Objective: We present two cases of late-onset progressive and episodic cerebellar ataxia with prolonged courses of complex diagnostic work-up and management prior to genetic Spinocerebellar ataxia 27B (SCA27B) diagnosis.

Background: SCA27B is a recently identified autosomal-dominant cause of late-onset cerebellar ataxia (LOCA) due to (GAA)_>250 intronic repeat expansion of fibroblast growth factor 14¹. Prevalence estimates suggest SCA27B accounts for 10-60% of undiagnosed LOCA. Age of onset ranges from 50-70s with slowly progressive pancerebellar syndrome including prominent gait ataxia and cerebellar oculomotor abnormalities (notably downbeat nystagmus). Up to half of patients report episodes of vertigo, visual disturbances, or dysarthria.^{2, 3}

Method: Case 1 is a woman who presented with unsteadiness and falls at age 80. She also experienced progressive dysphagia, dysarthria, impaired hand dexterity, and wheelchair dependence. She had episodes of leg weakness, diplopia, or dysarthria several times a day. Family history was negative. At age 85, exam revealed saccadic pursuit, dysarthria, clumsy rapid alternating movements, dysmetria on finger to nose, and overshoot on finger chase.

Case 2 is a man who presented at age 50 with 1-minute episodes of dysarthria. Over 10 years he developed progressive imbalance and daily, multi-hour episodes of diplopia and dysarthria. Triggers included exercise, heat, stress, reading, alcohol, and caffeine. Family history included a mother with ataxia in 70s, maternal grandfather with Parkinson’s disease, and sibling with tremor. At age 60, exam revealed downbeat nystagmus, thumb tremor, mild past-pointing on finger pursuit, and 3+ patellar reflexes.

Results: Case 1 and 2 received genetic diagnosis of SCA27B at 5 and 10 years after symptom onset, respectively.  Collectively they underwent broad work up; they received presumptive diagnoses including TIA, seizure, functional neurological disorder, multifactorial gait disorder, and autoimmune encephalitis before clinical course and genetic testing revealed the final diagnosis. Both cases started 4-aminopyridine; case 1 had improvement in dysarthria and case 2 had decrease in episode frequency/severity.

Conclusion: Due to late-onset and episodic symptoms, SCA27B may be easily mistaken for more prevalent conditions; clinical index of suspicion should be increased in cases of unexplained LOCA.

References: 1. Pellerin D, Danzi MC, Wilke C, et al. Deep Intronic FGF14 GAA Repeat Expansion in Late-Onset Cerebellar Ataxia. New England Journal of Medicine 2023;388(2):128-141.
2. Clément G, Puisieux S, Pellerin D, Brais B, Bonnet C, Renaud M. Spinocerebellar ataxia 27B (SCA27B), a frequent late-onset cerebellar ataxia. Revue Neurologique 2024;180(5):410-416.
3. Pellerin D, Danzi MC, Renaud M, et al. Spinocerebellar ataxia 27B: A novel, frequent and potentially treatable ataxia. Clinical and Translational Medicine 2024;14(1).

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MDS Abstracts - https://www.mdsabstracts.org/abstract/sca27b-an-elusive-diagnosis-with-effective-symptomatic-treatment-two-cases-of-late-onset-progressive-and-episodic-cerebellar-ataxia/