Downbeat nystagmus as an initial clinical sign in spinocerebellar ataxia type 6

Dear Editor, Spinocerebellar ataxia type 6 (SCA6) is characterized clinically by progressive cerebellar ataxia with cerebellar atrophy, dysarthria, and oculomotor disturbance, and genetically by CAG repeat expansion in the α1A-voltage-dependent calcium channel gene (CACNA1A) on chromosome 19p13 [1]. SCA6 accounts for 15% of all cases of SCAs worldwide [1]. Downbeat nystagmus (DBN) is defined as a vertical spontaneous nystagmus, marked by downward fast phases [2], and is frequently observed in SCA6 [3]. However, the presence of DBN as an initial clinical sign for SCA6 is unknown [3, 4], particularly as accurate SCA6 diagnosis is difficult in isolated DBN cases. Herein, we report an SCA6 patient with DBN as an initial clinical sign, who lacked all other clinically distinguishing signs and symptoms of SCA and who had no family history of SCA. A 71-year-old woman visited our hospital because of positionalor motion-induced dizziness followed by continuous dizziness over 2 months. Her medical history was hypertension, dyslipidemia, and Graves’ disease at 2 years prior, but no cancer. She did not drink alcohol. She had been taking nifedipine, fluvastatin, and thiamazole for 2 years. She had no family history of SCA. Initial neurological examination revealed isolated DBN, but no cerebellar ataxia and dysarthria. The DBN amplitude increased during lateral gaze. Smooth pursuit and saccade were normal. She had no other cranial nerve disturbance, sensory disturbance, or limb weakness, and her deep tendon reflex and gait were normal. As she had only isolatedDBNwithout cerebellar ataxia and no family history of SCA, we initially considered her illness as cerebellar tumor, Arnold–Chiari malformation, cerebrovascular disease, paraneoplastic neurologic syndromes, or Wernicke’s encephalopathy. Laboratory studies showed normal vitamin B1, B12, and glucose. Serum electrolytes, renal and liver function, and complete blood count were normal. Carcinoembryonic antigen, alpha-fetoprotein, pro-gastrin-releasing peptide, and thyroid-stimulating hormone were within normal limits. No onconeural antibodies (anti-glutamic acid decarboxylase, Zic4, titin, SOX1, recoverin, Tr, Hu, Yo, Ri, Ma2/Ta, CV2, amphiphysin) were detected in serum. Chest radiograph andwhole-body computed tomography scans were unremarkable. No cerebellar or brainstem atrophy was observed on magnetic resonance imaging (MRI) (Fig. 1a– c). Therefore, initial differential diagnosis was excluded on the basis of laboratory and imaging studies. As the etiology of DBN was undetermined, we tentatively diagnosed the patient as idiopathic DBN and started clonazepam, betahistine, difenidol, and ifenprodil therapy. However, she visited the hospital irregularly, as these treatments failed to improve her symptoms. Three years after the initial visit, neurological examination revealed DBN in the sitting and supine positions (Online Resource 1). The DBN amplitude increased in lateral gaze and decreased in downgaze. Smooth pursuit was slightly saccadic in all directions, while saccade was normal. The horizontal vestibulo-ocular reflex was impaired. Cancelation of the vestibulo-ocular reflex was also impaired (Online Resource 1). Furthermore, the patient had a mild wide-based gait, and she was unable to perform tandem stand with eye open and stand on one foot. Nevertheless, limb Electronic supplementary material The online version of this article (doi:10.1007/s10072-017-2973-y) contains supplementary material, which is available to authorized users.