Comment on

• The natural history of multiple sclerosis: a geographically based study. 5. The clinical features and natural history of primary progressive multiple sclerosis.

  Cottrell DA, Kremenchutzky M, Rice GP, Koopman WJ, Hader W, Baskerville J, Ebers GC. Cottrell DA, et al. Brain. 1999 Apr;122 ( Pt 4):625-39. doi: 10.1093/brain/122.4.625. Brain. 1999. PMID: 10219776
• The natural history of multiple sclerosis: a geographically based study. 6. Applications to planning and interpretation of clinical therapeutic trials in primary progressive multiple sclerosis.

  Cottrell DA, Kremenchutzky M, Rice GP, Hader W, Baskerville J, Ebers GC. Cottrell DA, et al. Brain. 1999 Apr;122 ( Pt 4):641-7. doi: 10.1093/brain/122.4.641. Brain. 1999. PMID: 10219777