Primary lateral sclerosis

**Definition:** Primary lateral sclerosis (PLS) is a rare motor neuron disease variant which presents with slowly progressive UMN signs, such as spastic gait, brisk deep tendon jerks, and extensor plantar responses. Onset is most commonly with spastic paraparesis, but patients typically progress to develop upper limb and bulbar involvement. The characteristic feature of PLS is the complete absence of involvement of lower motor neuron involvement. When LMN signs develop during the course of the disease, the diagnosis will change to ALS, and they are considered a spectrum of the same disorder.