Electrodiagnostic Evaluation of Lumbosacral Radiculopathy

Anatomy and Physiology

Eleven pairs of spinal nerves (5 lumbar, 5 sacral, and 1 coccygeal) emerge from the spinal cord in the lumbosacral region of the spine.[3] They exit the spinal cord as dorsal and ventral nerve roots, carrying sensory and motor axons. As the tract moves distally, the roots merge into a mixed sensory and motor nerve. Further distally, the mixed motor and sensory nerve divide again into dorsal and ventral primary rami, with the dorsal rami supplying the paraspinal muscles and the ventral rami supplying the lumbosacral plexus. The sensory fibers' cell body resides outside the spinal cord, within the dorsal root ganglia. Knowing the anatomic location of the dorsal root ganglia is important in understanding the reasoning behind the common findings on nerve conduction studies and needle electromyography when diagnosing lumbosacral radiculopathy. The lesion is typically proximal to the dorsal root, in or around the spinal cord, affecting the ventral root more than the dorsal root. This lesion typically yields normal sensory findings on the nerve conduction portion of the electrodiagnostic study, as the dorsal root ganglion is unaffected. On the other hand, the cell body of motor fibers within the ventral root resides within the spinal cord; thus, a lesion in this area yields pathological electromyographic potentials (fibrillations and positive sharp waves) commonly seen in lumbosacral radiculopathy.

L2-L4 Radiculopathies

L2-L4 radiculopathies are typically grouped. It is challenging to distinguish an isolated spinal lesion due to the broad L2-L4 myotomal overlap of the anterior thigh muscles (as well as adjacent root overlap).[4] 

• L2: Sensory loss in the anterolateral thigh may suggest L2 radiculopathy.
• L3: Weakness in hip adduction and flexion and sensory loss on the medial aspect of the thigh may suggest L3 radiculopathy.
  • If a patient presents with medial thigh sensory changes and weakness, the clinician must not overlook other conditions that may present similarly, such as obturator neuropathy or femoral neuropathy.[3]
• L4: The most common level of lumbosacral disc herniation is L4.
  • Clinical presentation may include numbness and tingling in the medial lower leg and diminished patellar reflexes.

L5 Radiculopathy

Diminished sensation along the anterolateral aspect of the lower leg and dorsum of the foot, accompanied by foot drop and difficulty with heel walking, are common features that may suggest L5 radiculopathy. It is essential to know about other causes of foot drop and sensory changes along the lateral lower leg; additional causes include common fibular and sciatic neuropathy and lumbosacral plexopathy.[3]

S1 Radiculopathy

A patient with difficulty toe walking, a diminished or absent Achilles reflex, or weakness in plantar flexion may represent S1 radiculopathy. Of note, sciatic neuropathy and lower lumbosacral plexopathy may present similarly to S1 radiculopathy. An abnormal H-reflex (a recordable monosynaptic S1 reflex response elicited with electrical stimulation) observed when stimulating the tibial nerve during the nerve conduction study can be seen in S1 radiculopathy. The H-reflex response is sensitive, although not specific for detecting an S1 radiculopathy.[4] The nerve roots exit below their corresponding vertebral level in the lumbosacral spine—for example, the L4 nerve root exits at the L4-L5 disc space.

Indications

Electrodiagnostic testing is indicated in a patient with sensory and motor signs and symptoms in a limb without imaging studies to explain the pathology. Signs and symptoms may include numbness, pain, or paresthesia along a dermatomal distribution, weakness along a myotomal distribution, and diminished or absent deep tendon reflexes corresponding to a specific nerve root.[5][6]

Contraindications

Although nerve conduction studies and needle electromyography are considered safe diagnostic procedures, a few precautions and safety measures should be addressed before performing. It is imperative to review patients' medication lists, as taking an anticoagulant is classified as a relative contraindication to performing needle electromyography.[7] However, needle electromyography is contraindicated if a patient has an uncontrolled bleeding disorder. Needle electromyography should also be avoided in areas of active skin infections such as cellulitis and in limbs that present with lymphedema, as it can act as a nidus for infection.[8] Nerve conduction studies are contraindicated in patients with cardiac defibrillators. For patients with cardiac pacemakers, while nerve conduction studies are permissible, it is advised that the electrical stimulation not be performed directly over the implant. It is best practice to discuss the risks of the diagnostic studies with each patient undergoing a diagnostic procedure and involve their primary doctor to ensure a safe test administration.[9]

Clinical Significance

Nerve Conduction Studies

• Sensory: Sensory nerve action potentials (SNAPs) are typically normal in radiculopathy because the lesion is proximal to the corresponding dorsal root ganglion. Because of this, the axon and its cell body remain intact. If the SNAP is abnormal, other lesions must be considered.

• Motor: Most muscles in the extremities receive innervation from more than one nerve root; typically, there is overlap with the root levels above or below. Therefore, it is unlikely that the amplitude of a compound muscle action potential (CMAP) will be abnormal in motor nerve conduction studies in isolated radiculopathy.

The above indicates that the nerve conduction study is useful in ruling out other causes of conditions similar to lumbosacral radiculopathy. The diagnostic findings are mostly through the needle electromyography portion of the exam.

Needle Electromyography

In diagnosing lumbosacral radiculopathy, needle electromyography is the most useful diagnostic study. Findings of positive sharp waves and or fibrillations illustrate spontaneous activity. The examiner should perform the needle electromyography on at least 3 muscles, specifically one paraspinal muscle and 2 limb muscles supplied by the same nerve root but innervated by different peripheral nerves. Spontaneous activity found in all muscles sampled following the above protocol (along with normal SNAP and CMAP findings) supports a diagnosis of lumbosacral radiculopathy. The utility of testing the paraspinal muscles in the work-up of lumbosacral radiculopathy is its use in deciphering between radiculopathy and a plexopathy, as needle electromyography of the paraspinal muscles is typically normal in a pure lumbar plexopathy.[10]

Enhancing Healthcare Team Outcomes

Lumbosacral radiculopathy has a prevalence of about 3% to 5% of the population.[3] The performance of nerve conduction studies and needle electromyography is helpful in its diagnosis. Pain is subjective and warrants an extensive evaluation to identify its source. Often, patients initially present to their primary care physician, who needs to understand the role of a physiatrist in diagnosing and treating pain disorders. Lumbosacral radiculopathy is best managed by an interdisciplinary team, including a physiatrist, primary care clinician, orthopedist or neurosurgeon, interventional pain specialist, psychology professional, physical and occupational therapists, and acupuncturist.[11]