Electrodiagnostic Evaluation of Cervical Radiculopathy

Anatomy and Physiology

Patients with cervical radiculopathy commonly complain of neck pain, with radiation to the arm. This is often accompanied by numbness and tingling caused by the impinged sensory nerve's dermatome distribution. If there is motor nerve involvement, patients will also complain of weakness in the involved nerve's myotomal distribution.[2]

On the physical exam, it is essential to test motor strength, sensation, and reflexes bilaterally. On inspection, it is necessary to evaluate muscle atrophy as well. Although not every patient will exhibit the same symptoms for radiculopathies, it is common to see decreased reflexes related to the affected nerve root, weakness in muscles innervated by the affected nerve root, and sensory symptoms in the dermatomal pattern. Diminished reflexes, particularly involving the triceps by a C7 nerve root involvement, are the most common neurological finding.[3] These findings come with limitations due to the subjective nature of the exam. However, electromyography (EMG) can show a change when there is a small percent loss of motor axons, which can be missed on a physical exam.

A thorough understanding of anatomy is essential for radiculopathy evaluation. Spinal nerves are sensory and motor nerves mixed, which form the dorsal and ventral nerve rootlets distal to the dorsal root ganglion in intervertebral foramina. After the nerve exits the foramen, it divides into dorsal and ventral ramus. The ventral ramus forms the cervical and brachial plexuses that supply the extremity. The dorsal ramus innervates the paraspinal muscles and the skin of the neck and back of the neck with an overlapping innervation.[4][5] The dorsal root ganglia contain the cell body of the sensory axons. Each cervical root exits at the foramen superior to the corresponding vertebrae down to the C8 nerve root.

Indications

The diagnosis of cervical radiculopathy can be made based on a thorough history and physical exam. Electrodiagnostic evaluation can be used to confirm the diagnosis and to exclude peripheral nerve diseases that mimic radiculopathy. Electrodiagnostic findings are useful for patients with atypical symptoms, those who present weakness secondary to pain, and those where the imaging findings do not show a focalizing lesion.[4] Before performing any diagnostic study, a comprehensive review of the patient's history and clinical course, and a complete physical exam must be performed. 

It is essential to inspect for muscle atrophy on physical exam, test for upper extremities' motor strength and sensation, and test bilateral deep tendon reflexes. Special tests such as the Spurling test, where the neck is extended, and side bent to apply pressure onto the exiting nerve root can also aid in diagnosis if it reproduces the neuropathic radiating symptoms.[4] Spurling test on its own is specific but has a low sensitivity for cervical radiculopathy; however, when combined with other tests such as axial traction, a positive test increase the likelihood of cervical radiculopathy.[4][6] Hoffman sign and Babinski response indicate an upper motor neuron disorder. Electrodiagnostic studies in cervical radiculopathy can only evaluate the C4-C8 and T1 roots as they are the only with representation in the upper extremity. The C7 nerve root is the most common radiculopathy, followed by the C6 nerve root.

Contraindications

Performing electrodiagnostic studies in patients with cervical radiculopathy have a few absolute contraindications. Needle EMG is contraindicated in those with severe bleeding disorders.[7] Needles should also never be inserted into areas of active soft tissue infection. Nerve conduction studies (NCS) are contraindicated in patients with implanted cardiac defibrillators or connected to external defibrillators. The electrical stimulation should not be performed directly or near the device.

Equipment

The equipment used in electrodiagnostic studies for cervical radiculopathy include:

• EMG/NCS machine
• Needle electrodes
• Surface electrodes (active, reference, and ground)
• Amplifiers
• Filters

Technique or Treatment

The physiatrist/technician orients the patient on the indications of the electrodiagnostic studies that will be performed to confirm the diagnosis of cervical radiculopathy and get consent after complications are explained. EMG needle testing and sensory/motor NCS in proximal and distal muscles are performed bilaterally to compare findings.[8] The extremity temperature should be at or above 32 degrees Celsius, as colder temperatures can increase amplitudes, prolonged the latencies, and slowed conduction velocities. A notch filter should be used to reduce electrical interference. Unnecessary equipment in the room must be turned off.

For electrodiagnostic studies, there are three electrodes used to record nerve potentials. These are the active electrode, reference electrode, and ground electrode. The active electrode is placed, covering the muscle belly directly. The reference electrode is placed on a nearby tendon or bone, approximately three to four centimeters from the tested muscle. The ground electrode is important to minimize artifact when recording potentials. It is placed between the stimulating and recording electrode.

For the needle exam, the needle should be inserted quickly into the muscle belly. Monitor for exertional activity. Electrical stimulation occurs via a cathode and anode, along with the use of conduction gel. A cathode is a negative pole, and an anode is a positive node. Electrical impulses are created by stimulating the tested nerve, and the action potential is recorded onto the screen. Several factors are measured on the potential created, such as amplitude and onset latency. When testing for cervical radiculopathy, six muscle groups are tested, where one must be a cervical paraspinal muscle.

Complications

The risk of complications is low in electrodiagnostic studies.[9] A few complications that can occur include:

• Infection from the needle puncture
• Bleeding from the needle puncture
• Discomfort (if significant to the patient, the test should be terminated)

Clinical Significance

Sensory Nerve Conduction Studies

Patients with cervical radiculopathy will often present with pain, numbness, and tingling in the upper extremities. In most radicular processes, the sensory nerve action potential will be normal in amplitude and latency. This occurs because the compression at the nerve root occurs proximal to the sensory dorsal root ganglion.[4][5] If an anomaly is noted on the exam, an entirely different diagnosis or a coexisting pathology may be the cause. In cervical radiculopathy, this may be carpal tunnel syndrome or ulnar nerve entrapment. In the setting of a sensory NCS with abnormalities, it is important to consider other lesions such as peripheral nerve entrapment, peripheral neuropathy, cervical stenosis, or brachial plexopathy.

Most common sensory conduction areas evaluated:

• Median thumb C6
• Median index C6–C7
• Radial thumb C6–C7
• Ulnar little finger C8

Motor Nerve Conduction Studies

Compound muscle action potential amplitudes are a measure of motor fiber activation upon muscle stimulation. Latency is a measure of the speed of the fastest muscle fibers. As with sensory NCS, the motor NCS will be normal in cervical radiculopathy. This is because only a fraction of nerve fascicles within the nerve root are affected.[5] However, if there is sufficient root compression where distal parts of axons are not continuous with the cell body, axon loss ensues. This is called Wallerian degeneration. If more than 50% of the motor axons are injured, the amplitude may be reduced compared with the contralateral limb.[5] While NCS may not directly point to cervical radiculopathy, it is important to test them to consider other differential diagnoses.

Most common motor conduction areas evaluated:

• Axillary (deltoid) C5–6
• Musculocutaneous (biceps) C5–6
• Ulnar (hypothenar) C8
• Median (thenar) T1>C8

Electromyography Needle Study

Electromyography is useful in localizing cervical radiculopathy and predicting prognosis. Only sensory changes and demyelinating components are usually found in early cervical radiculopathy; thus, EMG will not detect acute radiculopathy. As discussed earlier, if severe radiculopathy occurs, Wallerian degeneration may occur. This will show fibrillation potentials and positive sharp waves on an EMG due to muscle fibers spontaneously firing. This has a characteristic visual appearance and a distinct auditory sound as well. Spontaneous activity is the most objective evidence of a nerve lesion. Fibrillation potentials are seen first at the more proximal muscles (paraspinal muscles), and then at the limb muscles.[5] Polyphasic motor units may be present if reinnervation has occurred with the sprouting of new collateral axons. However, the diagnosis of cervical radiculopathy should not be solely made by the presence of polyphasic potentials.[10]

Spontaneous activity of the paraspinal muscles is often seen on EMG within the first five to seven days.[11] However, some studies have shown this can extend up to six weeks for more distal portions of limbs. Reinnervation is most commonly seen most proximally with paraspinal muscles first and then progressing to distal muscles in the extremity with time.[5] Fibrillation potentials may be seen in normal individuals' cervical paraspinal muscles in up to 12%, and are nonspecific for cervical radiculopathy. Paraspinal muscles fibrillations and positive sharp waves are also found in motor neuron disease and inflammatory myopathies.[4] In the post-spinal surgery setting, fibrillation potentials have no diagnostic significance as they remain abnormal for years after the surgery. Abnormalities in the paraspinal muscles can differentiate radiculopathy from plexopathy as the proximal dorsal rami innervate them before the plexus is formed.

Specific muscle testing is essential for cervical radiculopathy. The selection of muscles is critical to determine if a lesion is at the root, plexus, or peripheral nerve entrapment. The physical exam is important in selecting muscles, particularly muscles that appear weak on the exam. To definitively diagnose cervical radiculopathy, one paraspinal muscle and two muscles from a different peripheral nerve innervated by the same root must have positive EMG findings.[4][11] This criterion must be met entirely for a diagnosis of cervical radiculopathy. A negative EMG does not rule out cervical radiculopathy.

Most common muscles tested:

• Brachioradialis C5
• Deltoid C5–6
• Biceps C5–6
• Flexor carpi radialis C6
• Triceps C6–7
• First dorsal interosseous C8-T1
• Flexor pollicis longus C8
• Abductor digiti minimi C8-T1

Each root will have specific muscles to be tested and with typical NCS.[4][5]

• C5 Radiculopathy: Primary innervation of the deltoid and biceps is from the C5 root with secondary innervation from the C6 root. Sensory NCS can not reliably assess the C5 nerve root. If the rhomboids show EMG changes on the exam, the C5 root is probably affected as the C5 root primarily innervates it with a small contribution from the C4 root.
• C6 Radiculopathy: The C5 and C6 nerve roots provide nearly equal innervation to the brachioradialis. The C6-C8 roots supply the triceps. The median-innervated thumb and lateral antebrachial cutaneous are examined for sensory NCS recording. There is no reliable motor NCS to be recorded.
• C7 Radiculopathy: The triceps, flexor carpi radialis, and pronator trees are typically affected with C7 radiculopathy. There is no reliable motor NCS to be recorded.
• C8 Radiculopathy: The abductor digiti minimi and flexor pollicis longus are typically affected with C8 radiculopathy. The first dorsal interosseous has a more substantial contribution from the T1 nerve root. The motor NCS recording is performed from the first dorsal interosseous.
• T1 Radiculopathy: The first dorsal interosseous, abductor pollicis brevis, and opponens pollicis are affected with T1 radiculopathy.

Enhancing Healthcare Team Outcomes

Cervical radiculopathy is a relatively common condition seen in the outpatient setting, particularly after trauma or a fall. Patients will present with symptoms of pain, numbness, or tingling. A standard workup includes imaging such as cervical x-rays, cervical computed tomographic (CT) scan, cervical magnetic resonance imaging (MRI), and an electrodiagnostic workup. MRI is the gold standard to evaluate herniated disc. It is also beneficial for identifying other pathologies like tumors, infections, and demyelinating disorders. A cervical CT scan is the study of choice for distinguishing an osteophyte's presence compressing the nerve root, especially if combined with myelography. Imaging solely is not sufficient in diagnosing cervical radiculopathy. Electrodiagnostic studies, along with a thorough history and physical exam, are important to diagnose a patient.

The care of a patient with cervical radiculopathy involves several medical professionals, including physiatrists, neurologists, neurosurgeons, physical therapists, occupational therapists, social workers, and case managers. Cervical radiculopathy is initially managed conservatively with physical and occupational therapists' assistance, which typically resolves 75-90% of patients' symptoms.[12] Those who still feel pain can consider epidural steroid injections by pain management specialists, or surgical anterior or posterior discectomy by neurosurgeons.

Studies have shown a communication gap between the patient's imaging results and actual clinical findings. Abnormal findings on cervical MRI can be shown without clinical symptoms, thus not needing an intervention.[13] Coordinated care and strong interprofessional communication between healthcare professionals and the patient are essential to maximizing outcomes.