Causes of Numbness on the Radial Side of the Arm
Numbness on the radial (thumb-side) portion of the arm is most commonly caused by radial nerve compression at the spiral groove of the humerus from prolonged pressure, cervical radiculopathy (particularly C6 or C7 nerve root compression), or brachial plexus injury from excessive arm positioning or stretch.
Primary Etiologies
Radial Nerve Compression
- Prolonged pressure on the radial nerve in the spiral groove of the humerus is the most direct cause of radial-sided arm numbness 1.
- This commonly occurs from:
- Direct compression against hard surfaces during sleep or prolonged positioning 1
- Perioperative positioning injuries when the arm is improperly padded or positioned 1
- Automated blood pressure cuff placement (though evidence is mixed, with 39% of consultants agreeing this increases radial neuropathy risk) 1
- The radial nerve is particularly vulnerable where it wraps around the humerus in the spiral groove 1
Cervical Radiculopathy (C6 or C7)
- Cervical nerve root compression from disc herniation or facet joint spondylosis produces radial-sided arm symptoms 2, 3.
- C6 radiculopathy typically affects the lateral forearm and thumb 2, 3
- C7 radiculopathy (the most commonly affected root) can also produce radial-sided symptoms, though pain and sensory patterns overlap significantly between C6 and C7 4
- Important caveat: Symptoms from C6 and C7 radiculopathy are diffuse and overlap extensively, making clinical differentiation difficult based on symptom location alone 4
- Associated findings include neck pain, dermatomal sensory changes, and potential motor weakness in myotomal distribution 2, 3
Brachial Plexus Injury
- Upper brachial plexus injuries (C5-C6 roots) affect proximal shoulder and upper arm function and can produce radial-sided numbness 5.
- Positioning-related causes include:
- The prone position allows greater tolerance of arm abduction due to different shoulder mechanics 1
Perioperative and Iatrogenic Causes
Positioning Injuries
- Radial nerve palsy from surgical positioning requires documentation of positioning details, evaluation for additional nerve injuries, and conservative treatment with splinting and physical therapy 6.
- Risk factors include:
Equipment-Related Compression
- Automated blood pressure cuffs may contribute to radial neuropathy risk, though consensus is lacking (39% of consultants agree, 41% uncertain) 1
- Blood pressure cuffs should be placed below the antecubital fossa to reduce upper extremity neuropathy risk 1
Diagnostic Approach
Clinical Evaluation
- Perform a simple postoperative assessment of extremity nerve function for early recognition of peripheral neuropathies 1.
- Examine for:
- Sensory distribution: radial-sided forearm, thumb, and dorsal first web space
- Motor function: wrist extension, finger extension, thumb extension
- Associated neck pain or cervical range of motion limitations (suggests radiculopathy) 2, 3
- Timing of onset (immediate postoperative suggests positioning injury) 7
Imaging Considerations
- Three-view wrist radiography (PA, lateral, oblique) should be obtained when traumatic radial nerve injury is suspected to rule out associated bony injury 6.
- For suspected cervical radiculopathy, MRI is the preferred imaging modality to identify nerve root compression, though it has high rates of abnormalities in asymptomatic individuals 1, 2
- Electrodiagnostic studies serve as an extension of the neurologic examination and are useful for atypical symptoms or nonfocal imaging findings 3
Key Clinical Pitfalls
- Do not rely solely on symptom location to differentiate C6 from C7 radiculopathy—pain and sensory symptoms overlap extensively 4
- Cervical spine imaging shows degenerative changes in most patients over 30 years old that correlate poorly with symptoms 1
- Most perioperative positioning injuries show spontaneous recovery over weeks to months with conservative management 7
- Unremitting pain or new symptoms during follow-up warrants immediate reevaluation for nerve compression or complications 8