What muscles should be sampled during Electromyography (EMG) for C5, C6, C7, and C8/T1 radiculopathy?

EMG Muscle Sampling for Cervical Radiculopathy

For diagnostic EMG evaluation of cervical radiculopathy, sample the following muscles based on the suspected root level: C5 (deltoid, infraspinatus, biceps, brachioradialis), C6 (pronator teres, extensor carpi radialis longus/brevis), C7 (triceps, flexor carpi radialis, pronator teres), and C8/T1 (first dorsal interosseous, abductor digiti minimi, abductor pollicis brevis, flexor pollicis longus). 1, 2

C5 Radiculopathy - Specific Muscles to Sample

• Deltoid muscle - shows weakness or denervation potentials in 100% of surgically confirmed C5 radiculopathy cases 1, 2
• Infraspinatus muscle - demonstrates abnormalities in 100% of C5 cases 1, 2
• Biceps brachii - affected in 90% of C5 radiculopathy patients 1, 2
• Brachioradialis - shows denervation in approximately 89% of C5 cases 1, 2

The C5 pattern is relatively stereotyped with consistent involvement of these shoulder girdle and proximal arm muscles. 2

C6 Radiculopathy - Specific Muscles to Sample

• Pronator teres - demonstrates weakness and/or denervation in 100% of surgically confirmed C6 cases 1, 2
• Extensor carpi radialis longus (ECRL) - shows denervation potentials in C6 radiculopathy 1
• Extensor carpi radialis brevis (ECRB) - affected in 100% of C6 cases based on EMG findings 1

Critical caveat: C6 radiculopathy has the most variable EMG presentation among cervical roots - approximately 50% of patients show patterns similar to C5 (with pronator teres involvement distinguishing them), while the other 50% show patterns identical to C7 radiculopathy. 2 This variability makes C6 the most challenging root level to diagnose electrodiagnostically.

C7 Radiculopathy - Specific Muscles to Sample

• Triceps brachii - shows weakness and/or denervation in 100% of surgically confirmed C7 cases 1, 2
• Flexor carpi radialis (FCR) - demonstrates denervation potentials in 100% of C7 radiculopathy 1, 2
• Pronator teres - affected in approximately 22% of C7 cases, showing overlap with C6 1, 2
• Anconeus - consistently involved in C7 radiculopathy 2
• Extensor digitorum - shows weakness/denervation in approximately 44% of C7 cases 1

The C7 pattern is relatively stereotyped with consistent involvement of these muscles. 2

C8/T1 Radiculopathy - Specific Muscles to Sample

• First dorsal interosseous - demonstrates fibrillation potentials in surgically confirmed C8 radiculopathy 2
• Abductor digiti minimi - consistently affected in C8 root lesions 2
• Abductor pollicis brevis - shows denervation in C8 radiculopathy 2
• Flexor pollicis longus - involved in C8 root compression 2
• Extensor indicis proprius - demonstrates abnormalities in C8 cases 2

The C8 pattern is relatively stereotyped with consistent involvement of intrinsic hand muscles. 2

Key Technical Considerations

• Needle EMG is the most important electrodiagnostic test for radiculopathy, with modest sensitivity but high specificity that complements spine imaging. 3
• Only classify studies as abnormal based on fibrillation potentials (either positive sharp waves or biphasic spike forms) on needle electrode examination, not based on other EMG parameters. 2
• EMG has a sensitivity of approximately 80% and specificity of 70% for radiculopathy diagnosis. 4

Important Clinical Pitfalls

• Do not rely solely on a single muscle - the diagnosis requires a pattern of involvement consistent with a specific root level rather than isolated muscle abnormalities. 2, 3
• C6 radiculopathy requires particular caution due to its highly variable presentation that can mimic either C5 or C7 patterns; pronator teres involvement helps distinguish C6 from C5. 2
• Overlap exists between adjacent root levels - ECRL shows denervation in both C5 and C6 cases, ECRB in C5/C6/C7, and pronator teres in both C6 and C7. 1
• Preoperative EMG has mixed utility in predicting surgical outcomes and should be used primarily for diagnostic localization rather than prognostication. 5