What are the signs and symptoms of nerve injury after an intramuscular injection in the arm?

Signs and Symptoms of Nerve Injury from Intramuscular Injection in the Arm

Nerve injury from intramuscular injection in the arm most commonly presents with pain, paresthesias (tingling), muscle weakness, and sensory disturbance in the distribution of the affected nerve, with the radial and axillary nerves being most vulnerable to injury in this location.

Clinical Presentation

The neurological manifestations of injection-induced nerve injury can range from minor transient symptoms to severe deficits 1:

• Immediate to early symptoms (within minutes to hours):

  • Sharp, shooting pain at the injection site radiating along the nerve distribution 2
  • Burning or electric shock-like sensations 3
  • Immediate weakness in the affected muscle groups 4

• Delayed symptoms (hours to weeks):

  • Progressive muscle atrophy, particularly of the deltoid muscle with axillary nerve injury 4
  • Persistent paresthesias (tingling, numbness) in the sensory distribution 2
  • Motor weakness or paralysis in the affected nerve territory 3, 2
  • Sensory disturbance ranging from mild numbness to complete sensory loss 1

Specific Nerve Injuries in the Arm

Radial Nerve Injury

The radial nerve is vulnerable when injections are given in the lateral or posterior arm 3:

• Wrist drop (inability to extend the wrist and fingers)
• Weakness of thumb extension
• Sensory loss over the dorsal first web space
• All four reported cases in one series showed eventual recovery, though one required surgical neurolysis 3

Axillary Nerve Injury

The axillary nerve is at risk with deltoid injections 4:

• Significant deltoid muscle atrophy (visible within 4 weeks) 4
• Severe shoulder abduction weakness
• Sensory loss over the lateral shoulder (badge area)
• Loss of shoulder contour due to deltoid wasting 4

Pathophysiology and Timing

Pathological changes in the nerve can occur as early as 30 minutes after injection 5:

• Direct neurotoxic effect on both the axon and Schwann cells 5
• Breakdown of the blood-nerve barrier 5
• Apoptosis, demyelination, and axonal degeneration may develop 5
• The mechanism involves direct mechanical trauma from the needle and/or chemical injury from the injectate 4, 5

Electrodiagnostic Findings

When nerve conduction studies and electromyography are performed (typically ≥60 days after onset) 2:

• Evidence of axonal damage with reinnervation in 33% of patients 2
• Axonal damage without reinnervation in 67% of patients, indicating poorer prognosis 2
• Acute denervation changes on needle EMG confirm the diagnosis 4

Prognosis

The prognosis for injection-induced nerve injury in the arm is generally poor, with only 28% of patients achieving good recovery 2:

• Recovery occurred in all four radial nerve palsy cases in one series (one requiring surgery, three spontaneous) 3
• The presence of reinnervation on EMG is a favorable prognostic indicator 2
• Absence of reinnervation on EMG suggests poor recovery potential 2

Critical Anatomical Considerations

To minimize risk of axillary neuropathy, practitioners should understand that 4:

• The anterior branch of the axillary nerve is vulnerable to injury during deltoid injections
• Proper injection technique requires careful attention to anatomical landmarks
• The recommended site for arm injections is the deltoid muscle in the lateral aspect of the upper arm, though even this carries risk 6, 7

Common Pitfall

The most important pitfall is delayed recognition of nerve injury, as early identification and appropriate management are crucial to reduce neurological deficit and maximize recovery 1. Any patient reporting immediate severe pain during injection, electric shock sensations, or developing weakness or sensory changes after arm injection should be promptly evaluated with clinical examination and, if indicated, electrodiagnostic studies 4, 2.