Neuralgic amyotrophy

Neuralgic amyotrophy (also referred to as brachial neuritis or Parsonage-Turner syndrome) is a self-limiting inflammatory disorder of the brachial plexus, that mainly affects males between 20–30 years of age. There are two clinically similar, yet etiologically distinct forms of neuralgic amyotrophy (NA). Idiopathic NA is more common, often unilateral, and non-recurrent. It is thought to be an immune-mediated process triggered by preceding viral infection, immunization, or trauma. Hereditary NA is a rare, autosomal dominant condition that often affects both shoulders and is characterized by recurrent symptoms. In both types, patients present with acute onset of excruciating shoulder pain that lasts for weeks followed by patchy lower motor neuron paresis of the proximal muscles in the affected arm. Diagnosis is clinical, with nerve conduction studies and needle electromyography performed to identify which nerves are affected. Treatment is mainly supportive and involves analgesia and physical therapy. Most patients recover complete muscle strength within 2 years.

• Incidence: 1–3 cases per 100,000 population per year
• Peak age: 20–30 years
• Sex: ♂ > ♀

References:^[1][2][3][4]

Epidemiological data refers to the US, unless otherwise specified.

• Idiopathic NA (more common)
  • Likely immune-mediated
  • Potential triggers: recent viral infection/immunization; ; mechanical strain on the arm/shoulder; trauma; surgery; childbirth; vasculitis (temporal arteritis, PAN, SLE)
• Hereditary NA
  • Rare
  • Autosomal dominant

References:^{[1][3][4][5][6]}

The exact pathophysiology is unknown, but it is believed to be multifactorial.

• Environmental factors (e.g., viral infection, immunization, surgery) → activation of the immune system
• Mechanical factors (e.g., repetitive strain, recent strenuous exercise) → repeated stretching of the brachial plexus nerves → predisposition of the brachial plexus to immune-mediated injury

A combination of the above factors → patchy inflammation of the brachial plexus → axonal injury of the affected nerves → severe burning pain, followed by paresis of the muscles supplied by the affected nerve

References:^[3]

• Shoulder pain
  • Acute phase
    • Sudden onset of excruciating, burning pain which does not decrease with NSAIDs
    • Mostly unilateral (esp. affects the right shoulder ), radiates to the arm and/or the neck
    • Usually lasts for several weeks
  • Chronic phase
    • Continuous pain lasting ∼ 1 year or
    • Intermittent exacerbations of shooting pain, lasting for a few hours; brought on by certain shoulder positions/actions (arm abduction/elevation)
• Progressive weakness and atrophy of the shoulder/arm muscles
  • Occurs ∼ 2 weeks after the acute onset of pain
  • Patchy involvement of more than one nerve of the brachial plexus
  • Different degrees of lower motor neuron paresis of muscles innervated by the same peripheral nerve
  • Nerves commonly affected:
    • Suprascapular nerve; → weak abduction and external rotation of the shoulder
    • Long thoracic nerve → winging of the scapula
    • Axillary nerve; → weak abduction of the arm
    • Subscapular nerve; → weak internal rotation of the shoulder
• Sensory loss: hypoesthesia: (numbness); paresthesia or dysesthesia (painful sensation to light touch) in the arm, forearm, and/or hand
• Autonomic dysfunction: edema, excessive sweating, faster growth of hair and nails on the affected limb

References:^{[1][3][4][5][6][7][8][9]}

NA is mainly a clinical diagnosis.

• Assessment of damage localization and severity
  • Nerve conduction studies: show conduction block in the affected nerves
  • Needle electromyography: show axonal degeneration; help differentiate between muscular and neural causes of weakness
• Exclusion of differential diagnoses
  • Chest x-ray: to rule out Pancoast tumor or thoracic outlet syndrome
  • MRI of the cervical spine: to rule out cervical disc disease/degeneration
  • MRI of the shoulder: to rule out rotator cuff tears/impingement

References:^{[3][4][6][7][8]}

• Upper limb weakness and/or proximal muscle atrophy
  • Pancoast tumor
  • Rotator cuff tendinitis/tear
  • Cervical radiculopathies; , mononeuritis multiplex, amyotrophic lateral sclerosis, thoracic outlet syndrome, acute poliomyelitis
• Acute severe pain: myocardial infarction, herpes zoster neuritis

References:^[1][3][4][5]

The differential diagnoses listed here are not exhaustive.

There is no specific causative treatment for NA. The condition is self-limiting and is treated symptomatically.

• Analgesia
  • Acute phase: combination of a long-acting NSAID (e.g., diclofenac) and an opiate (e.g., morphine)
  • Chronic phase: gabapentin, carbamazepine, or amitriptyline
• Physical therapy
  • Initiated after acute pain has subsided
  • Prevents/minimizes muscular atrophy and preserves normal range of motion of the shoulder joint
• Surgery: nerve grafts or tendon transfers are possible; only considered in protracted disease courses (> 2 years)

References:^{[2][3][4][6][7][10][11]}

• Prognosis is variable
  • Some patients recover complete muscle strength and are pain-free within 1–2 years
  • Some may have pain and paresis for many years, causing significant functional disability
• Recurrence
  • Idiopathic NA: often non-recurrent
  • Hereditary NA: ∼ 75% of patients will have recurrent NA

References:^[1][3][4][9]