Brachial neuritis (BN), also known as neuralgic amyotrophy or Parsonage-Turner syndrome, is a rare syndrome of unknown etiology affecting mainly the lower motor neurons of the brachial plexus and/or individual nerves or nerve branches. BN usually is characterized by the acute onset of excruciating unilateral shoulder pain, followed by flaccid paralysis of shoulder and parascapular muscles several days later. The syndrome can vary greatly in presentation and nerve involvement. [1, 2] (See images below.)
Brachial neuritis (BN) exists in an inherited and an idiopathic form. In the idiopathic version, the pathophysiology is unknown, but the condition is generally thought to be an immune system–mediated inflammatory reaction against nerve fibers of the brachial plexus. [3, 4, 5, 6, 7] Axonopathy with subsequent Wallerian degeneration appears to predominate, but proximal conduction block has also been described in over 33% of cases in the series by Lo and Mills.  The inherited form is autosomal dominant and has been linked to mutations in the SEPT9 gene on chromosome 17q. [9, 10, 11] Septins are involved in the formation of the cytoskeleton and in cell division, but how these mutations result in BN is unknown.
A study by Arányi et al described the ultrasonographic characteristics of brachial neuritis, finding the following abnormalities in a group of 14 patients:
Focal or diffuse nerve or fascicle enlargement
Incomplete nerve constriction
Complete nerve constriction with torsion
The investigators suggested that inflammation-related constriction precedes torsion, with the torsion encouraged by limb rotation. 
A study by Ferrante and Wilbourn of 281 patients with sporadic brachial neuritis found through electrodiagnostic testing that out of 379 assessable events, 174 (46%) involved a single nerve, with another 205 (54%) being multifocal. The investigators also found that most bouts of brachial neuritis purely involved motor nerves, with the second greatest number involving mostly motor nerves and the fewest involving a more even mix of sensory and motor nerves. 
The incidence of brachial neuritis is approximately 1-2 cases per 100,000 person-years.
In the United Kingdom, the incidence of brachial neuritis is approximately 3 per 100,000 person-years.  Brachial neuritis has also been described in many countries around the world, although specific rates of incidence have not been reported.
A prospective cohort study from the Netherlands suggested that the incidence of brachial neuritis is many times higher than currently thought. Using data from two primary care institutions, the study, by van Alfen and colleagues, found that out of 492 patients with new-onset neck, shoulder, or arm complaints, 14 were confirmed to have brachial neuropathy, putting the 1-year incidence rate for the condition at 1 per 1000 persons. According to the investigators, this indicated an incidence for the condition that is 30-50 times greater than currently believed. 
Brachial neuritis is not a fatal condition, although the phrenic nerve may be involved. The risk of 'significant' residual disability in the involved limb after 2 years is approximately 10-20%.  . A recent survey suggested that up to half of patients are left with residual shoulder pain and decreased endurance. 
Brachial neuritis occurs predominantly in males, with the male-to-female ratio for the condition ranging from 2:1 to 4:1.
Brachial neuritis has been reported in individuals from age 3 months to 74 years; however, the condition's prevalence is highest in young to middle-aged adults. Onset in childhood should be considered suggestive of hereditary brachial neuritis. 
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