Idiopathic
Post-infectious
Viral
Epstein-Barr, CMV, influenza, parvovirus, coxsackie, HIV, Polio
Bacterial
Lyme, Leptospirosis, TB, Salmonella
Post-immunization
Compressive
Neoplasm
Pancoast tumor
Post-traumatic
Penetrating or blunt neck/shoulder trauma
Traction injury
Birth trauma
Erb's palsy
Klumpke's paralysis
Connective tissue disorders
Autoimmune disorders
Lupus
Polyarteritis nodosa
Temporal arteritis
Iatrogenic
Post-surgical
Medication induced
Radiation
Hereditary Neuralgic Amyotrophy
Rotator Cuff Injury
Adhesive capsulitis
Calcific tendinitis
Thoracic outlet syndrome
ALS
Poliomyelitis<unk> 
Viral
Epstein-Barr, CMV, influenza, parvovirus, coxsackie, HIV, Polio
Bacterial
Lyme, Leptospirosis, TB, Salmonella
Epstein-Barr, CMV, influenza, parvovirus, coxsackie, HIV, Polio
Lyme, Leptospirosis, TB, Salmonella
Neoplasm
Pancoast tumor
Pancoast tumor
Penetrating or blunt neck/shoulder trauma
Traction injury
Birth trauma
Erb's palsy
Klumpke's paralysis
Erb's palsy
Klumpke's paralysis
Lupus
Polyarteritis nodosa
Temporal arteritis
Post-surgical
Medication induced
Radiation
Carpal tunnel syndrome
Pronator teres syndrome
Anterior interosseous neuropathy
Ulnar neuropathy
Radial neuropathy at the spiral groove (ie. "Saturday night palsy")
Posterior interosseous neuropathy
Suprascapular neuropathy
Long thoracic neuropathy
Axillary neuropathy
Spinal accessory neuropathy
Musculocutaneous neuropathy
Brachial plexopathy<unk> 
Cervical radiculopathy<unk>
Parsonage-Turner syndrome
Idiopathic
Down syndrome
Post-infectious
Kounis syndrome
Viral
Sheehan's syndrome
Epstein-Barr, CMV, influenza, parvovirus, coxsackie, HIV, Polio