O'Brien wrote that while on assignment in Asia on February 12th a case containing camera gear fell on his arm. The next day, the arm was sore and swollen, and he sought medical care the day after that when it was even worse.
He was diagnosed with acute compartment syndrome and a doctor recommended an emergency procedure to relieve the pressure in the arm caused by blocked blood flow.
However, during the surgery, complications from the compartment syndrome caused O'Brien to lose blood pressure, and the doctor made the call to amputate the arm just above the elbow. O'Brien wrote it was a choice between "a life and a limb," and that he's grateful to be alive.
The former CNN science and space correspondent covers science for PBS NewsHour.