Lumbosacral radiculopathy is a pathological disorder affecting the nerve root in the lumbosacral region of the spinal cord. Radiculopathy is commonly the result of compression of the nerve root from a structural lesion (i.e., herniated nucleus pulposus, calcified facet joint, or vertebral osteophyte), but may also result from irritation secondary to an infection, tumor, or surrounding tissue inflammation.[1]

Numbness, tingling, weakness, and radiating neuropathic type pain are all common symptoms of radiculopathy, which typically present unilaterally.[2] Common physical exam findings are muscle weakness in a specific myotomal distribution, sensory diminishment or loss in a specific dermatomal distribution, and diminishing or loss of deep tendon reflex relative to a particular root level. Patient signs and symptoms can vary, hence why electrodiagnostic evaluation is a critical tool for diagnosis. The history and physical examination are essential to establish a focused differential of neurological conditions; the electrodiagnostic examination assists in locating and characterizing the neurophysiologic type and severity of the lesion. A complete electrodiagnostic evaluation includes two steps; a nerve conduction study along with a needle electromyography study.

In the lumbosacral region of the spine, there are eleven pairs of spinal nerves (five lumbar, five sacral, and one coccygeal) that emerge from the spinal cord.[3] They exit the spinal cord as dorsal and ventral nerve roots, carrying sensory and motor axons, respectively. As the tract moves distally, the roots merge to become 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 cell body of sensory fibers resides outside of 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 so 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 not affected. 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 are typically grouped together. 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] Sensory loss in the anterolateral thigh may suggest L2 radiculopathy. Weakness in hip adduction and flexion, along with sensory loss on the medial aspect of the thigh, may suggest L3 radiculopathy. Of note, 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]

The most common level of lumbosacral disc herniation occurs at L4. Clinical presentation of numbness and tingling at the medial lower leg may occur. Diminished patellar reflexes may also present.

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. Of note, it is essential to be knowledgeable on 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]

A patient presenting with difficulty toe walking, a diminished or absent Achilles reflex, and/or weakness in plantar flexion may represent S1 radiculopathy. Of note, sciatic neuropathy and lower lumbosacral plexopathy may present similar to S1 radiculopathy. An abnormal H-reflex (a recordable mon-synaptic 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 the detection of an S1 radiculopathy.[4]

Of note, in the lumbosacral spine, the nerve roots exit below their corresponding vertebral level. For example, the L4 nerve root exits at the L4-L5 disc space.

Electrodiagnostic testing is indicated in a patient who presents 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 or diminished or absent deep tendon reflexes corresponding to a specific nerve root.[5][6]

Although nerve conduction studies and needle electromyography are considered safe diagnostic procedures, there are a few precautions and safety measures that should be addressed prior to their performance. It is imperative to review patients’ medication lists, as taking an anticoagulant is classified as a relative contraindication to performing needle electromyography.[7] 

If a patient has an uncontrolled bleeding disorder, however, needle electromyography is contraindicated. Needle electromyography should also be avoided in areas of active skin infections such as cellulitis and should also be avoided 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 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 administration of the test.[9]