Spinocerebellar ataxias 27B (SCA27B) is caused by an expansion of ≥ 250 GAA triplets in the FGF14 gene and accounts for 15% of cerebellar ataxias (around 500 patients in France). It is a late-onset form often presenting paroxysmal episodes of ataxia and/or diplopia. The disease progresses slowly, with an average increase of 0.10 points/year on the Friedreich's Ataxia Rating Scale (FARS) - Functional Staging and by 0.23 points/year on the Scale for the Assessment and Rating of Ataxia (SARA). To date, no treatment has been proven to be effective in these patients. Three open-label studies using 4-aminopyridine, have shown improvements in visual symptoms and gait in a total of 36 out of 44 patients, although these improvements were evaluated through diverse methodologies. In a subgroup of patients (n=7), administration of 4-aminopyridine resulted in a reduction in FARS - Functional Staging, ranging from 0.5 to 2 points. Notably, this beneficial effect rapidly disappearing in all patients stopping the drug. 4-aminopyridine, a potassium channel blocker, may involve restoration of cerebellar Purkinje cell rhythmic firing property, impaired with the loss of FGF14 function. Although these results appear very promising, the positive effect of 4-aminopyridine is reported only in restricted sample sizes and open-label experiences. Therefore, a robust clinical trial is necessary to provide the level of evidence required for a definitive conclusion on the benefit-risk of fampridine and before introducing the treatment into the regular patient clinical management. Hence, to confirm the beneficial effect of 4-aminopyridine treatment, this study will compare fampridine 10 mg bid (sustained-release form) to placebo during a 3-month treatment in a randomized, double-blind, multicenter, placebo-controlled study, on functional handicap in SCA27B cerebellar ataxia patients.