What lab work is recommended for a patient presenting with symptoms suggestive of Parsonage-Turner syndrome?

Laboratory Workup for Parsonage-Turner Syndrome

For patients with suspected Parsonage-Turner syndrome, routine laboratory testing is generally not diagnostic, but serological testing for Lyme disease should be performed, and electromyography (EMG) is essential to confirm the diagnosis and localize nerve involvement. 1, 2

Essential Diagnostic Testing

Electrophysiological Studies

• EMG and nerve conduction studies are the cornerstone of diagnosis, demonstrating denervation patterns in affected muscles and confirming involvement of specific nerves (most commonly the long thoracic, axillary, and suprascapular nerves). 1, 3, 4
• Testing typically shows acute denervation changes in shoulder girdle muscles, with the supraspinatus and infraspinatus being most frequently affected. 3, 4
• Serial neurologic examinations are preferred over repeated EMG/nerve conduction studies for monitoring disease activity once the diagnosis is established, as repeated testing is invasive and unnecessary for stable patients. 5

Serological Testing for Infectious Triggers

• Lyme disease serological testing should be performed routinely in all patients presenting with Parsonage-Turner syndrome, as neuroborreliosis can manifest as acute brachial neuritis. 2
• Four documented cases showed Parsonage-Turner syndrome as the first manifestation of Lyme disease, with positive serology in serum and cerebrospinal fluid. 2
• Consider testing for other infectious triggers based on clinical context (recent viral illness, immunizations, or travel history). 1

Additional Laboratory Considerations

Cerebrospinal Fluid Analysis

• If Lyme disease is suspected or neurological symptoms suggest central involvement, lumbar puncture may reveal lymphocytosis and protein elevation. 2
• Three of four patients with Lyme-associated Parsonage-Turner syndrome showed CSF abnormalities including lymphocytosis and elevated protein. 2

Screening for Underlying Causes

• A thorough workup should include screening for metabolic disorders, inflammatory conditions, and malignancy-related syndromes when polyneuropathy patterns are present. 5
• Basic metabolic panel and complete blood count may help exclude other causes of acute neuropathy but are not diagnostic for Parsonage-Turner syndrome itself. 5

Imaging Studies (Not Laboratory, But Critical)

MRI of Brachial Plexus and Shoulder

• MRI is highly sensitive for detecting denervation changes and shows T2 hyperintensities in affected muscles, with nerve hourglass swellings and constrictions visible. 1, 4
• The supraspinatus and infraspinatus muscles demonstrate signal abnormalities most commonly. 4
• MRI may be instrumental in accurately diagnosing the syndrome when clinical presentation is atypical. 4

Ultrasound

• Diaphragm ultrasound improves diagnosis when phrenic nerve involvement is suspected, as this nerve is commonly affected outside the brachial plexus. 1
• Ultrasound can also identify nerve hourglass swellings and constrictions. 1

Common Pitfalls to Avoid

• Do not rely solely on routine laboratory tests (CBC, metabolic panel) to make or exclude the diagnosis, as Parsonage-Turner syndrome is primarily a clinical and electrophysiological diagnosis. 1, 3
• Do not skip Lyme serology, even in the absence of reported tick bite or erythema migrans, as only one of four documented cases reported these classic features. 2
• Avoid repeated EMG studies for monitoring once diagnosis is confirmed; clinical examination is sufficient and less invasive. 5
• Do not perform superficial biopsies if vasculitis is in the differential, as these miss deeper pathology. 6