How is ALS medically classified?
Making a proper diagnosis in ALS is complicated because symptoms can vary in each patient. For greater accuracy, physicians have classified every known form:

1. Classical ALS
   a progressive neurological disease characterized by a deterioration of upper and lower motor neurons (nerve cells). This type of ALS affects more than two-thirds of those with the disease.
2. Primary Lateral Sclerosis (PLS)
   a progressive neurological disease in which the upper motor neurons (nerve cells) deteriorate. If the lower motor neurons are not affected within two years, the disease usually remains a pure upper motor neuron disease. This is the rarest form of ALS.
3. Progressive Bulbar Palsy (PBP)
   a condition that starts with difficulties in speaking, chewing and swallowing due to lower motor neuron (nerve cell) deterioration. This disorder affects about 25% of those with ALS.
4. Progressive Muscular Atrophy (PMA)
   a progressive neurological disease in which the lower motor neurons (nerve cells) deteriorate. If the upper motor neurons are unaffected within two years, the disease usually remains a pure lower motor neuron disease.
5. Familial
   a progressive neurological disease that affects more than one member of the same family. This type of ALS accounts for a very small number of people with ALS in the United States (between 5 and 10 percent).