What are the next steps if I experience right leg muscle weakness and right arm weakness after a flu shot?

Immediate Evaluation for Post-Influenza Vaccination Neurologic Symptoms

You should seek immediate medical evaluation to rule out Guillain-Barré syndrome (GBS), a rare but serious neurologic complication that can occur after influenza vaccination, with urgent neurologic assessment including nerve conduction studies if symptoms progress or involve respiratory muscles. 1

Understanding the Risk and Timeline

• The estimated risk of GBS after influenza vaccination is extremely low, approximately 1-2 additional cases per 1 million persons vaccinated, based on post-1976 surveillance data. 1
• GBS cases typically peak around 2 weeks after vaccination when an association exists. 1
• Most post-vaccine neurologic adverse events involving limb weakness are due to peripheral neuropathy, which generally has a good prognosis with complete improvement within a few months without sequelae. 2

Immediate Clinical Assessment Required

Seek urgent evaluation if you experience:

• Progressive weakness ascending from legs to arms or involving both sides of the body (classic GBS pattern). 1
• Difficulty breathing, swallowing, or speaking (indicating potential respiratory muscle involvement requiring emergency care). 1
• Loss of reflexes or sensory changes accompanying the weakness. 2
• Symptoms that worsen over hours to days rather than remaining stable. 2

Diagnostic Workup Your Physician Should Perform

• Neurologic examination focusing on motor strength grading, deep tendon reflexes, sensory testing, and cranial nerve function. 2
• Nerve conduction studies and electromyography (EMG) if GBS is suspected, typically performed at 1-2 weeks after symptom onset for optimal diagnostic yield. 3
• Cerebrospinal fluid analysis may show characteristic albuminocytologic dissociation (elevated protein with normal cell count) in GBS, though this may not appear until the second week. 1

Alternative Diagnoses to Consider

• Localized peripheral neuropathy affecting specific nerves (such as radial nerve palsy) can occur after vaccination, typically presenting within 12-16 hours and often resolving with conservative management over several months. 3
• Polyneuropathy in the extremities was the most common neurologic adverse event (78.6% of cases) in pediatric studies, with motor weakness being the predominant feature. 2
• Injection site-related nerve injury (SIRVA - Shoulder Injury Related to Vaccine Administration) if the weakness is isolated to the arm where injection occurred. 4

Expected Clinical Course and Management

• If diagnosed with post-vaccine peripheral neuropathy (not GBS), expect gradual improvement over 2-6 months with supportive care including physical therapy and splinting as needed. 2, 3
• Most cases of polyneuropathy following influenza vaccination demonstrate complete recovery without permanent sequelae. 2
• GBS, if confirmed, typically requires hospitalization for monitoring of respiratory function and may require intravenous immunoglobulin (IVIG) or plasmapheresis treatment. 1

Important Caveats

• The temporal association between vaccination and symptom onset does not automatically establish causation—coincidental neurologic illness unrelated to vaccination must be excluded. 1
• The benefits of influenza vaccination substantially outweigh the extremely low risk of GBS, particularly for high-risk individuals. 1
• If GBS is confirmed to have occurred within 6 weeks of influenza vaccination, you generally should not receive influenza vaccine in future years unless you are at high risk for severe influenza complications, in which case the decision requires careful risk-benefit discussion with your physician. 1