Case Presentation: A 55 year old male with metastatic lung cancer presented to the emergency department with right arm pain, weakness and right hand numbness. Patient had brain metastasis previously treated surgically and with radiotherapy. His last treatment with radiation was to the left cerebellum, 3 months prior. Patient was started on pembrolizumab and vorinostat two weeks ago. Patient stated that his symptoms started 4 days ago. He noted pain in his arm with weakness and right hand numbness. He noted difficulty with fine motor skills of his right hand. He took oxycodone for the pain but did not note any significant relief. CT head was negative for any new intracranial abnormalities. Patient denied headaches, vision changes, balance disturbance or lower extremity weakness. Brachial plexus MRI was performed which was negative for infiltrative or space-occupying compressive mass lesion involving the right axillo-subclavian neurovascular bundle. Brain MRI revealed scattered foci of increased FLAIR signal in pericallosal white matter bilaterally with classic appearance of demyelinating plaque sclerosis. Patient had known family history of multiple sclerosis. He was evaluated by neurology and further interpretation of the MRI suggested longstanding plaques of no significance. He underwent an EMG study consistent with denervation changes in the distribution of C8-T1 muscles. Patient was started on steroid tapering with prednisone 60mg over 2 weeks. During his two week follow-up the patient’s examination showed improvement in both motor and sensory function.

Discussion: Parsonage-Turner Syndrome (PTS), also referred to as idiopathic brachial plexopathy or neuralgic amyotrophy, is a rare disorder consisting of a complex constellation of symptoms. It has abrupt onset of unilateral shoulder pain followed by progressive neurologic deficits of motor weakness, dysesthesias, and numbness. The etiology of the syndrome is unclear. It has been reported in various clinical situations like postoperatively, postinfectious, posttraumatic, and postvaccination. PTS is a rare and possibly new finding in immunotherapy related adverse effects.

Conclusions: PTS is a condition that typically presents suddenly with disabling pain and is often difficult to diagnose in its acute state. Postvaccination is a common etiology of this entity. The pain in PTS is generally self-limiting, and the natural history of PTS leads to a generally favorable functional recovery. Early diagnosis with a optimal clinical evaluation, electrodiagnostics, and relevant imaging can allow for aggressive short-term pain management, help outline proper therapy, and provide comfort to both patient and physician in establishing a diagnosis. Proper diagnosis can also avoid unnecessary additional testing or surgical exploration. Limited follow-up EMG testing can be useful in demonstrating early electrodiagnostic signs of recovery which can precede clinical evidence of recovery. This finding can help alleviate the patient’s psychological concerns about their disability and functional recovery and also provide the treating medical team with information regarding the patient’s prognosis and, ultimately, recovery. As new advances in immunotherapy treatment for different neoplasms are readily available, atypical and typical forms of both common and uncommon immune mediated neuropathies will be seen. Early recognition by hospitalists is paramount for the prognosis and rapid improvement of those conditions.