Opisthotonos refers to a phenomenon where an individual's body is rigid and arched backward due to extreme muscle spasms. Some synonyms of opisthotonos include tetanus, back arching, muscle spasms, and rigidity. Other synonyms may include tonic immobility, trismus, lockjaw, and stiff neck. Opisthotonos is typically associated with conditions such as meningitis, encephalitis, and tetanus. However, several other illnesses and injuries may result in similar symptoms, including head and spinal cord injuries and certain drug reactions. It is essential to seek immediate medical attention in case of experiencing opisthotonos or any related symptoms.