Hemimasticatory spasm (HMS) was originally named as Romberg spasm 1987 by Gowers WR [1]. It is a relatively rare type of unilateral trigeminal nerve (TN) motor branch dysfunction, whose pathogenesis is currently unclear [2]. It causes unilateral paroxysmal contractions in the jaw-closing muscles that prevent voluntary closing of the mouth, which may persist during sleep, and is accompanied by muscle pain, with compensatory hypertrophy changes also often seen in the muscles affected by spasms (Fig. 18.1). The medial pterygoid, masseter, and temporalis muscles are affected by this condition [3, 4].