Rigidity of facial muscles 'Risus sardonicus' is associated with:

Generalized tetanus is characterized by lock-jaw or trismus due to spasm of masseter, which is the initial symptom and a dental surgeon is the first person often to be consulted. Dysphagia, stiffness or pain in the neck, shoulder or back muscles appear concurrently. Marked rigidity interferes with the movement of chest and impairs cough and swallowing reflexes. Laryngeal spasms may lead to asphyxia. The involvement of various muscles produces rigid abdomen and stiff proximal limb muscles. Hands and feet are relatively spared and sustained contraction of facial muscles results in a grimace or sneer called risus sardonicus. The contraction of muscles of the back produces an arched back called opisthotonus. The spasms occur repetitively or spontaneously or provoked by slight stimulation. Treatment Antitoxin is injected to neutralize circulating toxin and unbound toxin. Human tetanus immunoglobulin (TIG) 3000–6000 units IM individual doses. Though the optimum dose is not known, a 500 unit dose is as effective as higher doses.  Prophylaxis - Wound debridement and booster doses of TT.  Unimmunized individuals - Anti-tetanus serum 1500 units or TIG 250 units should be given.  Ref: Shafer's textbook of oral pathology 7th edition page 328