SPAST and hereditary spastic paraplegia: SPAST gene mutation is the most common cause of autosomal dominant pure HSP (SPG4), although in-depth studies of larger patient cohorts have also associated gene mutation with more complex forms of HSP with bulbar dysfunction, and sensory and lower motor neuron signs (McDermott et al., 2006; Sartucci et al., 2007; Kumar et al., 2012).