What is the definition or description of: primary lateral sclerosis?

Variation of ALS: Motor neuron disease or ALS presents in several variable forms, and is a disease of motor neuron degeneration in brain and spinal cord. In the primary lateral sclerosis variant (only 1-3% of all cases), the disorder affects the upper motor neuron only, and is focal on one side or the other. By definition, this form would be far less malignant.