Definition of stages
| Stage 1 | Functional involvement of one CNS region (symptom onset) |
| Stage 2 | Functional involvement of two CNS regions |
| Stage 3 | Functional involvement of three CNS regions |
| Stage 4 | Need for gastrostomy (4A) or non-invasive ventilation (4B) |
| Stage 5 | Death |
CNS, central nervous system.