What are the likely causes of arm pain post injection?

Arm Pain Post-Injection: Likely Causes

Arm pain following injection is most commonly caused by typical local inflammatory reactions to the injected substance (occurring in 65-78% of patients), with pain representing the most frequent adverse event, though nerve injury, vasovagal reactions, and rare immune-mediated complications must also be considered. 1

Primary Causes: Typical Local Reactions

Normal Inflammatory Response

• Pain at the injection site is the most common post-injection adverse event, reported in 65-78% of vaccine recipients and up to 50% of corticosteroid injection patients 1, 2
• Moderate to severe pain (incapacitating or preventing usual activities) occurs in 16-18% of patients receiving vaccines 1
• Post-injection pain flares occur in approximately 9-12.9% of patients, representing the most frequent delayed adverse event 3, 4
• Most injection-related pain resolves within 1-2 days, with 83% of patients experiencing resolution by day 5 2

Mechanism of Pain

• Local reactions result from immune response to injected antigens, tissue trauma from needle insertion, and irritant properties of the injectate 1
• Higher allergen concentration, injection volume, and number of antigens increase local reaction rates 1
• Glycerin preservatives in concentrations above 30% correlate with increased injection pain, particularly when total glycerin dose exceeds 0.05 mL 1

Secondary Causes: Neurological Injury

Direct Nerve Trauma

• Radial nerve palsy can occur from intramuscular injections into the arm, with documented cases showing both spontaneous recovery and requiring neurolysis 5
• Ulnar neuropathy at the upper arm level has been reported when injections are administered "sidearm" into the upper arm rather than proper deltoid positioning 6
• Nerve compression syndromes may develop, as documented in one case of radiating neck and arm pain with nerve compression diagnosed 12 days post-vaccination 1

Transient Nerve Block

• Transient sciatic nerve block occurs in 6.3% of sacral injections when local anesthetic is included, though this is anatomically specific to lower extremity injections 3

Tertiary Causes: Systemic and Immune-Mediated

Vasovagal Reactions

• Vasovagal reactions occur in 2.5% of patients, manifesting as dizziness, syncope, pain, and erythema at injection site 1, 3, 4
• These reactions resolve without sequelae but require patient counseling about fall risk 1, 3

Complex Regional Pain Syndrome (CRPS)

• CRPS represents a rare but serious complication, documented following COVID-19 vaccination with acute-onset severe arm pain and swelling 7
• This immune-mediated inflammatory response occurs in predisposed individuals and requires specific diagnostic criteria including electrodiagnostic and bone scan findings 7

Corticosteroid-Specific Effects

• Facial flushing and sweating occur in 1.6-2.3% of corticosteroid injections, typically resolving within 24-48 hours 3, 4
• Hyperglycemia affects diabetic patients and requires glucose monitoring for 1-3 days post-injection 4

Critical Diagnostic Approach

Immediate Assessment (Within 30 Minutes)

• Evaluate for immediate reactions including vasovagal symptoms, anaphylaxis risk, or acute neurological deficits 1
• Document pain severity, location, and any motor or sensory changes 5, 6

Early Period (Days 1-5)

• Expected pain trajectory: 50% of patients experience increased pain that resolves by day 2, with 83% resolution by day 5 2
• Pain persisting beyond 5 days or worsening pain warrants further evaluation 2
• Assess for signs of infection (rare but serious): warmth, erythema, fever, particularly in immunosuppressed patients 3, 4

Late Period (Beyond 1 Week)

• Persistent or progressive pain requires evaluation for nerve injury, CRPS, or infection 7, 5, 6
• Consider electrodiagnostic studies if neurological symptoms present 7, 6
• Three-phase bone scan may be indicated if CRPS suspected 7

Common Pitfalls to Avoid

• Do not dismiss persistent arm pain beyond 5 days as "normal" - this warrants investigation for nerve injury or CRPS 7, 5, 2
• Improper injection technique significantly increases nerve injury risk - injections must be administered into the middle deltoid with proper positioning, not "sidearm" into the upper arm 6
• Failing to counsel patients about expected pain trajectory leads to unnecessary anxiety - inform patients that 50% experience transient increased pain resolving by day 2 2
• Overlooking immunosuppressed status increases risk of serious infectious complications including epidural abscess and meningitis 3, 4