CMT or ALS?
CMT is a disease of the peripheral nerves, that is, the motor and/or sensory nerves, affecting the myelin (CMT type 1, 4 and X) or the nerve axon (CMT Type 2). Therefore the problem is between the neurons in the spinal column and the muscles. A nerve conduction study will generally reveal whether the disease is demyelinating or not, and if it is demyelinating the chance of ALS (which is not demyelinating) plus a demyelinating polyneuropathy is rare in the onset stages. Therefore, ALS is generally ruled out if the patient presents with demyelination.
ALS is a disease of the upper motor neurons and/or the lower motor neurons. CMT is a disease of the peripheral nerves and has no upper motor neuron (brain) involvement. Therefore, If the patient shows sign of UMN involvement (hyperreflexia, spasticity, etc.), then CMT can be ruled out.
The lower motor neurons are in the spinal column and are a step rostral (upstream) from the dorsal root ganglion, which is where the motor and sensory nerves coming from the spinal column split into two pairs, one pair going to the limbs and one pair going to the paraspinals.
If an EMG shows denervation in the paraspinals and in the myotome of that paraspinal, there is a strong presumption that the two areas are affected by one disease (that is, a disease affecting the motor neurons of the spinal column, which is called anterior horn cell disease) and it is diagnosed as ALS. If an EMG shows denervation in only the paraspinals or only the limbs, then the problem is distal (downstream) to the dorsal root ganglion (not in the spinal column) so ALS is ruled out.
As a practical matter, the muscles in the limbs frequently show denervation due to radiculopathies and other compression or nerve impingement problems, while the thoracic paraspinals rarely are affected by radiculopathies. So an EMG that shows denervation in a preponderance of thoracic paraspinals is a strong indication of ALS, especially if the myotomes of those paraspinals also show signs of denervation.
Progression
CMT is slower progressing than ALS. CMT is not considered a fatal disease and people with most forms of CMT have a normal life expectancy, though the path can be similar—cane, walker, wheelchair, PWC. There is no cure for either disease.