Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Parsonage Turner syndrome ?
    ### Response:
    Parsonage Turner syndrome is characterized by the sudden onset of shoulder and upper arm pain followed by progressive (worsening over time) weakness and/or atrophy of the affected area. Although the exact cause is unknown, researchers believe that most cases are due to an autoimmune response following exposure to an illness or environmental factor.
    