The brachial plexus is a network of nerves that conducts signals from the spine to the shoulder, arm, and hand.
Parsonage-Turner Syndrome, or brachial plexitis, causes inflammation of the brachial plexus without any obvious shoulder injury. This syndrome can begin with severe shoulder or arm pain followed by weakness and numbness.
Affected muscles can become weak and atrophied, and in advanced cases, paralyzed. Occasionally, there will be no pain and just paralysis, and sometimes just pain, not ending in paralysis.
The cause of PTS is unknown, and it affects around 10,000 people per year globally. It usually resolves itself in 18 months to 2 years.
Further reading : National Institute of Neurological Disorders and Stroke