To describe a Japanese patient with spinocerebellar ataxia type 27B（SCA27B）that was complicated by grip myotonia.

SCA27B is a newly identified, dominantly inherited, late-onset form of cerebellar ataxia. An intronic GAA repeat expansion in the FGF14 gene located on Chromosome 13 causes SCA27B.

An 80-year-old man with no family history of ataxia or parents' consanguineous marriage complained of ataxic gait at the time of his first visit to our department. His neurological examination revealed cerebellar ataxia, sensory neuropathy, and vestibular dysfunction. A chronic cough was not observed. The patient had repeatedly experienced transient dysarthria, and he was aware that opening his hands was not easy, which he had withheld for the past seven years. His difficulty in opening his hands gradually worsened. When he was 82, we noticed contracture of his hand muscles and grip myotonia. Needle electromyogram showed very short running myotonic discharge-like potential on flexor carpi ulnaris and dorsal interosseous muscles. The exercise test and repeated stimulation test were normal. We initially suspected cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS), but genetic analysis showed no evidence of biallelic repeat expansion in the intron region of the RFC1 gene. However, we found GAA repeat expansion (229 repeats) in the FGF14 gene and diagnosed the patient with SCA27B. A literature review showed that SCA27B may have a phenotype similar to CANVAS, but no cases of myotonia were reported.

Although this is a single case report, it is clinically significant for the following three reasons: 1) SCA27B is also found in Japanese patients; 2) SCA27B can exhibit a CANVAS-like clinical presentation; and 3) myotonia may be a novel clinical sign of SCA27B.