Answer: This is a good question; the easy answer is that primary and secondary progressive are meant to be different disease types, but this may not be entirely true.
Years ago a bunch of MS experts sat around a table and came up with clinical definitions of MS disease types. The four types agreed upon were relapsing remitting, secondary progressive, primary progressive and progressive relapsing. The reason for defining disease types was as follows:
- Different MS researchers were using different definitions and names of disease types in their publications making it difficult for others to duplicate or even understand their results.
- No one knew how to separate MS disease types based on pathology, imaging or a blood test (we still don’t) but we could separate MS based on different clinical behaviors that had been observed for over 100 years. It was felt that this would be a good place to start for identifying causation and mechanism of disease.
So this clinical classification scheme was developed and applied to all clinical research studies to characterize patient populations.
- Relapsing remitting MS is the most common and occurs in about 85 % of patients at onset and during the early years of the disease. This type is characterized by abrupt onset of symptoms and worsening function (called a relapse or exacerbation) reaching it’s worst point within 1-2 weeks in most patients before spontaneously remitting (recovering), although recovery is often incomplete. This phase of the disease can occur once (a single relapse) or go on for 10-20 years. If people worsen during this phase of the disease the worsening is presumed to be due to incomplete recovery from a relapse, although after many years it is often hard to know if this is the entire cause of worsening or if the person is now becoming secondary progressive (see # 2).
- Secondary Progressive MS ALWAYS FOLLOWS A RELAPSING REMITTING COURSE, by definition. In other words you can not be classified as secondary progressive unless you were once relapsing remitting or experienced at least one relapse. Approximately 80 % of relapsing remitting patients become Secondary progressive within 25 years (average is 10-15 years). The rest (20%) remain relapsing or more commonly just stable after many years. By definition patients have either no relapses or rare relapses during this phase of the disease but they all show slow progression of problems not associated with the abrupt changes that characterize relapses. The worsening is usually measured over intervals of 1-2 years.
- Primary Progressive MS tends to involve older individuals and is marked by a slow progressive course from onset WITHOUT any relapses or exacerbations (except those caused by infections). The most common presentation is an individual with a slowly developing problem walking that takes 12 to 24 months for their doctors to figure out. This type of MS occurs in about 10-15 % of MS patients. Interestingly, this form tends to involve males and females evenly unlike relapsing MS which is more common in females. The average age of diagnosis is over 40 unlike the younger age of onset with relapsing remitting MS. In fact the age of onset of secondary progressive and primary progressive MS seems to be about the same. This and some other characteristics has led to the hypothesis that the mechanism of injury (and therefore treatment) is the same for primary and secondary progressive MS; the difference may be that those individuals with primary progressive MS do not experience the inflammatory reactions in the brain that cause relapses and therefore the disease goes undetected for many years.
- Progressive relapsing is the rarest form in the classification scheme. This is defined as a person who is progressive from onset but then suddenly experiences a relapse. Relapses tend to be rare in this type and it is more of a progressive disease. The fact that cases like this exist lends credence to the notion that all these disease types overlap and are more accurately displayed along a spectrum instead of as discrete entities.
What we really need is a way to differentiate disease types based on pathology and mechanism of injury so we can use this information to apply the correct treatment to the correct person.
Hope this helps