Defects in ERLIN2 are the cause of spastic paraplegia autosomal recessive type 18 (SPG18) [MIM:611225]. A form of spastic paraplegia, a neurodegenerative disorder characterized by a slow, gradual, progressive weakness and spasticity of the lower limbs. Rate of progression and the severity of symptoms are quite variable. Initial symptoms may include difficulty with balance, weakness and stiffness in the legs, muscle spasms, and dragging the toes when walking. In some forms of the disorder, bladder symptoms (such as incontinence) may appear, or the weakness and stiffness may spread to other parts of the body. SPG18 is a severe form with onset in early childhood. Most affected individuals have severe psychomotor retardation. Some may develop significant joint contractures.