Hello Tom
Spinal muscular atrophy (SMA) is different than progressive muscular atrophy (PMA).
SMA is a condition found in infants, children and young adults. The time in the life that it is contracted dictates what type it is. They are categorized from Type I all the way to Type IV, with Type I being found in infants, Type II in children, Type III in teenagers and people in their twenties and Type IV is an adult form found in people older than 30. It is very, very, very slow in its progression and is typically not fatal.
PMA on the other hand, is fatal. This is a variant of
ALS where there is solely lower motor neuron involvement. It's progression isn't typically as fast as
ALS but the way it progresses is the same. It will march up and down limbs, have bulbar involvement and will make it's way to the respiratory muscles eventually.
There are certainly other conditions that cause lower motor neuron problems (peripheral neuropathy for example) that are not typically fatal, because they are not progressive (for the most part). They more or less hit you then stop. Flare-ups can certainly occur (and these flare-ups can be acute or chronic) but the progression isn't like
ALS or PMA, which is relentless; once
ALS or PMA start, they simply do not stop.
Why? 
Wright? 
