**1. Introduction**

Motor neuron diseases (MNDs) are a group of progressive neurodegenerative disorders associated with the degradation of the upper (UMN) and lower motor neurons (LMN), without affecting sensory and autonomic systems [1]. MNDs can be classified based on the pattern of motor neuron involvement; they encompass pure LMN syndromes, mixed upper and lower motor neuron diseases, and pure UMN syndromes [2]. MNDs patients display a large heterogeneity of clinical symptoms, including muscular weakness, atrophy, and corticospinal tract signs in varying combinations and severities, presenting a unique diagnostic challenge to clinicians [3]. In this chapter, we provide an overview of MNDs, in particular amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinobulbar muscular atrophy (SBMA), and hereditary spastic paraparesis (HSP) [2].
