Risus sardonicus, "malicious smirk", symptom of an infection with Clostridium tetani*: the facial muscles cramp and it looks like the person is smirking. He/She can`t loosen this grimace.
*Clostridium tetani is a spore-forming rod-shaped bacterium, anaerobic and Gram-positive, that produces toxines (tetanospasm) that split the synaptobrevin VAMP in the grey matter of the spinal cord. Thereby it influences the release of neurotransmitters and thus blocks inhibiting motor neurons. This causes the facial and extensor muscles to extremely and painfully cramp, which may even cause a vertebral fracture. The diagnosis "tetanus" can be made according to the symptomatology and evidence of toxines. The infection is rarely treatable, the wound can simply be thoroughly cut out or even amputated, so that the bacterium can`t further reproduce in it. Otherwise, sensory deprivation can be used to prevent stimulus-caused cramps, in extreme cases muscle relaxation during artificial respiration. Needless to say, the best solution would be to make use of the preventing vaccination, so that the disease won`t even arise.