Parsonage-Turner Syndrome is an idiopathic inflammatory plexopathy that primarily affects the brachial plexus. It is characterized by the sudden onset of excruciating shoulder pain, followed days or weeks later by profound muscle weakness and atrophy. For the board-certified physician, diagnosing PTS requires a high index of suspicion, as the initial pain often subsides just as the dramatic functional deficits appear.

1. Etiology and the “Immune Trigger”

While the exact cause is unknown, PTS is believed to be an immune-mediated response.

• Triggers: Roughly 50% of cases are preceded by an event such as a viral infection (URI, GI bug), vaccination (flu, tetanus), recent surgery, or extreme physical exertion.
• Demographics: Most commonly affects men between the ages of 20 and 50, though it can occur in any population.

2. Clinical Presentation: The Two-Phase Pattern

The boards will almost always present PTS as a two-phase chronological vignette:

• Phase 1: The Acute Pain Phase: Sudden, severe, “boring” or “stabbing” pain in the shoulder girdle. This pain is often worse at night and does not correlate with a specific movement. It typically lasts for 1 to 3 weeks.
• Phase 2: The Paralytic/Atrophic Phase: As the pain begins to resolve, the patient notices significant weakness and rapid muscle wasting.
  • Most Common Nerves Involved: The Long Thoracic Nerve (leading to scapular winging), the Suprascapular Nerve (weakness in abduction/external rotation), and the Axillary Nerve (deltoid weakness).
  • Phrenic Nerve: In rare cases, the phrenic nerve is involved, leading to unexplained dyspnea or hemidiaphragm elevation on X-ray.

3. Physical Examination Findings

• Scapular Winging: This is a classic high-yield finding. Weakness of the Serratus Anterior (Long Thoracic Nerve) causes the medial border of the scapula to lift off the rib cage when the patient pushes against a wall.
• Muscle Atrophy: Visible hollowing of the supraspinatus or infraspinatus fossae or flattening of the deltoid.
• Sensory Deficits: Usually mild or absent. If present, they often follow the distribution of the axillary nerve over the “regimental badge” area of the lateral shoulder.

4. Differential Diagnosis: The “Great Mimic”

• Cervical Radiculopathy (C5/C6): Radiculopathy usually involves neck pain and sensory changes that follow a strict dermatome. PTS is typically “patchy” and doesn’t follow a single root.
• Rotator Cuff Tear: While both cause weakness, PTS pain is more severe and is not mechanical (it hurts even when the arm is perfectly still).
• Brachial Plexitis (Traumatic): Requires a history of trauma (e.g., a “stinger” or traction injury).

5. Electrodiagnostic (EMG/NCS) Correlation

EMG is the most useful diagnostic tool, but timing is critical.

• NCS: May show decreased Sensory Nerve Action Potential (SNAP) amplitudes if the lesion is post-ganglionic (differentiating it from radiculopathy).
• EMG: Will show acute denervation (fibrillations and positive sharp waves) in the affected muscles. This usually takes 3–4 weeks to appear after the onset of weakness.
• The “Patchy” Hallmark: EMG will show involvement of multiple nerves from different trunks and cords of the plexus that do not fit a single root or peripheral nerve pattern.

6. Management and Prognosis

• Acute Phase: Treatment is primarily supportive with high-dose NSAIDs, opioids for severe pain, and sometimes a short course of oral corticosteroids to reduce the inflammatory response.
• Chronic Phase: Physical therapy is the cornerstone of management. The focus is on range of motion to prevent adhesive capsulitis and strengthening the compensatory muscles.
• Prognosis: Recovery is slow, often taking 12 to 36 months. While roughly 70–90% of patients recover significantly, some permanent weakness or atrophy may persist.

7. Functional Maintenance

The goal is to manage the “functional fallout” of the paralysis.

• Scapular Stabilization: Training the rhomboids and trapezius to compensate for a weak serratus anterior.
• Shoulder Mechanics: Monitoring for secondary impingement. As the deltoid or rotator cuff atrophies, the humeral head may not sit properly in the glenoid, leading to secondary mechanical pain.

---

High-Yield Board “Fast Facts”

• Clinical Pattern: Severe pain followed by rapid weakness and atrophy.
• Long Thoracic Nerve: The most commonly involved nerve in PTS (look for scapular winging).
• EMG Timing: Wait 3–4 weeks after the onset of weakness for the most accurate results.
• Sensation: Usually spared or very minimal compared to the degree of motor loss.
• Triggers: Often follows a viral illness or immunization.