Secondary progressive multiple sclerosis is a relentlessly progressive, usually ascending, disease process. Once secondary progression begins, regardless of how long the disease was present before secondary progression began, patients appear to progress at a uniform rate. Recent studies show that it responds poorly to medications effective in relapsing remitting disease, although these drugs decrease relapses and have a substantial effect on lesions seen on magnetic resonance imaging. Disruption of axonal transport is known to occur in demyelinated fibers. Synapses, vacated when axons are destroyed, cause sprouting in surviving terminal axons, resulting in metabolic overload in the terminal axons. This noninflammatory process would not be expected to be altered by current disease-altering therapies.
Herndon RM. Medical HypothesisWhy Secondary Progressive Multiple Sclerosis Is a Relentlessly Progressive Illness. Arch Neurol. 2002;59(2):301-304. doi:10.1001/archneur.59.2.301