The Utility of Electrodiagnostic Testing in Patients with Suspected Lumbar Radiculopathy

The intended audience of this article is healthcare providers who refer patients for EMG & NCV testing to assist with the diagnosis of lumbar radiculopathy.  This information may also be useful for patients with symptoms of radiating low back pain looking for information on their problem.

Patient number one is a 33-year-old female who has low back pain radiating to the left posterior lower leg down to the calf with tingling in the left calf.  The onset of these symptoms came after a motor vehicle accident.  Patient’s medical history includes a cesarean section and anemia.  An MRI performed two months after onset of symptoms was positive for a broad-based central L4-5 disc herniation indenting the ventral thecal sac.  The physical examination was normal.  Electrodiagnostic test consisting of nerve conduction studies and needle electromyography that was performed six months after the onset of symptoms revealed no abnormalities.

Patient number two is a 39-year-old male with a chief complaint of pain, numbness, and tingling in the bilateral lower extremities with greater involvement of the left leg.  He also reports muscle cramping on the inside of the legs.  He denies any significant previous medical history.  Patient notes onset of symptoms after a motorcycle accident.  An MRI of the lumbar spine performed 10 months after the accident was positive for multilevel chronic lumbar and joint disease with some moderate stenosis most remarkable at L3-4 and L4-5.  The physical examination was normal except for diminished sensation to light touch in the left dorsum of the foot and a positive nerve provocation test at the left fibular neck.  Electrodiagnostic test consisting of nerve conduction studies and needle electromyography that was performed one year after the motorcycle accident revealed isolated compromise of the left fibular nerve in the area between the back of the knee and fibular head with no evidence of motor axonal compromise of the nerve roots.

Patient number three is a 57-year-old male with complaints of numbness in the right anterior thigh that travels to the inside of the knee and weakness of the leg, reporting dragging of the foot during walking.  He reports only being able to sit for 40 minutes or stand for 10 minutes without discomfort.  His medical history includes prediabetes, high cholesterol that is managed with medication, coronary arterial sclerosis, angioplasty, cardiac stents, high blood pressure, and prior episode of radiating low back pain that resolved one year ago.  An MRI performed one month after onset of current symptoms revealed a stable, mild central canal stenosis at L3-L4 and L4-L5.  The physical examination of the right lower extremity revealed muscle wasting of the right quadriceps, decreased strength of the right knee extension and right ankle dorsiflexion, diminished patella reflex compared to the other side and a positive slump test in the right leg.  An electrodiagnostic test performed two months after onset of current symptoms showed evidence of acute mild motor axonal radiculopathic process of the right L4 nerve root and a chronic mild motor axonal radiculopathic process of the right L5 nerve root.  Nerve conduction studies of the right lower extremity were normal while needle EMG of the right quadriceps and abductor muscles demonstrated abnormal active spontaneous potentials and the right anterior tibialis and posterior tibialis demonstrated chronic motor unit morphological changes.

The above three actual cases encountered by the author of this article demonstrate the complex, confusing, and sometimes frustrating nature of electrodiagnostic examinations in patients with suspected lumbar radiculopathy.  The purpose of this article is to help explain the reason for unexpected results of electrodiagnostic examinations in patients with signs and symptoms of lumbar radiculopathy, to demonstrate how an electrodiagnostic test outcome can assist with the management of a patient regardless of the abnormality of the findings, and to discuss the components of a thorough and valuable electrodiagnostic exam.

To help explain the confusion we should review some established definitions.  Low back pain is pain occurring posteriorly in the region between the lower rib margin and the proximal thigh.¹  Low back pain is the most common reason for people to seek healthcare with the highest overall costs in the nation.² Lumbar radicular pain is neuropathic pain caused by a pathology of the sensory lumbar nerve roots that results in radiating pain in a lumbar dermatomal pattern.³  Approximately 13% to 40% of people will experience this during their lifetime.³  Lumbar radiculopathy is a pathology of the lumbar nerve root that produces a range of signs and symptoms including sensory disturbances, numbness, and motor deficit. ³ The term sciatica is often used synonymously with lumbar radiculopathy.  Radiculopathy can occur in the absence of radicular pain and radicular pain can occur in the absence of a radiculopathy.⁴  The prevalence of lumbar radiculopathy in outpatient populations has been reported to be between 3% and 5%.⁵  The incidence of radicular symptoms in patients with low back pain is between 12% to 40%.⁵ While the dysfunction associated with lumbar radiculopathy can be significant, it has been reported that the pain and related disabilities from lumbar radiculopathy typically resolve in about two weeks.⁶  Only 30% of patients with lumbar radiculopathy will continue to have signs for longer than one year in duration. ⁶ Patients with lumbar radiculopathy will often have abnormal findings on clinical examination that include impaired sensation, weakness, and diminished reflexes.⁷ Patients will often be referred for diagnostic tests that include MRIs and electrodiagnostic studies.  The consensus in the published literature is that electrodiagnostic tests have modest sensitivity at detecting lumbar radiculopathy. ⁷ The reason for the moderate sensitivity is because for the radiculopathy to affect parameters that are detectable by the electrodiagnostic exam there needs to be motor neuronal cellular involvement, axon fiber loss that is detectable by needle EMG and occur within a time dependent period to demonstrate observable electrophysiological changes.  Radiculopathy is a common referring diagnosis for electrodiagnostic (EDX) testing; however, up to 60% of EDX tests have normal results. ⁸ Electrodiagnostic testing is not recommended as a standard procedure in the diagnostic workup of a patient with suspected lumbar radiculopathy but can be most helpful in patients with normal MRI results or otherwise unexplained leg pain.  The American Academy of Neuro-Electrodiagnostic Medicine (AANEM) suggest not to perform needle EMG testing for isolated back pain after motor vehicle accidents as needle EMG is unlikely to be helpful.⁹ The AANEM also advises against performing nerve conduction studies without needle EMG. ⁹

The physical examination will often contain clinical signs that will predict an abnormal electrodiagnostic test. ⁷ Patients with impaired sensation of the anterior thigh or medial calf and foot; weakness of the hip flexors, hip adductors, or knee extensors; and diminished patella reflexes should be considered for L3 or L4 radiculopathies.  Patients with impaired sensation of the dorsal foot and great toe or lateral calf with weakness of the ankle and toe extensors, ankle inverters or evertors and hip abductors should be considered for L5 radiculopathies.  Patients with impaired sensation of the lateral foot posterior calf or plantar foot; weakness of the ankle plantar flexors, knee flexors, and/or hip extensors; or abnormal Achilles’ tendon reflex may have an S1 radiculopathy.  The signs that increase the likelihood of electrodiagnostic confirmation of lumbar radiculopathy are muscle weakness not due to pain, and loss of reflex.¹⁰ The symptoms with little predictive value regarding abnormal electrodiagnostic results are tingling, burning, and pain even when found in a single nerve root distribution. ¹⁰ The physiological reason for the predictability of clinical signs and symptoms is that the needle EMG exam primarily reflects integrity of the motor axons. For patients with symptoms or signs relating to muscle strength/contraction, needle EMG exam is the most useful component of the electrodiagnostic test at ruling in a radiculopathy.  In patients with signs and symptoms compatible with weakness of a specific nerve root level, as many as 82% had an abnormal EMG exam.¹¹ However, patients with clear sensory symptoms such as pain, paresthesia, or anesthesia of a specific dermatomal pattern and otherwise normal physical examination only had abnormal electrodiagnostic test results 41% of the time. ¹¹

During an electrodiagnostic examination there are many possible nerve conduction studies and needle EMG examination of muscles that can be performed.  The recommended protocol for the electrodiagnostic examination of patients with suspected lumbar radiculopathies has been established. Nerve conduction studies should consist of the fibular motor and tibial motor nerves and at least one sensory study in the distribution of sensory symptoms which could be either the saphenous, superficial fibular, or sural nerves. ¹²  Of the two late responses that are commonly performed in electrodiagnostic studies, the tibial H-reflex can aid in the confirmation of an S1 radiculopathy when the rest of the nerve conduction studies are normal, while the F-waves of the fibular and tibial nerves have been shown to have low sensitivity at detecting nerve root compromise.¹³ To minimize patient discomfort while optimizing diagnostic accuracy the electromyographer should first examine muscles in the relevant myotome of weakness in the limb. ¹² The muscles of the suspected myotome to be examined should be innervated by at least two different peripheral nerves. ¹² If abnormalities are found, the electromyographer should examine muscles in adjacent myotomes to exclude a widespread or diffuse lesion. ¹² If the findings are mild or equivocal, comparison with the contralateral asymptomatic muscle should be considered. ¹² In post spinal surgery settings, spontaneous potentials observed in the paravertebral muscles have less diagnostic significance. ¹² A muscle is considered significantly abnormal if neuropathic findings are present.⁷ Neuropathic findings include sustained spontaneous potentials, large amplitude, long duration motor unit action potentials, a significant amount of polyphasic motor unit action potentials, or reduced recruitment with fast firing motor units. ⁷ Needle EMG can rule in radiculopathy if neuropathic findings are present in two or more muscles innervated by the same nerve root with different peripheral nerves and muscles innervated by adjacent nerve roots are normal. ⁷ Certain muscles have been shown to have higher optimization at detecting involvement of that nerve root when compared with surgical findings.  The adductor longus and vastus lateralis are optimal for detection of L4 radiculopathies.¹⁴ The fibularis longus, tensor fascia latae, and posterior tibialis are most useful to rule in L5 radiculopathy. ¹⁴ The biceps femoris short head and gastrocnemius are often abnormal in patients with S1 radiculopathy. ¹⁴

While abnormal EMG that rules in lumbar radiculopathy signifies motor axonal denervation of the muscle fibers, the implications of a positive electrodiagnostic test for radiculopathy is generally favorable for outcomes.  Persons receiving conservative management including rehabilitation and epidural corticosteroid injections have demonstrated a better clinical response when electrodiagnostic tests have localized nerve root involvement to one level.¹⁵ In people who undergo spinal surgery, a positive electrodiagnostic exam performed prior to surgery is also associated with improved postsurgical outcomes. ¹⁵ In addition, when an electrodiagnostic exam for a patient with signs and symptoms of a lumbar radiculopathy does not have evidence of a radiculopathy but does rule in specific nerve compromise or polyneuropathic involvement, the patient can then receive the proper treatment to address those areas of deficit.  Lastly when the electrodiagnostic test of a patient with suspected lumbar radiculopathy is normal, rehabilitation can focus on possible musculoskeletal causes of dysfunction.  As with all diagnostic tests, the electrodiagnostic test itself is neither good nor bad but instead it is good when used appropriately and bad when utilized for patients who do not demonstrate signs or symptoms consistent with lumbar radiculopathy.

John Lugo, PT, DPT, ECS

EMG Solutions Clinical Education and Residency Coordinator

References: 

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