Pearls & Oy-sters: ATX-FGF14 Mimicking Autoimmune Pathology.

ATX-FGF14 (formerly spinocerebellar ataxia 27, OMIM #193003) is an autosomal dominant condition caused by a pathogenic variant in the fibroblast growth factor 14 ( FGF14 , OMIM #601515) gene located on chromosome 13. The phenotypic expression can vary in patients with the same genotype, often delaying diagnosis, especially in probands without known affected relatives and/or with limited available family history. We describe two cases of ATX-FGF14 in one family with a focus on the importance of differentiating episodic manifestations of neurogenetic conditions from inflammatory/autoimmune neurologic conditions. A 68-year-old male patient (Case 1) presented with episodic dysarthria, dizziness, imbalance, and encephalopathy, creating suspicion for a possible autoimmune etiology. At first evaluation, the patient reported no significant family history. Four years later, upon revisiting the family history, he noted that his 49-year-old niece (Case 2) had also developed neurologic symptoms of an unclear etiology. On evaluation, she had tremor and ataxia. Both patients also had coexistent evidence of systemic autoimmunity that likely contributed to the initial suspicion of neurologic autoimmunity, and neither had cerebellar or brainstem volume loss. Ultimately, their genetic testing revealed a pathogenic structural variant in the FGF14 gene, consistent with ATX-FGF14. These two cases highlight the importance of a detailed interval family history at each visit, especially in undiagnosed adult patients, as well as the importance of objectively analyzing the impact of immunotherapy diagnostic treatment trials to avoid unnecessary immunomodulatory medications.