Electrodiagnostic Evaluation of Spinal Stenosis

Anatomy and Physiology

In spinal stenosis, the anteroposterior diameter of the spinal canal is very significant. The average diameter of a cervical spinal cord is usually greater than or equal to 13mm. When there is a diameter noted to be less than 10mm, this is considered significant for cervical spinal stenosis. The average diameter of a lumbar spinal cord is usually greater than or equal to 15mm. When there is a diameter noted to be less than 12mm, this is considered significant for spinal stenosis. [3] 

Radiographic imaging, often magnetic resonance imaging (MRI) or computed tomography (CT), is done to help confirm your suspicion of spinal stenosis after doing a thorough history and physical exam. However, CT myelography is the recommended imaging pre-operatively. [4] There are many congenital or acquired factors why spinal stenosis can occur. Some examples include spondylolisthesis, intervertebral disc herniation, enlargement of the soft tissues in and around the canal, hypertrophy of the facet joints, or ligamentum flavum hypertrophy or laxity.[5][6]

Indications

Clinicians usually diagnose spinal stenosis from a thorough history and physical exam in addition to radiographic and electrodiagnostic studies. Electrodiagnostic studies are usually recommendations as part of the workup; however, not the gold standard for diagnosing spinal stenosis.[7] Electrodiagnostic testing is essential because it helps the physician to rule out other differentials such as lumbar radiculopathy, cervical radiculopathy, peripheral neuropathy, or entrapment neuropathy.[8] Electrodiagnostic findings in most patients, especially those with mild to moderate spinal stenosis, are normal or, at most, equivocal.

Contraindications

Performing electrodiagnostic studies in spinal stenosis patients have few absolute contraindications. Needle EMG is contraindicated in those with severe bleeding disorders. Needles should also never be inserted into areas of active soft tissue infection. NCS is contraindicated in patients with implanted cardiac defibrillators or if connected to external defibrillators. Patients require screening for pacemakers, and electrical stimulation should not be performed directly on or near the device itself. 

Technique or Treatment

Before performing any diagnostic study, a comprehensive review of the patient’s history and clinical course, as well as a complete physical exam, must be performed. The diagnostician will inform the patient bedside of the indications and overview of the studies, which will help diagnose spinal stenosis with electrodiagnostic testing properly. The diagnostician must thoroughly explain the risks and benefits of the exam to the patient and also get consent from the patient. One should ideally examine at least two extremities, performing both sensory and motor nerve conduction studies, as well as EMG needle testing in both proximal and distal muscles for comparison.

As with all nerve conduction studies, the temperature of the patient’s limbs should be kept warm at 32 degrees Celsius. Colder temperatures can cause mistakenly increased amplitudes, prolonged latencies, and slowed conduction velocities on NCS. A notch filter is necessary to minimize electrical interference, which can occur while doing the study bedside. If possible, all unnecessary machines turned off, including unplugging the clinic/hospital bed.

Complications

As with all electrodiagnostic studies in any setting and for any indication- the risk of complications is low. There is always a small risk of bleeding or introducing infection with needle studies.[9]

Clinical Significance

Sensory Nerve Conduction Studies

In a patient with spinal stenosis, sensory nerve conduction studies and amplitudes are usually normal because the sensory fibers travel through the dorsal root ganglion, which is outside the spinal canal.

Motor Nerve Conduction Studies

In a patient with spinal stenosis, motor nerve conduction studies and amplitudes are usually normal. Distal latencies should also be normal as the distal aspect of the nerve is not disturbed. However, if the disease has progressed to the point where axonal damage has occurred, you will see a decrease in amplitude of the CMAP on nerve conduction studies.

Late Responses

If the S1 nerve root is affected, one may find the H-reflexes to be prolonged or absent bilaterally. The F-reflex is routinely not performed in a spinal stenosis study as it does not provide specific information

EMG

For the needle EMG portion of the exam, it is essential to test multiple paraspinal levels bilaterally and multiple myotomes in both extremities. In spinal stenosis, often, there are findings of bilateral multilevel nerve root involvement. If there is a significant narrowing of the vertebral canal in the acute setting, you may find fibrillations and positive sharp waves at different levels bilaterally. If there is significant narrowing of the vertebral canal in the chronic setting, you may find large-amplitude, polyphasic, increased duration motor unit action potentials at different levels bilaterally.[10]

Electrophysiological Recordings

Electrophysiological recordings provide supplementary information to the neurological examination to help increase your suspicion of spinal stenosis. Somatosensory evoked potentials (SSEP) measure the speed of electrical conduction across the spinal cord. When the spinal cord has experienced considerably impinged, the electrical signals will travel slower than usual. Somatosensory evoked potentials (SSEP) can help confirm neurogenic claudication due to cauda equina involvement.[11] This test can help facilitate differentiating neurogenic from vascular claudication or musculoskeletal disorders of the lower extremities. Findings can aid physicians in differentiating patients whose symptoms are caused by degenerative spine disease with referred pain from true neurogenic spinal stenosis.[12][13]

Enhancing Healthcare Team Outcomes

Lumbar spinal stenosis is a condition often seen in the outpatient setting. Patients frequently come with complaints of pain and/or paresthesias in the extremities. Imaging studies, in addition to electrodiagnostic studies, are routinely ordered. Physicians must be cautious, as imaging findings might not correlate with the patient's symptoms. It is essential to take an interprofessional team including a team of physicians (physical medicine and rehabilitation, pain management, orthopedist, and/or neurosurgeons), physical therapists, occupational therapist, social workers, and case managers who can work together to coordinate mobilization with outpatient therapy and aggressive multifaceted rehabilitation so we can improve a patient's functional status. Typically there is a long and challenging recovery ahead for patients with spinal stenosis, but with a coordinated effort between the various medical disciplines and departments, the best outcomes for patients are achievable.[14]