Primary lateral sclerosis (PLS) is a progressive, degenerative disease of upper motor neurons (UMN) characterized by progressive spasticity (i.e., stiffness). It affects the lower extremities, trunk, upper extremities, and bulbar muscles (usually in that order).
Some individuals with this form of motor neuron disease never develop clinical lower motor neuron (LMN) signs. Most, however, do develop LMN signs later in their clinical course. This is referred to as “UMN-onset amyotrophic lateral sclerosis (ALS)”.
Pure primary lateral sclerosis and UMN-dominant ALS appear to have a more benign prognosis than typical ALS.
Survival for patients with UMN-dominant ALS was intermediate between that of primary lateral sclerosis and classic ALS.

The major clinical challenge that the presentation of PLS poses is distinguishing it from the more common form of motor neuron disease, amyotrophic lateral sclerosis (ALS), from hereditary spastic paraparesis (HSP), and from non-degenerative conditions that may present similarly early in their course.
PLS usually affects adults and is usually sporadic.

Adapted from Medscape Drugs & Diseases. Primary lateral sclerosis. Available at http://emedicine.medscape.com/article/1171782-overview. Accessed on February 15, 2016. To view the entire article and all other content on the Medscape Drugs & Diseases site, a free, one-time registration is required.