What are the causes of arm pain and weakness after an intramuscular (IM) injection?

Causes of Arm Pain and Weakness After IM Injection

Arm pain and weakness following intramuscular injection most commonly results from direct nerve injury during needle insertion, local inflammatory reactions at the injection site, or inadvertent injection into the subacromial-subdeltoid bursa (SIRVA), with nerve injury carrying the most serious implications for long-term morbidity.

Primary Mechanisms of Injury

Direct Nerve Trauma

• Direct needle trauma to peripheral nerves during injection causes immediate-onset numbness, pain, and weakness, particularly when injecting into limb sites rather than preferred truncal locations 1
• The radial nerve is vulnerable during arm injections, with 44% of injection-related nerve injuries affecting the radial nerve in one large case series 2
• Inadvertent injection near nerve bundles produces transient or persistent paresthesias that may progress to motor weakness 1
• Nerve injury from IM injection carries a poor prognosis, with only 28% of patients achieving good recovery in a 13-year retrospective study 2

Local Inflammatory Reactions

• Injection site pain and tenderness are the most common adverse events after IM administration, occurring in the majority of patients 3
• Local tissue reactions including swelling and erythema may compress adjacent nerves, contributing to secondary numbness and weakness 1
• Redness, induration, and swelling at the injection site can persist and mechanically impair nerve function 3

Shoulder Injury Related to Vaccine Administration (SIRVA)

• Unintentional injection of vaccine solution into the subacromial-subdeltoid bursa triggers a robust immune-mediated inflammatory reaction, causing shoulder pain and limited range of motion 4
• SIRVA results from improper injection technique with needle placement too high on the deltoid muscle 4
• This condition leads to shoulder dysfunction and ongoing pain that extends beyond typical injection site reactions 4

Clinical Evaluation Algorithm

Immediate Assessment (Within 48 Hours)

• Assess the distribution of numbness: dermatomal patterns suggest direct nerve injury, while diffuse patterns suggest systemic reaction 1
• Evaluate timing of onset: immediate symptoms indicate direct nerve trauma, while delayed onset (hours to days) suggests inflammatory compression 1
• Examine for associated weakness, pain radiating beyond the injection site, or loss of motor function—these indicate more serious nerve injury requiring urgent evaluation 1
• Inspect the injection site for swelling, erythema, warmth, or palpable mass suggesting hematoma or abscess formation 1

Neurophysiological Testing Indications

• Immediate neurological consultation for nerve conduction studies and electromyography is necessary if symptoms persist beyond 48 hours 1
• Needle EMG performed ≥60 days after onset can differentiate axonal damage with reinnervation (better prognosis) from axonal damage without reinnervation (poor prognosis) 2
• In one series, 67% of patients showed axonal damage without reinnervation on EMG, correlating with poor functional recovery 2

Imaging Considerations

• Consider ultrasound or MRI of the injection site if hematoma, abscess, or bursal inflammation is suspected 1
• Imaging can confirm subacromial-subdeltoid bursitis in cases of SIRVA 4

Management Strategy

Acute Phase (First 2-4 Weeks)

• Apply warm compresses to the injection site to reduce local inflammation 1
• Monitor for spontaneous resolution over 2-4 weeks, as most cases of mild nerve irritation resolve without intervention 1
• Document the event thoroughly and avoid future IM injections in the affected limb 1

Persistent Symptoms (Beyond 4 Weeks)

• Symptoms persisting beyond 4-6 weeks may indicate permanent nerve injury requiring ongoing management 1
• Consider surgical exploration and external neurolysis for intractable neuropathic pain, though results are often only partially successful 5
• Nerve injuries following IM injection carry a poor prognosis overall, with irreversible tissue damage occurring rapidly 6

Prevention Strategies

Injection Site Selection

• Inject into truncal sites (abdomen, buttocks) rather than limbs to minimize nerve injury risk 1
• When arm injection is necessary, use the mid-deltoid region and avoid injection too high on the shoulder (which risks SIRVA) 4
• The sciatic nerve is most commonly injured during gluteal injections due to its large size, though this is less relevant for arm injections 5, 2

Technical Considerations

• Use appropriate needle length to avoid inadvertent intramuscular delivery into deeper structures 1
• Rotate injection sites systematically to prevent repeated trauma to the same area 1
• Aspirate before injecting to ensure proper placement and avoid intravascular injection 1

Critical Pitfalls to Avoid

• Do not dismiss persistent weakness as a normal post-injection reaction—weakness is more typical of myositis or nerve injury than simple injection site pain 7
• Do not delay neurological consultation beyond 48 hours for persistent symptoms, as early intervention may improve outcomes 1
• Recognize that in developing countries, 86% of injection-related nerve injuries were caused by uncertified medical practitioners, emphasizing the importance of proper training 2
• Aseptic tissue necrosis (Nicolau syndrome) can occur from unintentional intra-arterial injection, presenting with severe immediate pain, swelling, and livid skin discoloration progressing to gangrene 6

Expected Outcomes

• Mild nerve irritation typically resolves within 2-4 weeks with conservative management 1
• Moderate to severe nerve injuries have only a 28% rate of good recovery, even with appropriate management 2
• Persistent symptoms beyond 4-6 weeks indicate likely permanent nerve injury 1
• The consequences of nerve injection injury are potentially devastating and may result in serious neurological disability and chronic neuropathic pain 5