IONM in Spinal Surgery

Introduction

IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

Symptoms and Diagnostics

Relevant clinical symptoms

1. Myelopathies. Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

2. Radiculopathies. Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

3. Foot drop. Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

4. Scoliosis. Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

5. Kyphosis.Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

6. Lordosis. Lordosis is an abnormal inward curvature of the lower spine.

Diagnostic Tests

1. Muscle strength exams. Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

2. Nerve conduction studies. Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

3. Imaging studies. CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

Lumbosacral spine surgery

Posterior lumbosacral interbody fusions. Lumbosacral fusions are usually performed to stabilize the spine and relieve pressure on the exiting nerve roots. The lumbosacral spine includes the L1-L5 levels as well as the sacrum (S1-S5 are five fused segments). Compression of the exiting nerve roots, or the spinal cord at the L1-2 level, can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Problems of the lumbar spine that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, lordosis and scoliosis.

The lumbar plexus is at risk for nerve injury during a fusion. At the upper lumbar levels, the nerves of the lumbar plexus are in most cases posterior to the surgical site. But the risk for injury to the lumbar plexus becomes greater if the surgical approach is below the L3 level. IONM of lower lumbar and sacral regions involves the monitoring of spinal nerve root function, primarily with EMG recordings and SSEPs. Ascending and descending spinal cord function is monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

Pedicle screw testing. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw and the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

Lateral lumbar interbody fusions. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach due to direct injury or prolonged use of retractors. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

Cervical spine surgery

In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

Thoracic spinal surgery

Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using pulse-train protocols that are similar to those used to generate MEPs. One study found that pedicle screw testing with triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874). Alternatively, another approach to direct pedicle screw testing has shown that pulse-train stimulation inside of the pedicle track with electromyography from lower limb muscles was very effective at detecting medial breaches from misplaced screws (Calancie et al., J Neurosurg Spine 20:675–691, 2014).

Scoliosis surgery

The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

Spinal tumor surgery

Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

Others

1. Interbody cages and bone grafts. For spinal decompressions involving a discectomy, the surgeon will remove a part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space between the two vertebral bodies with either a bone graft (e.g., autograft, allograft) or an interbody cage to maintain the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

2. Ondontoid (dens) fracture

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

Type I: avulsion fracture of the apex, in which a fragment tears away from the odontoid bone. Type II: fracture through the base of the dens, at the junction of the odontoid base and the body of C2. Type III: fracture extends into the body of the axis.