The genetics of ataxia: through the labyrinth of the Minotaur, looking for Ariadne's thread

Among the hereditary cerebellar ataxias (CAs), there are at least 36 different forms of autosomal dominant cerebellar ataxia (ADCAs), 20 autosomal recessive cerebellar ataxias (ARCAs), two X-linked ataxias, and several forms of ataxia associated with mitochondrial defects. Despite the steady increase in the number of newly discovered CA genes, patients, especially those with putative ARCAs, cannot yet be genotyped. Moreover, in daily clinical practice, ataxia may present as an isolated cerebellar syndrome or, more often, it is associated with a broad spectrum of neurological manifestations including pyramidal, extrapyramidal, sensory, and cognitive dysfunction. Furthermore, non-neurological symptoms may also coexist. A close integration between clinical records, neurophysiological, neuroradiological and, in some instances, biochemical findings will help physicians in the diagnostic work-up (including selection of the correct genetic tests) and may lead to timely therapy. Some inherited CAs are in fact potentially treatable, and the efficacy of the therapy is directly related to the severity of the cerebellar atrophy and to the time of onset of the disease. Most cases of CA are sporadic, and the diagnostic work-up remains a challenge. Detailed anamnesis and deep investigation of the family pedigree are usually enough to discriminate between acquired and genetic conditions. In the case of ADCA, molecular testing should be guided by taking into account the main associated symptoms. In sporadic cases, a multi-disciplinary approach is needed and should consider the following points: (1) onset and clinical course; (2) associated features; (3) neurophysiological parameters, with special attention to the occurrence of peripheral neuropathy; (4) neuroimaging results; and (5) laboratory findings. A late-onset sporadic ataxia, in which other possible causes have been excluded by following the proposed steps, might be attributable to metabolic disorders, which in some instances may be treatable. In this review, we will guide the reader through the labyrinth of CAs, and we propose a diagnostic flow chart.