1. Department of Neurology; St Vincent's University Hospital; Dublin Ireland
2. School of Medicine and Medical Science; University College Dublin; Belfield Dublin 4 Ireland
3. MRC Centre for Neuromuscular Diseases; UCL Institute of Neurology, National Hospital for Neurology; Queen Square London UK
4. Department of Neurology; Cork University Hospital; Cork Ireland
5. Department of Statistics; Trinity College Dublin; Ireland