Idiopathic neuralgic amyotrophy, commonly known as Parsonage-Turner syndrome (PTS) and also known as acute brachial neuritis or brachial neuralgia, is an atraumatic inflammatory condition involving the brachial plexus. It is characterized by severe shoulder pain followed by progressive muscle weakness, atrophy, and sensory deficits in a patchy distribution.

Causes / typical injury mechanism: PTS is an idiopathic condition and occurs spontaneously in about half of cases. Various antecedent events have also been associated with onset of the condition, including recent bacterial or viral infection, strenuous exercise, and pregnancy.

Classic history and presentation: Patients typically present with unilateral acute to subacute shoulder, arm, or axillary pain followed by variable weakness and sensory change, often involving the shoulder, neck, and upper arm. Pain is usually severe, continuous, and unilateral (right more often than left, but reported as bilateral in up to 30% of cases), lasting for days up to 2 months. Progressive muscle weakness and atrophy begins hours to weeks after the onset of pain, characteristically in a patchy upper brachial plexus distribution involving the periscapular and perihumeral muscles (70% of cases), with (50%) or without (20%) involvement of the long thoracic nerve. Sensory deficits including numbness and/or paresthesias occur in most cases, commonly over the lateral shoulder and/or arm. Rare cases can involve nerves outside of the brachial plexus, including the phrenic and recurrent laryngeal nerves, leading to respiratory issues. Most patients will experience moderate-to-complete recovery that occurs over months.

Incidence / prevalence: PTS has been considered a rare disease, with an estimated incidence of 1-3 / 100 000 persons, but recent studies suggest it may be underdiagnosed, with an incidence closer to 1 / 1000 persons.

• Age – All ages are affected, but most commonly between the ages of 20 and 60 years.
• Sex / gender – Men are affected more often than women (2:1).

Risk factors: Precipitating conditions are associated with 50% of cases and include –

• Recent viral or bacterial infection
• Vaccinations
• Surgery or other medical procedure (spinal tap, etc)
• Pregnancy and childbirth
• Strenuous exercise
• Trauma
• Hereditary neuralgic amyotrophy is caused by an autosomal dominant gene mutation that accounts for 10% of cases, while idiopathic PTS accounts for the other 90%.

Pathophysiology: The exact cause of PTS is not known but is presumed to be multifactorial with immune and biomechanical components. Microtrauma to the nerves of the brachial plexus may lead to increased permeability of the blood-nerve barrier, predisposing the nerves to multifocal immune-mediated inflammation and axonal degeneration. This condition can involve multiple motor and sensory nerves of the brachial plexus, with a predilection of motor nerves.

Grade / classification system:

• Phase 1: Sudden-onset, severe, unrelenting shoulder pain
• Lasts up to a few weeks (longer in men than women)
• Phase 2: Painless flaccid paralysis
• Occurs within 4 weeks of pain onset.
• "Patchy paresis" – differential involvement of muscles innervated by the same nerve is highly characteristic
• Phase 3: Slow recovery
• Gradual return of motor function for 6-18 months
• Duration of recovery could be proportional to the duration of phase 1