# Radiculopathy: Electrodiagnostic Localization

## Overview

### Definition and Role of Electrodiagnosis
Radiculopathy is dysfunction of a spinal nerve root causing motor, sensory, and/or reflex changes in the corresponding myotome and dermatome. Electrodiagnosis confirms physiologic nerve root dysfunction, determines severity and chronicity, and excludes mimicking conditions. NCS/EMG is the only diagnostic test that assesses the physiologic function of the nerve root (MRI shows anatomy only). Electrodiagnosis and MRI are complementary; abnormal MRI findings are frequently asymptomatic.

### Key Electrodiagnostic Principle
Radiculopathy is a preganglionic lesion: the dorsal root ganglion (DRG) lies within or just distal to the neural foramen. Because the DRG and its distal sensory axon are intact, the **SNAP is preserved** despite clinical sensory loss. This is the single most important electrodiagnostic feature distinguishing radiculopathy from plexopathy or peripheral neuropathy. Motor fibers originate from anterior horn cells within the spinal cord; when root damage affects motor axons, CMAP amplitude and EMG will be abnormal.

## Electrodiagnostic Protocol for Cervical Radiculopathy

### Sensory NCS
Median, ulnar, and radial sensory studies should be normal in pure radiculopathy. Abnormal SNAPs suggest plexopathy, peripheral neuropathy, or a combined lesion. Median sensory comparison studies should be included to screen for concurrent CTS (common coexistence).

### Motor NCS
Median motor (APB): C8-T1 contribution. Ulnar motor (ADM, FDI): C8-T1 contribution. CMAP amplitudes may be reduced in severe motor axon loss at C8-T1. Most cervical radiculopathies (C5-C7) do not significantly affect standard motor NCS because the recorded muscles have multi-root innervation.

### Needle EMG: Cervical Myotomal Screen

| Root | Key Muscles | Peripheral Nerve |
|------|-------------|-----------------|
| C5 | Deltoid, biceps, infraspinatus, rhomboids | Axillary, musculocutaneous, suprascapular, dorsal scapular |
| C6 | Biceps, brachioradialis, pronator teres, FCR | Musculocutaneous, radial, median |
| C7 | Triceps, pronator teres, FCR, EDC | Radial, median |
| C8 | FDI, ADM, FDP (2,3), FPL, EIP | Ulnar, median, radial (PIN)  |   |  T1  |  FDI, ADM, APB  |  Ulnar, median |

Sample at least 2 muscles from different peripheral nerves for the suspected root level. Include a muscle above and below the suspected level to define the extent. **Cervical paraspinal muscles**: fibrillations support radiculopathy but may be absent in foraminal lesions or present in spondylosis without radiculopathy.

## Electrodiagnostic Protocol for Lumbosacral Radiculopathy

### Sensory NCS
Sural and superficial peroneal sensory SNAPs should be normal (preganglionic lesion). Abnormal SNAPs suggest plexopathy (lumbosacral plexus) or peripheral neuropathy. Exception: L5 DRG may be intraspinal in some individuals, making the lesion potentially postganglionic.

### Motor NCS
Peroneal motor (EDB): L5-S1; may show reduced CMAP in severe L5 or S1 radiculopathy. Tibial motor (AH): S1-S2; may show reduced CMAP in S1 radiculopathy. F-waves: may be prolonged or absent with proximal motor axon involvement.

### H-Reflex
Tibial nerve H-reflex (recording from soleus) assesses the S1 reflex arc. Side-to-side latency difference >1.5 ms or absence on one side supports S1 radiculopathy. Most useful in acute S1 radiculopathy when needle EMG may still be normal. Not root-specific beyond S1 in routine clinical practice. Can be abnormal in polyneuropathy and sciatic neuropathy (non-specific).

### Needle EMG: Lumbosacral Myotomal Screen

| Root | Key Muscles | Peripheral Nerve |
|------|-------------|-----------------|
| L2-L3 | Iliacus, adductor longus, vastus medialis | Femoral, obturator |
| L3-L4 | Vastus medialis, vastus lateralis, tibialis anterior | Femoral, deep peroneal |
| L4 | Tibialis anterior, vastus lateralis, tensor fasciae latae | Deep peroneal, femoral, superior gluteal |
| L5 | Tibialis anterior, EHL, peroneus longus, tibialis posterior, gluteus medius | Deep peroneal, superficial peroneal, tibial, superior gluteal |
| S1 | Medial gastrocnemius, lateral gastrocnemius, biceps femoris (short head), gluteus maximus | Tibial, peroneal (sciatic), inferior gluteal |

L5 radiculopathy is the most common lumbosacral level affected. **Lumbosacral paraspinal muscles**: fibrillation potentials in the multifidus support radiculopathy. Paraspinal mapping (mini-PM technique) systematically samples multiple levels. Paraspinal fibrillations may be absent after posterior spine surgery (muscle disruption).

<image>Anatomical illustration of the lumbosacral spine showing L4, L5, and S1 nerve root exits with their corresponding myotomal muscle innervation patterns, demonstrating the principle that each muscle receives innervation from multiple roots and each root innervates muscles via different peripheral nerves</image>

## Timing of Electrodiagnostic Abnormalities in Radiculopathy

### Temporal Sequence After Acute Root Injury
**Week 1**: NCS and EMG may be entirely normal; H-reflex may be abnormal (S1). **Week 1-2**: paraspinal fibrillations may appear (shortest axon distance from root to muscle). **Week 2-3**: fibrillations appear in proximal limb muscles. **Week 3-6**: fibrillations appear in distal limb muscles.

**Weeks to months**: neurogenic MUAP changes develop as reinnervation occurs. **Optimal timing**: 3-4 weeks after symptom onset for maximum diagnostic sensitivity.

### Implications of Timing
Negative EMG in the first 2 weeks does not exclude radiculopathy. Paraspinal fibrillations appear first and resolve first (with successful reinnervation). Chronic radiculopathy may show MUAP changes without fibrillations (completed reinnervation). Active fibrillations in the setting of chronic MUAP changes suggest ongoing or recurrent denervation.

## Sensitivity and Limitations

### Sensitivity of EMG for Radiculopathy
Overall sensitivity: approximately 50-85% (varies by study and severity). Higher sensitivity for motor-predominant radiculopathy. Lower sensitivity for purely sensory radiculopathy (no motor axon loss to detect). Sensitivity depends on: timing of study, severity of root compression, number of muscles sampled.

### Limitations
Cannot detect pure sensory radiculopathy (SNAPs are normal in radiculopathy). Cannot identify the specific structural cause (disc herniation, foraminal stenosis, etc.). Paraspinal EMG is nonspecific (false positives from spondylosis, prior surgery, age-related changes). Multi-root innervation of muscles limits precision of single-root localization.

Mild or intermittent compression without axonal loss will produce a normal study. Negative EMG does not exclude radiculopathy.

### Comparison with MRI
MRI has higher sensitivity for structural pathology but lower specificity (many asymptomatic disc herniations). EMG has higher specificity for physiologically significant root dysfunction. Concordance between MRI and EMG findings strengthens diagnostic confidence. Discordance should prompt careful clinical correlation.

<image>Timeline diagram showing the sequential appearance of electrodiagnostic abnormalities after acute radiculopathy from week 0 to 6 months, including H-reflex changes, paraspinal fibrillations, proximal then distal limb fibrillations, and development of chronic neurogenic MUAP changes with reinnervation</image>

## Differential Diagnosis

### Plexopathy
SNAPs are abnormal (postganglionic lesion) -- the key distinguishing feature. Multiple root and peripheral nerve territories affected. Paraspinal muscles are normal (plexus is distal to the paraspinal innervation). Consider: diabetic amyotrophy (lumbosacral radiculoplexus neuropathy), tumor, radiation, trauma.

### Peripheral Neuropathy (Mononeuropathy)
Abnormal SNAPs in the distribution of the specific nerve. Abnormalities confined to muscles innervated by one peripheral nerve (not one root). Example: peroneal neuropathy at the fibular head mimics L5 radiculopathy but spares tibialis posterior and gluteus medius.

### Motor Neuron Disease
Fibrillations and neurogenic MUAPs in multiple myotomes across multiple limbs. Sensory NCS are entirely normal. Upper motor neuron signs on examination. No sensory symptoms.

### Polyradiculopathy
Multiple root levels affected bilaterally. Consider: spinal stenosis, CIDP, leptomeningeal disease, epidural abscess.

## Clinical Pearls

The preserved SNAP in radiculopathy is the most high-yield electrodiagnostic concept for board examinations. Always sample muscles from at least two different peripheral nerves at the suspected root level to confirm radiculopathy (cannot localize to a root with muscles from only one nerve). L5 radiculopathy is the most commonly encountered lumbosacral radiculopathy; the tibialis posterior (tibial nerve, L5) is a critical muscle to test to distinguish L5 radiculopathy from peroneal neuropathy. The H-reflex is the most sensitive early test for S1 radiculopathy.

Paraspinal fibrillations are supportive but not required for the diagnosis; they may be absent in foraminal stenosis or after surgery. A "chronic only" pattern (neurogenic MUAPs without fibrillations) may represent old, resolved radiculopathy or ongoing stable compression with compensated reinnervation. Bilateral S1 radiculopathy from central disc herniation or spinal stenosis can prolong H-reflexes bilaterally; compare to normative values rather than relying solely on side-to-side comparison. The electrodiagnostic study must be planned and interpreted in the context of the clinical examination; it is not a screening test.

## References

- AANEM Practice Parameter: Needle EMG in Lumbosacral Radiculopathy. Muscle Nerve. 1999;22(Suppl 8):S209-S211.
- Wilbourn AJ, Aminoff MJ. AAEM Minimonograph #32: The Electrodiagnostic Examination in Patients with Radiculopathies. Muscle Nerve. 1998;21(12):1612-1631.
- Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th Edition. Elsevier. 2021.
- Tsao B. The Electrodiagnosis of Cervical and Lumbosacral Radiculopathy. Neurol Clin. 2007;25(2):473-494.
- Fisher MA. Electrophysiology of Radiculopathies. Clin Neurophysiol. 2002;113(3):317-335.