What are the possible findings of radiculopathy on electromyography (EMG) and nerve conduction studies?

Updated: Aug 20, 2018
  • Author: Stephen Kishner, MD, MHA; Chief Editor: Jonathan P Miller, MD  more...
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Answer

Radiculopathy is a pathologic process that affects nerves at the root level, often presenting as pure sensory complaints since the sensory fibers are much larger and more easily injured, but sensorimotor or pure motor complaints are also possible. [6] A pure sensory lesion will have negative EMG and sensory NCS results, even though the patient is experiencing a clinical sensory deficit. Other possible findings include normal sensory nerve action potential (SNAP) and a compound motor action potential (CMAP) that is normal to reduced.

An H reflex could be abnormal with an S1 radiculopathy, but F waves are neither sensitive nor specific for a radiculopathy. EMGs show PSWs and fibrillation potentials in at least two different muscles innervated by two separate peripheral nerves with the same root.

Chronology of radiculopathy findings

Upon injury to a nerve root, the patient may begin to report clinical symptoms immediately, but abnormalities in EMG/nerve conduction studies do not appear right away. [3] Thus, although a physician may be tempted to obtain an EMG/nerve conduction study, the electrodiagnostic evidence may be insufficient to confirm or exclude the diagnosis.

A few days after the injury, there may be decreased recruitment and prolonged late responses. After one week, there may be decreased CMAP and some abnormal spontaneous activity in the paraspinals. At 2-3 weeks, abnormal spontaneous activity may be seen in the limbs and paraspinals, and this is when an EMG/nerve conduction study is appropriate. Around 5-6 weeks, reinnervation begins, demonstrated by increased amplitude from the reinnervated motor unit.

Table 1. Findings of Nerve Conduction Studies and EMG in Carpal Tunnel Syndrome [1] (Open Table in a new window)

Severity of Carpal Tunnel

Sensory Nerve Action Potential

Compound Motor Action Potential

Needle EMG Activity

Mild

Prolonged latency

Normal

Normal

Moderate

Prolonged latency and decreased amplitude

Prolonged latency

Normal

Severe

Absent

Prolonged latency and decreased amplitude

Abnormal activity

Table 2. Common Disorders [2] (Open Table in a new window)

Disorder

Distal Motor Latency

Distal Sensory Latency

Conduction Velocity

Amplitude of the Evoked Response

Motor neuron disease

Normal

Normal

Normal

Reduced

Axonal polyneuropathy

Normal

Normal

Slight decrease

Reduced

Demyelinating polyneuropathy

Prolonged

Prolonged

Decreased

Normal

Entrapment neuropathy

Normal (may be prolonged if this is a distal entrapment such as carpal tunnel syndrome)

Normal (May be prolonged if this is a distal entrapment such as carpal tunnel syndrome)

Decreased at the entrapment region

May be decreased when stimulating proximal to the site of entrapment

Radiculopathy

Normal

Normal

Normal

Motor response may be decreased

Myopathies

Normal

Normal

Normal

Decreased motor amplitudes


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