What tests are used to diagnose ulnar nerve damage in a patient presenting with numbness or weakness in the hand or arm?

Tests for Ulnar Nerve Damage

For diagnosing ulnar nerve damage, electrodiagnostic studies (nerve conduction studies with electromyography) are the primary diagnostic test, supplemented by high-resolution ultrasound or MRI with T2-weighted neurography for anatomical localization and identification of specific entrapment sites.

Clinical Assessment

Initial evaluation must include specific provocative maneuvers and motor/sensory testing:

• Elbow flexion test: Maintain elbow in full flexion for 3-5 minutes to reproduce symptoms of ulnar neuropathy 1, 2
• Tinel's sign: Percussion along the ulnar nerve course at the elbow (cubital tunnel) and wrist (Guyon's canal) to elicit paresthesias 1, 2
• Sensory testing: Assess for decreased sensation in the little finger and ulnar aspect of the ring finger, which distinguishes ulnar neuropathy from cervical radiculopathy 3
• Motor strength testing: Evaluate hand intrinsics (interossei, lumbricals), flexor carpi ulnaris, and flexor digitorum profundus to the ring and little fingers 1, 2
• Inspection for muscle atrophy: Look for wasting of the first dorsal interosseous and hypothenar eminence, and digital clawing 1
• McGowan's grading system: Apply this standardized classification to confirm diagnosis and establish prognosis 1

A focused preoperative physical assessment can identify patients at increased risk for perioperative peripheral neuropathies 4, 5.

Electrodiagnostic Studies (Primary Diagnostic Test)

Nerve conduction studies with electromyography are moderately sensitive and essential for confirming ulnar neuropathy:

• Diagnostic value: Electrodiagnostic studies support the diagnosis, particularly in patients with atypical presentation 5, 6
• Differentiation of injury type: These studies distinguish between demyelinating versus axonal injury patterns, which is secondary to localization but important for treatment planning 6
• Identification of axonal degeneration: Reduced sensory nerve action potential amplitude on electromyography indicates axonal degeneration 6
• Classic finding: The "sural sparing pattern" helps differentiate ulnar neuropathy from other peripheral neuropathies and polyneuropathies 5
• Prognosis assessment: Results determine follow-up monitoring strategies based on location and severity of the lesion 6

Important limitation: While electrodiagnostic studies confirm ulnar neuropathy, their ability to precisely localize the site of nerve injury is often limited 7.

Imaging Studies

MRI (Reference Standard)

MRI without IV contrast using T2-weighted neurography is the reference standard for imaging ulnar nerve pathology:

• Findings: Shows high signal intensity and nerve enlargement at the site of entrapment 5, 6
• Anatomical localization: Provides superior localization of ulnar injury and identification of specific anatomical pathology causing nerve entrapment compared to electrodiagnostic studies 7
• Distinction of entrapment sites: Can reliably distinguish ulnar nerve entrapment under the humero-ulnar arcade (cubital tunnel) from nerve injury at the retro-epicondylar groove 7
• Dedicated neurography sequences: In cases of suspected traumatic nerve injury, MRI with dedicated neurography sequences (e.g., diffusion-weighted imaging) improves visualization of injured nerves 4

Ultrasound (Effective Alternative)

High-resolution ultrasound is an effective alternative with excellent diagnostic accuracy:

• Accuracy: Sensitivity of 77-79% and specificity of 94-98% for assessing cross-sectional area and nerve thickness 5, 6
• Advantages: Provides greater localization of ulnar injury and identification of specific anatomical pathology causing nerve entrapment 7
• Shear-wave elastography: This advanced ultrasound technique has demonstrated 100% specificity and sensitivity for diagnosing ulnar neuropathy at the elbow 5, 6

Radiography

Plain radiographs have limited but specific utility:

• Evaluation of bony abnormalities: Radiographic examination identifies congenital deformities, bony hypertrophy, or structural abnormalities that may contribute to nerve compression 1
• Not for soft tissue diagnosis: Radiographs alone cannot diagnose ulnar neuropathy but help exclude contributing skeletal pathology 1

Algorithm for Test Selection

Follow this structured approach:

1. Start with clinical examination: Perform elbow flexion test, Tinel's sign, sensory testing, and motor strength assessment 1, 2

2. Proceed to electrodiagnostic studies: Order nerve conduction studies with electromyography to confirm the diagnosis and assess severity 5, 6, 7

3. Add imaging for anatomical localization:

   • First choice: High-resolution ultrasound for cost-effectiveness and dynamic assessment 5, 6, 7
   • Alternative: MRI with T2-weighted neurography if ultrasound is inconclusive or surgical planning is needed 5, 6, 7
   • Consider shear-wave elastography: If available, this provides the highest diagnostic accuracy 5, 6

4. Obtain radiographs: If congenital deformities or bony pathology is suspected based on clinical examination 1

Common Pitfalls to Avoid

• Do not rely on clinical tests alone: Clinical tests (sustained head-lift, hand grip, tongue depressor tests) have sensitivities of only 10-30% and positive predictive values less than 50% 4
• Avoid misdiagnosis as "cubital tunnel syndrome": This term is often erroneously applied to all ulnar neuropathies at the elbow; precise localization distinguishes cubital tunnel entrapment from retro-epicondylar groove injury 7
• Do not assume bilateral symptoms indicate nerve entrapment: Diabetes, smoking, alcohol consumption, rheumatoid arthritis, and hypothyroidism are risk factors that typically produce bilateral symptoms and may represent systemic neuropathy rather than focal entrapment 3
• Consider differential diagnoses: Cervical radiculopathy and thoracic outlet syndrome can mimic ulnar neuropathy; electrodiagnostic studies and imaging help differentiate these conditions 3

Monitoring During Procedures

For patients undergoing anesthesia or procedures, quantitative neuromuscular monitoring is essential:

• Ulnar nerve monitoring: The ulnar nerve is the most useful site for neuromuscular monitoring during procedures where neuromuscular blocking drugs are used 4
• Periodic assessment: Upper extremity position should be periodically assessed during procedures to prevent ulnar nerve injury 4, 5, 8
• Alternative sites: If thumb movement is impeded, use electromyography devices, compressomyography, or alternative sites such as facial or tibial nerve 4