months prior to admission, which showed multiple dot-like white matter lesions with unclear cause. On a repeated brain MRI, a new lesion presenting as a 1.5-cm-sized highly enhancing mass attached to the right frontal skull was found. A sharply marginated lytic skull defect was also evident on skull X-ray, which corresponded to the lesion mass. Given these new radiological findings, a systemic review of the patient’s medical history for rare secondary causes of cerebellar ataxia was performed, with particular attention to her past ‘diabetes insipidus’. The mass, lytic lesion of the skull, white matter lesion, diabetes insipidus, and cerebellar ataxia all suggested a final diagnosis of Langerhans cell histiocytosis (LCH), which was confirmed histopathologically. This is a rare case of late-onset LCH with an unusual initial symptom which underlines the importance of carefully reviewing the patient’s medical history and broadening the search for etiologies beyond the nervous system.
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